Help needed to assign a study from arraylist to student arraylist

[Iwan van den Hoek]

Hi,

I have been following a course in Java and with my latest assignment, I am running into a nullpointer exception each time I want to create a new student or Scholer with a given study or course.
It is a GUI based program. It consists of 7 classes and one GUI class. The superclass is StudenAdministratie that has assosiactions with Student and Study. Student has 2 subclasses RegulierStudent and Scholer. Studie has 2 subclasses CPP and Opleiding.
In Student administration I have 2 Arraylists one for students and one for Study's and courses. A student can be a RegulierStudent with a study or a scholer with a course, but not both.
The GUI has 4 tabs, the first tab is to create a student with a study chosen from a jcombobox.
The second tab creates a scholer with a course chosen from a jcombobox
The third tab is to add points or modules to the existing students and show the info
the fourth tab is toString that shows all the info per student.

I have written all the code but am stuck on the maakStudent Method and maakScholer method.
Any help is welcome

I have added the Gui as attachment, sorry for all the code I wanted to add it as zip file but that is not allowed.

package studentadmin;

import java.util.ArrayList;

public class StudentenAdministratie {

	private ArrayList<Student> studenten = null;
	private ArrayList<Studie> opleidingen = null;
	private double aantalPunten = 0.0;
	private int aantalModules = 0;
	Opleiding opleiding;
	CPP cpp;

	/**
	 * Creates an empty ArrayList with Students Creates an empty ArrayList with study's and courses
	 * 
	 * @author Iwan van den Hoek
	 */

	public StudentenAdministratie() {
		studenten = new ArrayList<Student>();
		opleidingen = new ArrayList<Studie>();
		mijnInit();
	}

	/**
	 * Initiates all the course and studies to be added to the ArrayList opleidingen
	 */
	private void mijnInit() {
		CPP java = new CPP("JAVA", 6);
		CPP softwarearchitect = new CPP("SOFTWAREARCHITECT", 4);
		CPP systeemontwikkelaar = new CPP("SYSTEEMONTWIKKELAAR", 3);
		Opleiding wiskunde = new Opleiding("WISKUNDE", 160);
		Opleiding informatica = new Opleiding("INFORMATICA", 200);
		opleidingen.add(java);
		opleidingen.add(softwarearchitect);
		opleidingen.add(systeemontwikkelaar);
		opleidingen.add(wiskunde);
		opleidingen.add(informatica);

	}
	/**
	 * Makes a new Student with a study
	 * 
	 * @param naam Name of the student
	 * @param richting Name of the Study
	 * @return
	 */
	public String maakStudent(String naam, String richting) {
		RegulierStudent student = new RegulierStudent(naam, opleiding);

		for (Studie opleiding : opleidingen) {
			if ((opleiding instanceof Opleiding) &&
                      (richting.equals(opleiding.getNaamStudie()))) {
				student.setStudie(opleiding);
				studenten.add(student);
			}
		}
		return naam;
	}

	public String maakScholer(String naam, String cursus) {
		Scholer scholer = new Scholer(naam, cpp);
		for (Studie opleiding : opleidingen) {
			if ((opleiding instanceof Opleiding) &&
                      (cursus.equals(opleiding.getNaamStudie()))) {
				scholer.setStudie(opleiding);
				studenten.add(scholer);
			}
		}

		return naam;

	}
	/**
	 * Add points to the student following a study
	 * 
	 * @param naam Name of the student
	 * @param aantalPunten points given to the student
	 */
	public void voegPuntenToe(String naam, double aantalPunten) {
		for (Student student : studenten) {
			if ((student instanceof RegulierStudent) &&
                      (naam.equals(student.getNaam()))) {
				student.setTotaalBehaaldePunten(
                         ((RegulierStudent) student).addPunten(aantalPunten));
			}
		}

	}
	/**
	 * Adds an achieved module to a student following a course
	 * 
	 * @param naam Name of the student
	 * @param aantalModules modules given to the student
	 */
	public void voegModulesToe(String naam, int aantalModules) {
		for (Student student : studenten) {
			if ((student instanceof Scholer) &&
                      (naam.equals(student.getNaam()))) {
				student.setTotaalBehaaldeModules(
                      ((Scholer) student).verhoogModules());
			}
		}
	}
	/**
	 * This methods fills the third tab with student info, when points or modules are added
	 * 
	 * @param naamStudent Name of the student
	 * @return
	 */
	public String infoPerStudent(String naamStudent) {
		String infoPerStudent = null;
		for (Student student : studenten) {
			if ((student instanceof RegulierStudent) && 
                       (naamStudent.equals(student.getNaam()))) {
				infoPerStudent = student.getNaam() +
                         ", " + student.getStudie().getNaamStudie() + ", " +
                          student.getTotaalBehaaldeAantalPunten() + " punten, "
						+ student.isGeslaagdOpleiding(
                                       student.getTotaalBehaaldeAantalPunten(), 200);
			} else if ((student instanceof Scholer) &&
                           (naamStudent.equals(student.getNaam()))) {
				infoPerStudent = student.getNaam() + ", " +
                                 student.getStudie().getNaamStudie() + ", " +
                                 student.getTotaalBehaaldeModules() + " modules, " +
                                 student.isGeslaagdCpp(student.getTotaalBehaaldeModules(), 6);
			}
		}
		return infoPerStudent;
	}
	/**
	 * Shows all the info per student on the fourth tab
	 */
	public String toString() {
		String alleStudenten = "";
		for (Student student : studenten) {
			if (student instanceof RegulierStudent) {
				alleStudenten = alleStudenten + student.getNaam() +
                             ", " + student.getStudie().getNaamStudie() + ", " +
                             student.getTotaalBehaaldeAantalPunten() + " punten, "
						+ student.isGeslaagdOpleiding(
                                 student.getTotaalBehaaldeAantalPunten(), 200) + "\n";
			} else {
				alleStudenten = alleStudenten + student.getNaam() +
                                ", " + student.getStudie().getNaamStudie() + ", " +
                                student.getTotaalBehaaldeModules() + " modules, "
						+ student.isGeslaagdCpp(
                                             student.getTotaalBehaaldeModules(), 6) + "\n";
			}
		}
		return alleStudenten;
	}

