Java GUI loan and payment calculator help

[mike saggese]

hey there, I'm new to Java and I need help with this assignment: Write a Java program to calculate and display monthly and total payments given interest starting from 3% to 6% (with an increment of 0.125%). The user inputs are loan amount and loan period (i.e., number of years).

Here's what I have so far

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.JLabel;
import javax.swing.JButton;
import javax.swing.JTextArea;
import javax.swing.JScrollPane;
import java.awt.*;
import java.awt.event.*;

public class TestCalc{
	
	static JFrame window;
	static JPanel p;
	static JLabel label1, label2;
	static JTextField JTextField1, JTextField2;
	static JButton b;
	static JTextArea textArea;
	static JScrollPane scrollPane;
	static ButtonHandler handler;
	
	public static void main(String args[])
	{
		window = new JFrame(); // create window
		p = new JPanel();
		
		 //Create the first label.
        label1 = new JLabel("Loan Amount", JLabel.LEFT);
		p.add( label1);
		
		//Set the position of its text, relative to its icon:
        label1.setVerticalTextPosition(JLabel.BOTTOM);
		label1.setHorizontalTextPosition(JLabel.LEFT);
		
		// construct textfield with 10 columns
		JTextField1 = new JTextField( 10 );
		p.add( JTextField1 );
		
		//Create the other labels.
        label2 = new JLabel("Number of Years");
		p.add(label2);
		
		// construct textfield with default text
		JTextField2 = new JTextField(3);
		p.add( JTextField2 );
		
		label1.setToolTipText("A label containing both image and text");
        label2.setToolTipText("A label containing only text");
		
		b = new JButton("Show Table" );
		p.add(b);
		
		textArea = new JTextArea(20, 40);
        textArea.setEditable(false);
        scrollPane = new JScrollPane(textArea);
		p.add( scrollPane );
		
		// create new ButtonHandler for button event handling
		handler = new ButtonHandler();
		b.addActionListener( handler );
		
		window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
		window.add(p);
		window.setSize( 520, 360); // set frame size
		window.setVisible( true ); // display frame	
	} // end main
	
	// inner class for button event handling
	private static class ButtonHandler implements ActionListener
	{
		// handle button event
		public void actionPerformed( ActionEvent event )
		{
		String temploan = JTextField1.getText();
		double principle;
		int lifeofloan;
		textArea.setText(temploan );
		}
	}	
} // end class TestCalc

I also have this code as reference to what I want to do but isn't exactly how I want it done

import javax.swing.JOptionPane; // include the JOptionPane library in swing 
public class MonPay  // class header
  {  // begin class
     public static void main(String args[ ] )  // main method header
      {  // begin main method
        System.out.println("Loan Monthly Payment Program \n "); // display program title
        String strAmount, strIRate, strYears;        // declare variables to be entered by user as strings
        double amount, iRate, years, monPay, totalPay;   // declare variables to be used in the formula
        strAmount = 
            JOptionPane.showInputDialog(" Enter the amount of the Loan $: "); // enter amount as a string 
        strIRate = 
            JOptionPane.showInputDialog("Enter the % interest rate: "); // enter % interest as a string 
        strYears = 
            JOptionPane.showInputDialog("Enter the # of years: "); // enter years as a string 
        amount =  Double.parseDouble(strAmount); // convert the input string strAmount to number
        iRate = Double.parseDouble(strIRate); // convert the input string strIRate into a real number 
        years = Double.parseDouble(strYears); // convert the input string strYears into a real number
        System.out.println("Enter the amount of the Loan $: " + amount + "\n"); // echo input
        System.out.println("Enter the % interest rate: " + iRate +"\n");
        System.out.println("Enter the # of years: " + years +"\n"); 
        monPay = (amount * iRate / 1200) / 
                      ( 1 - Math.pow( 1 / ( 1 + iRate / 1200), 12 * years)) ;   // calculate payment                     
        monPay = (int)(monPay * 100 + 0.5 ) / 100.0;  // round distance to the nearest 1/100 
        totalPay = monPay * 12 * years; // calculate the total pay back for the loan
        totalPay = (int)(totalPay * 100 + 0.5 ) / 100.0;  // round total to the nearest 1/100 
        System.out.println("The monthly payment is : $" + monPay +"\n"); // display monthly payment
        System.out.println("The total amount paid is : $" + totalPay +"\n" ); // display total payment
        System.exit( 0 ); // exit the system to close all windows
     }  // end main method
  } // end class