	/**
	 * Getters and Setters for the points and modules
	 * 
	 * @return
	 */
	public double getAantalPunten() {
		return this.aantalPunten;
	}

	public int getAantalModules() {
		return this.aantalModules;
	}
	public void setAantalModules(int aantalModules) {
		this.aantalModules = aantalModules;
	}
	public void setAantalPunten(double aantalPunten) {
		this.aantalPunten = aantalPunten;
	}
}

package studentadmin;

abstract public class Student {
	private String naam = "";
	private Studie studie;
	private double totaalBehaaldePunten = 0.0;
	private int totaalBehaaldeModules = 0;

	/**
	 * Constructor for student
	 * 
	 * @author Iwan van den Hoek
	 * @param naam
	 */
	public Student(String naam) {
		this.naam = naam;

	}

	/**
	 * Method to see if the student has achieved teh required points and has passed the exam
	 * 
	 * @param punten Given points
	 * @param maxAantalPunten max points for the study
	 * @return
	 */
	public String isGeslaagdOpleiding(double punten, double maxAantalPunten) {
		if (punten < maxAantalPunten) {
			String geslaagdOpleiding = "niet geslaagd";
			return geslaagdOpleiding;
		} else {
			String geslaagdOpleiding = "geslaagd";
			return geslaagdOpleiding;
		}

	}
	/**
	 * Method to see if the student has achieved the required modules and has passed the course
	 * 
	 * @param modules Given modules
	 * @param maxAantalModules max modules per course
	 * @return
	 */
	public String isGeslaagdCpp(int modules, int maxAantalModules) {
		if (modules < maxAantalModules) {
			String geslaagdCpp = "niet geslaagd";
			return geslaagdCpp;
		} else {
			String geslaagdCpp = "geslaagd";
			return geslaagdCpp;
		}
	}

	/**
	 * Getters and Setters
	 * 
	 * @return
	 */
	public String getNaam() {
		return naam;
	}

	public void setNaam(String naam) {
		this.naam = naam;
	}

	public Studie getStudie() {
		return this.studie;
	}

	public void setStudie(Studie studie) {
		this.studie = studie;
	}
	public void setTotaalBehaaldePunten(double totaalBehaaldePunten) {
		this.totaalBehaaldePunten = totaalBehaaldePunten;
	}
	public void setTotaalBehaaldeModules(int totaalBehaaldeModules) {
		this.totaalBehaaldeModules = totaalBehaaldeModules;
	}
	public double getTotaalBehaaldeAantalPunten() {
		return totaalBehaaldePunten;
	}

	public int getTotaalBehaaldeModules() {
		return totaalBehaaldeModules;
	}

}

package studentadmin;

abstract public class Studie {

	protected String naamStudie;
	private Opleiding opleiding;
	private CPP cpp;

	/**
	 * Costructor for Study's and courses
	 * 
	 * @author Iwan van den Hoek
	 * @param naamStudie
	 */
	public Studie(String naamStudie) {
		this.naamStudie = naamStudie;
	}

	public String toString() {
		return naamStudie;

	}
	/**
	 * Getters and Setters
	 * 
	 * @param naamStudie
	 */
	public void setNaamStudie(String naamStudie) {
		this.naamStudie = naamStudie;
	}

	public void setOpleiding(Opleiding opleiding) {
		this.opleiding = opleiding;
	}

	public void setCpp(CPP cpp) {
		this.cpp = cpp;
	}

	public String getNaamStudie() {
		return this.naamStudie;
	}
	public Opleiding getOpleiding() {
		return opleiding;
	}

	public CPP getCpp() {
		return cpp;
	}
}

package studentadmin;

public class Scholer extends Student {
	private CPP cpp;
	private int behaaldeModules;
	private int totaalAantalModules;

	/**
	 * Construcotor to create a new Scholer with course
	 * 
	 * @author Iwan van den Hoek
	 * @param naamStudie Name of the Student
	 * @param cpp The selected course
	 */
	public Scholer(String naamStudie, CPP cpp) {
		super(naamStudie);
		this.cpp = cpp;
	}
	/**
	 * Method to increase the achieved modules by 1
	 * 
	 * @return
	 */
	public int verhoogModules() {
		totaalAantalModules = totaalAantalModules + 1;
		return totaalAantalModules;

	}
	/**
	 * Getters and Setters
	 * 
	 * @return
	 */
	public int getBehaaldeModules() {
		return behaaldeModules;
	}
	public void setBehaaldeModules(int behaaldeModules) {
		this.behaaldeModules = behaaldeModules;
	}

	public CPP getCpp() {
		return cpp;
	}

}

package studentadmin;

public class RegulierStudent extends Student {

	private double behaaldePunten;
	private Opleiding opleiding;
	private double totaalAantalPunten;
	/**
	 * Constructor to create a Student with a study
	 * 
	 * @author Iwan van den Hoek
	 * @param naam Name of the student
	 * @param opleiding The chosen study
	 */
	public RegulierStudent(String naam, Opleiding opleiding) {
		super(naam);
		this.opleiding = opleiding;
	}
	/**
	 * Method to add points to the Student for the study
	 * 
	 * @param aantalPunten
	 * @return
	 */
	public double addPunten(double aantalPunten) {
		totaalAantalPunten = totaalAantalPunten + aantalPunten;
		return aantalPunten;

	}
	/**
	 * Getters and Setters
	 * 
	 * @return
	 */
	public double getBehaaldePunten() {
		return behaaldePunten;
	}
	public void setBehaaldePunten(double behaaldePunten) {
		this.behaaldePunten = behaaldePunten;
	}

	public Opleiding getOpleiding() {
		return opleiding;
	}

}

package studentadmin;
class Opleiding extends Studie {

	private double maxAantalPunten;

	/**
	 * Constructer for a study
	 * 
	 * @author Iwan van den Hoek
	 * @param naamStudie name of the study
	 * @param maxAantalPunten maximum points per study
	 */
	public Opleiding(String naamStudie, double maxAantalPunten) {
		super(naamStudie);
		this.maxAantalPunten = maxAantalPunten;
	}

	/**
	 * Getters and Setters
	 * 
	 * @return
	 */
	public double getMaxAantalPunten() {

		return maxAantalPunten;
	}

	public void setMaxAantalPunten(double maxAantalPunten) {
		this.maxAantalPunten = maxAantalPunten;
	}

	public String toString() {
		return naamStudie;

	}

}

package studentadmin;
class CPP extends Studie {

	private int maxAantalModules;
	/**
	 * Construtor for a course
	 * 
	 * @author Iwan van den Hoek
	 * @param naamStudie Name of the course
	 * @param maxAantalModules max modules per course
	 */
	public CPP(String naamStudie, int maxAantalModules) {
		super(naamStudie);
		this.maxAantalModules = maxAantalModules;

	}
	/**
	 * Getters and Setters
	 * 
	 * @return
	 */
	public int getMaxAantalModules() {
		return maxAantalModules;
	}

	public void setMaxAantalModules(int maxAantalModules) {
		this.maxAantalModules = maxAantalModules;
	}

	public String toString() {
		return naamStudie;

	}

}

[Jeanne Boyarsky]

Iwan,
That's a lot of code! Can you narrow it down to create a Small Self Contained Example? This let you get rid of the classes that aren't causing problems. Also, I don't see a main method so I'm not clear on how you run the code or what line the error occurs on.

[Iwan van den Hoek]

Jeanne Boyarsky wrote:Iwan,
That's a lot of code! Can you narrow it down to create a Small Self Contained Example? This let you get rid of the classes that aren't causing problems. Also, I don't see a main method so I'm not clear on how you run the code or what line the error occurs on.

Hi Jeanne,

Thank you for responding, I am not familiar with SSCE.

I did attach the GUI code as it was alot of code, I could send you the complete source code.
I can't attach .java or .txt files so I have placed in a code fragment sorry.
The need for the classes is too understand the inheritance within OO Java.

The Gui has a Pull down menu with a String Value if the String value corresponds with a Study in the Arraylist, it needs to add it to the student with that study to the Student Arraylist
The code that is not working is In StudentAdministratie Class:

/**
	 * Makes a new Student with a study direction
	 * using an ArrayList  opleidingen
	 * @param naam Name of the student
	 * @param richting Name of the Study
	 * @return
	 */
	public String maakStudent(String naam, String richting) {
		RegulierStudent student = new RegulierStudent(naam, opleiding);

		for (Studie opleiding : opleidingen) {
			if ((opleiding instanceof Opleiding) && (richting.equals(opleiding.getNaamStudie()))) {
				student.setStudie(opleiding);
				studenten.add(student);
			}
		}
		return naam;
	}

This part of the Gui Code executes it:

 /**
     * Creates the actions needed per button in the Gui
     */
    private void studentButtonAction() {
 
        admin.maakStudent(studentTextField.getText(), (String) opleidingComboBox.getSelectedItem());
        studentTextField.setText("");
        opleidingComboBox.setSelectedIndex(-1);
 
    }

[Jeanne Boyarsky]

I don't want the complete code. I want you to narrow down the problem. This consists of telling us exactly where the error is and narrowing down the problem. Do you need a GUI to reproduce the problem? How much of the code can you remove and have the problem still exist?

I don't expect you to be familiar with what a SSCCE is. That's why I provided a link describing it. This is a valuable troubleshooting technique.

[Knute Snortum]

You say the code is "not working." Could you be more specific?

[Iwan van den Hoek]

Knute Snortum wrote:You say the code is "not working." Could you be more specific?

The Method to create a student as I have described is resulting in a nullpointer exception.

We were given a GUI and we need to create the code to use the GUI.

The Guis has a Text box, here you fill in the name of the student. With the pulldown menu from the Jcombbox you select a study.
You press teh button and it should execute the maakStudent method and add the student with the given study to an ArrayList studenten.

The moment I hit the button/execute the method it throws a Nullpointer exception

If you need the Gui code just let me know.

[Knute Snortum]

Can you post the full message of the error, including the stack trace?

[Iwan van den Hoek]

Knute Snortum wrote:Can you post the full message of the error, including the stack trace?

Here is the error:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at studentadmingui.StudentAdminFrame.studentButtonAction(StudentAdminFrame.java:75)
at studentadmingui.StudentAdminFrame.access$3(StudentAdminFrame.java:73)
at studentadmingui.StudentAdminFrame$4.actionPerformed(StudentAdminFrame.java:379)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$500(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

[Knute Snortum]

The important part of the stack trace for you is here:
  studentadmingui.StudentAdminFrame.studentButtonAction(StudentAdminFrame.java:75)
It say that the error occurred on line 75 of a class called StudentAdminFrame in a method called studentButtonAction. Could you post this class or at least the method?

[Iwan van den Hoek]

Knute Snortum wrote:The important part of the stack trace for you is here:
  studentadmingui.StudentAdminFrame.studentButtonAction(StudentAdminFrame.java:75)
It say that the error occurred on line 75 of a class called StudentAdminFrame in a method called studentButtonAction. Could you post this class or at least the method?

Hi Knutte,

Here is the GUI code:

package studentadmingui;

import java.awt.Rectangle;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTabbedPane;
import javax.swing.JTextArea;
import javax.swing.JTextField;

import studentadmin.StudentenAdministratie;

public class StudentAdminFrame extends JFrame {

	private static final long serialVersionUID = 1L;
	private JPanel jContentPane = null;
	private JScrollPane jScrollPane = null;
	private JTextArea uitvoerGebied = null;
	private JButton toonAlleKnop = null;
	private JTabbedPane mijnTabbladenPanel = null;
	private JPanel voegStudenttoePanel = null;
	private JPanel voegScholertoePanel = null;
	private JPanel studentPanel = null;
	private JPanel alleStudentenPanel = null;
	private JLabel bestaandenaamLabel = null;
	private JTextField bestaandeNaamVeld = null;
	private JLabel infoLabel = null;
	private JTextField studentInfoVeld = null;
	private JLabel nieuwepuntenLabel = null;
	private JTextField puntenVeld = null;
	private JLabel uitlegLabel = null;
	private JButton moduleKnop = null;
	private JLabel opleidingLabel = null;
	private JLabel nstudentLabel = null;
	private JComboBox<String> opleidingComboBox = null;
	private JLabel studentLabel = null;
	private JTextField studentTextField = null;
	private JButton studentButton = null;
	private JComboBox<String> scholingComboBox = null;
	private JLabel scholerLabel = null;
	private JTextField scholerTextField = null;
	private JButton scholerButton = null;
	private StudentenAdministratie admin = null;

	/**
	 * This is the default constructor
	 */
	public StudentAdminFrame() {
		super();
		initialize();
	}

	/**
	 * This method initializes this
	 * 
	 * @return void
	 */
	private void initialize() {
		this.setSize(466, 347);
		this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		this.setContentPane(getJContentPane());
		this.setTitle("StudentAdministratie");
	}

	/**
	 * Creates the actions needed per button in the Gui
	 */
	private void studentButtonAction() {

		admin.maakStudent(studentTextField.getText(), (String) opleidingComboBox.getSelectedItem());
		studentTextField.setText("");
		opleidingComboBox.setSelectedIndex(-1);

	}

	private void scholerButtonAction() {
		admin.maakScholer(scholerTextField.getText(), (String) scholingComboBox.getSelectedItem());
		scholerTextField.setText("");
		scholingComboBox.setSelectedIndex(-1);
	}

	private void toonAlleKnopAction() {
		uitvoerGebied.setText("");
		uitvoerGebied.setText(admin.toString());
	}

	private void puntenVeldAction() {
		admin.voegPuntenToe(bestaandeNaamVeld.getText(), Double.parseDouble(puntenVeld.getText()));
		studentInfoVeld.setText(admin.infoPerStudent(bestaandeNaamVeld.getText()));
		bestaandeNaamVeld.setText("");
		puntenVeld.setText("");
	}

	private void moduleKnopAction() {
		admin.voegModulesToe(bestaandeNaamVeld.getText(), getDefaultCloseOperation());
		studentInfoVeld.setText(admin.infoPerStudent(bestaandeNaamVeld.getText()));
		bestaandeNaamVeld.setText("");
	}

	/**
	 * This method initializes jContentPane
	 * 
	 * @return javax.swing.JPanel
	 */
	private JPanel getJContentPane() {
		if (jContentPane == null) {
			infoLabel = new JLabel();
			infoLabel.setBounds(new Rectangle(14, 278, 424, 33));
			infoLabel.setText("");
			jContentPane = new JPanel();
			jContentPane.setLayout(null);
			jContentPane.add(getMijnTabbladenPanel(), null);
			jContentPane.add(infoLabel, null);
		}
		return jContentPane;
	}

	/**
	 * This method initializes jScrollPane
	 * 
	 * @return javax.swing.JScrollPane
	 */
	private JScrollPane getJScrollPane() {
		if (jScrollPane == null) {
			jScrollPane = new JScrollPane();
			jScrollPane.setBounds(new Rectangle(6, 65, 382, 121));
			jScrollPane.setViewportView(getUitvoerGebied());
		}
		return jScrollPane;
	}

	/**
	 * This method initializes uitvoerGebied
	 * 
	 * @return javax.swing.JTextArea
	 */
	private JTextArea getUitvoerGebied() {
		if (uitvoerGebied == null) {
			uitvoerGebied = new JTextArea();
			uitvoerGebied.setEditable(false);
		}
		return uitvoerGebied;
	}

	/**
	 * This method initializes toonAlleKnop
	 * 
	 * @return javax.swing.JButton
	 */
	private JButton getToonAlleKnop() {
		if (toonAlleKnop == null) {
			toonAlleKnop = new JButton();
			toonAlleKnop.setText("Toon alle studenten");
			toonAlleKnop.setBounds(new Rectangle(11, 8, 147, 20));
			toonAlleKnop.addActionListener(new java.awt.event.ActionListener() {
				public void actionPerformed(java.awt.event.ActionEvent e) {
					toonAlleKnopAction();
				}
			});
		}
		return toonAlleKnop;
	}

	/**
	 * This method initializes mijnTabbladenPanel
	 * 
	 * @return javax.swing.JTabbedPane
	 */
	private JTabbedPane getMijnTabbladenPanel() {
		if (mijnTabbladenPanel == null) {
			mijnTabbladenPanel = new JTabbedPane();
			mijnTabbladenPanel.setBounds(new Rectangle(13, 19, 423, 224));
			mijnTabbladenPanel.addTab("nieuwe student", null,
					getVoegStudenttoePanel(), null);
			mijnTabbladenPanel.addTab("nieuwe scholer", null,
					getVoegScholertoePanel(), null);
			mijnTabbladenPanel.addTab("studentinfo", null, getStudentPanel(), null);
			mijnTabbladenPanel.addTab("alle studenten", null,
					getAlleStudentenPanel(), null);
		}
		return mijnTabbladenPanel;
	}

	/**
	 * This method initializes voegStudenttoePanel
	 * 
	 * @return javax.swing.JPanel
	 */
	private JPanel getVoegStudenttoePanel() {
		if (voegStudenttoePanel == null) {
			studentLabel = new JLabel();
			studentLabel.setBounds(new Rectangle(15, 55, 91, 24));
			studentLabel.setText("Naam student");
			nstudentLabel = new JLabel();
			nstudentLabel.setBounds(new Rectangle(16, 16, 146, 24));
			nstudentLabel.setText("Selecteer een opleiding");
			voegStudenttoePanel = new JPanel();
			voegStudenttoePanel.setLayout(null);
			voegStudenttoePanel.add(nstudentLabel, null);
			voegStudenttoePanel.add(getOpleidingComboBox(), null);
			voegStudenttoePanel.add(studentLabel, null);
			voegStudenttoePanel.add(getStudentTextField(), null);
			voegStudenttoePanel.add(getStudentButton(), null);
		}
		return voegStudenttoePanel;
	}

	private JPanel getVoegScholertoePanel() {
		if (voegScholertoePanel == null) {
			scholerLabel = new JLabel();
			scholerLabel.setBounds(new Rectangle(16, 54, 116, 25));
			scholerLabel.setText("Naam scholer");
			opleidingLabel = new JLabel();
			opleidingLabel.setBounds(new Rectangle(16, 16, 173, 25));
			opleidingLabel.setText("Selecteer een CPP-Opleiding");
			voegScholertoePanel = new JPanel();
			voegScholertoePanel.setLayout(null);
			voegScholertoePanel.add(opleidingLabel, null);
			voegScholertoePanel.add(getScholingComboBox(), null);
			voegScholertoePanel.add(scholerLabel, null);
			voegScholertoePanel.add(getScholerTextField(), null);
			voegScholertoePanel.add(getScholerButton(), null);
		}
		return voegScholertoePanel;
	}

	/**
	 * This method initializes studentPanel
	 * 
	 * @return javax.swing.JPanel
	 */
	private JPanel getStudentPanel() {
		if (studentPanel == null) {
			uitlegLabel = new JLabel();
			uitlegLabel.setBounds(new Rectangle(16, 8, 334, 19));
			uitlegLabel.setText("Geef enter om invoer te bevestigen");
			nieuwepuntenLabel = new JLabel();
			nieuwepuntenLabel.setBounds(new Rectangle(14, 63, 256, 20));
			nieuwepuntenLabel.setText("Punten behaald (alleen reguliere opleiding) ");
			bestaandenaamLabel = new JLabel();
			bestaandenaamLabel.setBounds(new Rectangle(14, 35, 86, 20));
			bestaandenaamLabel.setText("Studentnaam");
			studentPanel = new JPanel();
			studentPanel.setLayout(null);
			studentPanel.add(bestaandenaamLabel, null);
			studentPanel.add(getBestaandeNaamVeld(), null);
			studentPanel.add(getStudentInfoVeld(), null);
			studentPanel.add(nieuwepuntenLabel, null);
			studentPanel.add(getPuntenVeld(), null);
			studentPanel.add(uitlegLabel, null);
			studentPanel.add(getModuleKnop(), null);
		}
		return studentPanel;
	}

	/**
	 * This method initializes alleStudentenPanel
	 * 
	 * @return javax.swing.JPanel
	 */
	private JPanel getAlleStudentenPanel() {
		if (alleStudentenPanel == null) {
			alleStudentenPanel = new JPanel();
			alleStudentenPanel.setLayout(null);
			alleStudentenPanel.add(getToonAlleKnop(), null);
			alleStudentenPanel.add(getJScrollPane(), null);
		}
		return alleStudentenPanel;
	}

	/**
	 * This method initializes bestaandeNaamVeld
	 * 
	 * @return javax.swing.JTextField
	 */
	private JTextField getBestaandeNaamVeld() {
		if (bestaandeNaamVeld == null) {
			bestaandeNaamVeld = new JTextField();
			bestaandeNaamVeld.setBounds(new Rectangle(114, 36, 151, 20));
		}
		return bestaandeNaamVeld;
	}

	/**
	 * This method initializes studentInfoVeld
	 * 
	 * @return javax.swing.JTextField
	 */
	private JTextField getStudentInfoVeld() {
		if (studentInfoVeld == null) {
			studentInfoVeld = new JTextField();
			studentInfoVeld.setBounds(new Rectangle(10, 152, 392, 27));
			studentInfoVeld.setEditable(false);
		}
		return studentInfoVeld;
	}

	/**
	 * This method initializes puntenVeld
	 * 
	 * @return javax.swing.JTextField
	 */
	private JTextField getPuntenVeld() {
		if (puntenVeld == null) {
			puntenVeld = new JTextField();
			puntenVeld.setBounds(new Rectangle(284, 63, 47, 20));
			puntenVeld.addActionListener(new java.awt.event.ActionListener() {
				public void actionPerformed(java.awt.event.ActionEvent e) {
					puntenVeldAction();
				}
			});
		}
		return puntenVeld;
	}

	/**
	 * This method initializes moduleKnop
	 * 
	 * @return javax.swing.JButton
	 */
	private JButton getModuleKnop() {
		if (moduleKnop == null) {
			moduleKnop = new JButton();
			moduleKnop.setBounds(new Rectangle(14, 91, 328, 21));
			moduleKnop.setText("Module behaald (alleen CPP)");
			moduleKnop.addActionListener(new java.awt.event.ActionListener() {
				public void actionPerformed(java.awt.event.ActionEvent e) {
					moduleKnopAction();
				}
			});
		}
		return moduleKnop;
	}

	/**
	 * This method initializes opleidingComboBox
	 * 
	 * @return javax.swing.JComboBox
	 */
	private JComboBox getOpleidingComboBox() {
		if (opleidingComboBox == null) {
			String opleiding[] = {"Wiskunde", "Informatica"};
			opleidingComboBox = new JComboBox(opleiding);
			opleidingComboBox.setSelectedIndex(-1);
			opleidingComboBox.setToolTipText("");
			opleidingComboBox.setBounds(new Rectangle(195, 16, 200, 24));
		}
		return opleidingComboBox;
	}

	/**
	 * This method initializes studentTextField
	 * 
	 * @return javax.swing.JTextField
	 */
	private JTextField getStudentTextField() {
		if (studentTextField == null) {
			studentTextField = new JTextField();
			studentTextField.setBounds(new Rectangle(195, 55, 197, 24));
		}
		return studentTextField;
	}

	/**
	 * This method initializes studentButton
	 * 
	 * @return javax.swing.JButton
	 */
	private JButton getStudentButton() {
		if (studentButton == null) {
			studentButton = new JButton();
			studentButton.addActionListener(new ActionListener() {
				public void actionPerformed(java.awt.event.ActionEvent e) {
					studentButtonAction();
				}
			});
			studentButton.setBounds(new Rectangle(16, 103, 153, 24));
			studentButton.setText("Voeg student toe");
		}
		return studentButton;
	}

	/**
	 * This method initializes scholingComboBox
	 * 
	 * @return javax.swing.JComboBox
	 */
	private JComboBox getScholingComboBox() {
		if (scholingComboBox == null) {
			String cursus[] = {"Java", "SoftwareArchitect", "SysteemOntwikkelaar"};
			scholingComboBox = new JComboBox(cursus);
			scholingComboBox.setSelectedIndex(-1);
			scholingComboBox.setBounds(new Rectangle(210, 16, 167, 25));
		}
		return scholingComboBox;
	}

	/**
	 * This method initializes scholerTextField
	 * 
	 * @return javax.swing.JTextField
	 */
	private JTextField getScholerTextField() {
		if (scholerTextField == null) {
			scholerTextField = new JTextField();
			scholerTextField.setBounds(new Rectangle(210, 54, 166, 25));
		}
		return scholerTextField;
	}

	/**
	 * This method initializes scholerButton
	 * 
	 * @return javax.swing.JButton
	 */
	private JButton getScholerButton() {
		if (scholerButton == null) {
			scholerButton = new JButton();
			scholerButton.setBounds(new Rectangle(14, 103, 163, 25));
			scholerButton.setText("Voeg scholer toe");
			scholerButton.addActionListener(new java.awt.event.ActionListener() {
				public void actionPerformed(java.awt.event.ActionEvent e) {
					scholerButtonAction();
				}
			});
		}
		return scholerButton;
	}

	public static void main(String[] args) {
		StudentAdminFrame fr = new StudentAdminFrame();
		fr.setVisible(true);
	}

} // @jve:decl-index=0:visual-constraint="10,10"

[Knute Snortum]

So something on this line can throw a NullPointerException (NPE). Do you know what it is?

admin.maakStudent(studentTextField.getText(), (String) opleidingComboBox.getSelectedItem());

[Iwan van den Hoek]

Knute Snortum wrote:So something on this line can throw a NullPointerException (NPE). Do you know what it is?

admin.maakStudent(studentTextField.getText(), (String) opleidingComboBox.getSelectedItem());

Hi Knutte,

I don't know exactly what the problem is, I think it is a combination of the method maakStudent and the studentbuttoaction.
the mthod maakStudent is causing the Nullpointer I think.

I have to add a student based on the textfield in the Gui to the Studenten ArrayList and check the already filled Opleidingen Arraylist for the Study based in the selected pulldown menu.
I have to combine the student with the study from the arraylist.

Hope this clarify it.

[Iwan van den Hoek]

Hi Knutte,

I have discoverd that I am doing something wrong with the construcor in the class RegulierStudent
This is the constructor but the Opleiding is null;

public RegulierStudent(String naam, Opleiding opleiding) {
        super(naam);
        this.opleiding = opleiding;
    }

I should do something like this I think:

 public RegulierStudent(String naam, Opleiding opleiding) {
        super(naam);
        this.opleiding = opleiding.getOpleiding();
    }

[Knute Snortum]

It looks like studentTextField can be null. Maybe you didn't call getStudentTextField() before studentButtonAction()?

[Knute Snortum]

this.opleiding = opleiding.getOpleiding();

I don't think that's necessary, and it could throw a NPE.

[Iwan van den Hoek]

Knute Snortum wrote:

this.opleiding = opleiding.getOpleiding();

I don't think that's necessary, and it could throw a NPE.

Hi Knutte,

I am calling the getText and I could be that I am doing something wrong with the maakStudent.

I need to add the new student in for loop and then search for the correct study.

I just need to figure out how to do that

public String maakStudent(String naam, String richting) {
		RegulierStudent student = new RegulierStudent(naam, opleiding);//This needs to be in the for loop

		for (Studie opleiding : opleidingen) {
			
			if ((opleiding instanceof Opleiding) && (richting.equals(opleiding.getNaamStudie()))) {
				
				student.setStudie(opleiding);
				studenten.add(student);
			}

[Iwan van den Hoek]

Knute Snortum wrote:

this.opleiding = opleiding.getOpleiding();

I don't think that's necessary, and it could throw a NPE.

Hi Knutte,

I think it should be something like this although it's not working:

public String maakStudent(String naam, String richting) {

		for (Studie opleiding : opleidingen) {

			if ((opleiding instanceof Opleiding) && (richting.equals(opleiding.getNaamStudie()))) {
				RegulierStudent student = new RegulierStudent(naam, (Opleiding) opleiding);
				student.setStudie(opleiding);
				studenten.add(student);
			}
		}
		return naam;

[Knute Snortum]

Knute Snortum wrote:It looks like studentTextField can be null. Maybe you didn't call getStudentTextField() before studentButtonAction()?

I still think this is the problem.

[Iwan van den Hoek]

Knute Snortum wrote:

Knute Snortum wrote:It looks like studentTextField can be null. Maybe you didn't call getStudentTextField() before studentButtonAction()?

I still think this is the problem.

Hi knuute,

In the Gui I execute the following code

private void studentButtonAction() {

		admin.maakStudent(studentTextField.getText(), (String) opleidingComboBox.getSelectedItem());
		studentTextField.setText("");
		opleidingComboBox.setSelectedIndex(-1);

	}

I tried to put studentTextField.getText(); above he admin.maakStudent, but still no go.

[Knute Snortum]

I think studentTextField is null.

[Iwan van den Hoek]

Knute Snortum wrote:I think studentTextField is null.

Hi Knute,

How can I solve that problem?

[Campbell Ritchie]

I have come late to this discussion after Knute has done all the work.
Don't initialise fields to null. Don't rely on methods to test for nullity; obviously you are trying to use that field before it is given a real value. Remove the == null test from all methods. Write a constructor for your class and initialise all fields, including that text component, to real values.

Remember field means a piece of information contained in the class or object, not a kind of text component.

[Carey Brown]

You might want to try

admin.maakStudent(getStudentTextField().getText(), (String) opleidingComboBox.getSelectedItem());

[Iwan van den Hoek]

Campbell Ritchie wrote:I have come late to this discussion after Knute has done all the work.
Don't initialise fields to null. Don't rely on methods to test for nullity; obviously you are trying to use that field before it is given a real value. Remove the == null test from all methods. Write a constructor for your class and initialise all fields, including that text component, to real values.

Remember field means a piece of information contained in the class or object, not a kind of text component.

Hi Campbell,

Thank you for joining the discussion:

We got the studentAdminFrame form the teacher and I know creating null doesn't help with testing.
The text field is like this, what is wrong with this:

private JTextField getStudentTextField() {
		if (studentTextField == null) {
			studentTextField = new JTextField();
			studentTextField.setBounds(new Rectangle(195, 55, 197, 24));
				}
		return studentTextField;
	}

[Iwan van den Hoek]

Carey Brown wrote:You might want to try

admin.maakStudent(getStudentTextField().getText(), (String) opleidingComboBox.getSelectedItem());

Hi Carey,

Thank you for joining the discussion, I tried your code, but it still gets the Null Pointer:

I changed the method maakStudent, but I think there is something wrong there too:

public String maakStudent(String naam, String richting) {

		for (Studie opleiding : opleidingen) {
			if ((opleiding instanceof Opleiding) && (richting.equals(opleiding.getNaamStudie()))) {
				RegulierStudent student = new RegulierStudent(naam, (Opleiding) opleiding.getOpleiding());
				student.setStudie(opleiding);
				studenten.add(student);
			}
		}
		return naam;
	}

[Carey Brown]

I think I didn't take it far enough. Try

admin.maakStudent(getStudentTextField().getText(), (String) getOpleidingComboBox().getSelectedItem());

admin.maakStudent(studentTextField.getText(), (String) opleidingComboBox.getSelectedItem());

In your original code (above) for this, if you are getting a NPE on this line, it can only be one of:

admin

studentTextField

opleidingComboBox

Insert three print statements for these variables just before line 75 to see which one is null.

[Campbell Ritchie]

Iwan van den Hoek wrote:. . . what is wrong with this:
. . .

Everything.

You are confusing the field and a local variable. You are doing something in a method which ought to be done in the constructor. You are using setBounds which suggests you are not using a layout correctly. The only thing which would make that method worse would be to give it public access.

The method with the for‑each loop is an incorrect use of a List. In fact it suggests to me you do not want a List at all, but a different data structure.

[Knute Snortum]

You really shouldn't have the initialization of the fields in the getters. You have an initialize() method; why not use it to initialize your fields?

    //...
    private JTextField studentTextField;
    //...
    private void initialize() {
        this.setSize(466, 347);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setContentPane(getJContentPane());
        this.setTitle("StudentAdministratie");
        //...
        studentTextField = new JTextField();
        studentTextField.setBounds(new Rectangle(195, 55, 197, 24));
        //...
    }

[Iwan van den Hoek]

Campbell Ritchie wrote:

Iwan van den Hoek wrote:. . . what is wrong with this:
. . .

Everything.

You are confusing the field and a local variable. You are doing something in a method which ought to be done in the constructor. You are using setBounds which suggests you are not using a layout correctly. The only thing which would make that method worse would be to give it public access.

The method with the for‑each loop is an incorrect use of a List. In fact it suggests to me you do not want a List at all, but a different data structure.

Okay thank you for pointing that out to me so clearly.
I will notify the Teacher that his code is wrong.

I don't want a different data structure.

In my first post I explained the assignment and what I want to achieve.
I want to add a Student with a string to an Arraylist and add the study that is chosen from the JcomboBox with that student. the study and courses are in a different arraylist opleidingen. So I need to go through that ArrayList and apoint the correct study to the student so the name of the student and the corresponding study are added to the Arraylist studenten.

Hope this help

[Iwan van den Hoek]

Carey Brown wrote:I think I didn't take it far enough. Try

admin.maakStudent(getStudentTextField().getText(), (String) getOpleidingComboBox().getSelectedItem());

admin.maakStudent(studentTextField.getText(), (String) opleidingComboBox.getSelectedItem());

In your original code (above) for this, if you are getting a NPE on this line, it can only be one of:

admin

studentTextField

opleidingComboBox

Insert three print statements for these variables just before line 75 to see which one is null.

Hi Carey,

Thank you, I will try that and let you know the outcome

[Knute Snortum]

I will notify the Teacher that his code is wrong.

Okay, maybe don't do that, but be aware that there are better ways to code this.

I don't want a different data structure.

Well okay, but what you are describing would really lend itself to a Map of some kind. Your "loop through the students" use of a List will work fine, though (if more slowly).

[Iwan van den Hoek]

Knute Snortum wrote:

I will notify the Teacher that his code is wrong.

Okay, maybe don't do that, but be aware that there are better ways to code this.

I don't want a different data structure.

Well okay, but what you are describing would really lend itself to a Map of some kind. Your "loop through the students" use of a List will work fine, though (if more slowly).

Hi Knutte and Carey,

I have tested it with System.out.println and admin was causing the Nullpointer. Whoohoo

So the Nullpointer is gone, but now I need to get the correct info in it and that is not going as planned.
What could I do to fix the below method to get a new student with the corresonding study in the ArrayList

public String maakStudent(String naam, String richting) {
		
		for (Studie opleiding : opleidingen) {
			if ((opleiding instanceof Opleiding) && (richting.equals(opleiding.getNaamStudie()))) {
				RegulierStudent student = new RegulierStudent(naam, (Opleiding) opleiding);
				student.setStudie(opleiding);
				studenten.add(student);
			}
		}
		return naam;
	}

[Knute Snortum]

Well, what's not happening? I take it that the student isn't getting added to the students array. If so, then when I look at this code...

if ((opleiding instanceof Opleiding) && (richting.equals(opleiding.getNaamStudie()))) {

... I see two things. If opleiding isn't an instance of Opleiding or richting doesn't equal the NaamStudie.

So why does opleiding have to be an instance of Opleiding? From a syntactical point of view, it doesn't. Being a Studie is good enough. Is there a logical reason? Using instanceof is often a red flag that the design is poor.

Next, you pass in richting and test it against NaamStudie. What stops there from being no name equal to richting? Or several names? You might want to use an enum for this rather than a String.

So again, to debug, use System.out.println (SOP) to print whether opleiding is an instance of Opleiding and if richting equals the NaamStudie. But keep in mind that there are better ways to accomplish what you're trying to do.

[Iwan van den Hoek]

Knute Snortum wrote:Well, what's not happening? I take it that the student isn't getting added to the students array. If so, then when I look at this code...

if ((opleiding instanceof Opleiding) && (richting.equals(opleiding.getNaamStudie()))) {

... I see two things. If opleiding isn't an instance of Opleiding or richting doesn't equal the NaamStudie.

So why does opleiding have to be an instance of Opleiding? From a syntactical point of view, it doesn't. Being a Studie is good enough. Is there a logical reason? Using instanceof is often a red flag that the design is poor.

Next, you pass in richting and test it against NaamStudie. What stops there from being no name equal to richting? Or several names? You might want to use an enum for this rather than a String.

So again, to debug, use System.out.println (SOP) to print whether opleiding is an instance of Opleiding and if richting equals the NaamStudie. But keep in mind that there are better ways to accomplish what you're trying to do.

Hi Knute,

Thank you for replying again.

I am not sure if the use of instanceof is a good thing, I guess not as you are more experienced.

All I need to accomplish is to check the ArrayList opleidingen with the given String Value from the JCombBox and then connect the study with the Student so it is added to the ArrayList studenten.

If there is a better way to do that I would love to know

[Iwan van den Hoek]

Knute Snortum wrote:Well, what's not happening? I take it that the student isn't getting added to the students array. If so, then when I look at this code...

if ((opleiding instanceof Opleiding) && (richting.equals(opleiding.getNaamStudie()))) {

... I see two things. If opleiding isn't an instance of Opleiding or richting doesn't equal the NaamStudie.

So why does opleiding have to be an instance of Opleiding? From a syntactical point of view, it doesn't. Being a Studie is good enough. Is there a logical reason? Using instanceof is often a red flag that the design is poor.

Next, you pass in richting and test it against NaamStudie. What stops there from being no name equal to richting? Or several names? You might want to use an enum for this rather than a String.

So again, to debug, use System.out.println (SOP) to print whether opleiding is an instance of Opleiding and if richting equals the NaamStudie. But keep in mind that there are better ways to accomplish what you're trying to do.

Well You are right again, it is not doing anything.

So it justs skips the for loop as it doesn't equal anything. I also tried the following but nothing.

public String maakStudent(String naam, String richting) {

		for (Studie opleiding : opleidingen) {
			if (richting.equals(opleiding.getNaamStudie())) {
				System.out.println("test");
				RegulierStudent student = new RegulierStudent(naam, (Opleiding) opleiding);
				System.out.println("test2");
				// student.setStudie(opleiding);
				studenten.add(student);
				System.out.println("test3");
			}
		}
		System.out.println("test4");
		return naam;
	}

[Liutauras Vilda]

Iwan van den Hoek, I am looking only at your latest post and have a feeling, that you didn't decompose your method enough. In effect of that, your method does something different than its name suggests. And most likely it does more than it should, so your confusion is absolutely understandable.

Could you please explain in english, what this method suppose to be doing?

[Iwan van den Hoek]

Liutauras Vilda wrote:Iwan van den Hoek, I am looking only at your latest post and have a feeling, that you didn't decompose your method enough. In effect of that, your method does something different than its name suggests. And most likely it does more than it should, so your confusion is absolutely understandable.

Could you please explain in english, what this method suppose to be doing?

Hi Liutauras,

Thank you joining the discussion, I really appreciate it.
Here is what I want to accomplish with the method.

In Student administration I have 2 Arraylists one for students and one for Study's and courses. A student can be a RegulierStudent with a study or a scholer with a course, but not both.
I have to add a student based on the textfield in the Gui to the studenten ArrayList and check the already filled opleidingen Arraylist for the Study based on the selected String value in the pulldown menu from the Jcombobox.
if the chosen String value corresponds with a Value in the opleidingen ArrayList I want to add the student with that particular study/course to the studenten Arraylist.

So basically I want to create a new Student with the selected study and add it to the studenten ArrayList, so that I can manage the student, add points and get info on the student etc.

Hope this helps

[Liutauras Vilda]

No no, wait. That is the problem. You're telling how you need to do it without knowing what you need to do. Don't use any Java terms, we're not interested in those - we know some ourselves.
Don't tell anything about the GUI, problem most likely is not there. Don't even use your variable names in explanation, these could be not accurately chosen and they are in other language.

Iwan van den Hoek wrote:A student can be a RegulierStudent with a study or a scholer with a course, but not both.

Not sure understood that. Is regular student the one who enrolled to a full study programme, and the other type is the one who only enrolled to a one particular module to study?

Elaborate and expand more the quote below, this one is the way closer to what we want to hear from you.

Iwan van den Hoek wrote:... I want to create a new Student with the selected study...

Before you tell us in non technical language what is the exact task, please read tutorials StopCoding and WhatNotHow.

Note: The way you tried to solve this exercise, gave you the result you have now, if you're going to continue solving that way, the result going to be the same. So read those tutorials, and describe the task in plain english (in details, but without technical terminology).

[Carey Brown]

Iwan van den Hoek wrote:In Student administration I have 2 Arraylists one for students and one for Study's and courses. A student can be a RegulierStudent with a study or a scholer with a course, but not both.
I have to add a student based on the textfield in the Gui to the studenten ArrayList and check the already filled opleidingen Arraylist for the Study based on the selected String value in the pulldown menu from the Jcombobox.
if the chosen String value corresponds with a Value in the opleidingen ArrayList I want to add the student with that particular study/course to the studenten Arraylist.

In pseudo code (is this correct?)

Find study/course from studenten
IF FOUND
  Find student from list of students
  IF FOUND
    there is a problem (?)
  ELSE
    add a new student with the corresponding study/course
ELSE
  we have a problem (?)

[Knute Snortum]

In the opleiding combo box you're putting:

String opleiding[] = {"Wiskunde", "Informatica"};

But in the opleidingen you're putting:

        Opleiding wiskunde = new Opleiding("WISKUNDE", 160);
        Opleiding informatica = new Opleiding("INFORMATICA", 200);

This is the problem with using Strings the way you are. You should be using an enum.