If anyone can help me make heads or tails of anything that would be great thanks

[Steve Luke]

Do you have a problem or question with the code? Are you stuck someplace? TellTheDetails (<-link)

[mike saggese]

Steve Luke wrote:Do you have a problem or question with the code? Are you stuck someplace? TellTheDetails (<-link)

Yes. My gripe is where do I put the interest rate and monthly payment and total payment formula for the show table button to calculate the loan?

[Steve Luke]

My suggestion would be in a different class. Have a class whose sole purpose is to maintain a balance, total term, take a percentage rate as a value, and generate the payment amount.

I would create a small gui-less example (like the second code) to test out the class to make sure that the class does what you need it to do.

Then I would use the information learned from the gui-less example (i.e. what methods have to be called in what order) to generate the functionality of your actionPerformed method in your GUI.

[mike saggese]

Steve Luke wrote:My suggestion would be in a different class. Have a class whose sole purpose is to maintain a balance, total term, take a percentage rate as a value, and generate the payment amount.

I would create a small gui-less example (like the second code) to test out the class to make sure that the class does what you need it to do.

Then I would use the information learned from the gui-less example (i.e. what methods have to be called in what order) to generate the functionality of your actionPerformed method in your GUI.

I did a little more with the code. How about this?

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.JLabel;
import javax.swing.JButton;
import javax.swing.JTextArea;
import javax.swing.JScrollPane;
import java.awt.*;
import java.awt.event.*;

public class TestCalc{
	
	static JFrame window;
	static JPanel p;
	static JLabel label1, label2;
	static JTextField JTextField1, JTextField2;
	static JButton b;
	static JTextArea textArea;
	static JScrollPane scrollPane;
	static ButtonHandler handler;
	
	public static void main(String args[])
	{
		window = new JFrame(); // create window
		p = new JPanel();
		
		 //Create the first label.
        label1 = new JLabel("Loan Amount", JLabel.LEFT);
		p.add( label1);
		
		//Set the position of its text, relative to its icon:
        label1.setVerticalTextPosition(JLabel.BOTTOM);
		label1.setHorizontalTextPosition(JLabel.LEFT);
		
		// construct textfield with 10 columns
		JTextField1 = new JTextField( 10 );
		p.add( JTextField1 );
		
		//Create the other labels.
        label2 = new JLabel("Number of Years");
		p.add(label2);
		
		// construct textfield with default text
		JTextField2 = new JTextField(3);
		p.add( JTextField2 );
		
		label1.setToolTipText("A label containing both image and text");
        label2.setToolTipText("A label containing only text");
		
		b = new JButton("Show Table" );
		p.add(b);
		
		textArea = new JTextArea(20, 40);
        textArea.setEditable(false);
        scrollPane = new JScrollPane(textArea);
		p.add( scrollPane );
		
		// create new ButtonHandler for button event handling
		handler = new ButtonHandler();
		b.addActionListener( handler );
		
		window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
		window.add(p);
		window.setSize( 520, 360); // set frame size
		window.setVisible( true ); // display frame	
	} // end main
	
	// inner class for button event handling
	private static class ButtonHandler implements ActionListener
	{
		// handle button event
		public void actionPerformed( ActionEvent event )
		{
		String LoanAmount = JTextField1.getText();
		String NumberofYears = JTextField2.getText();
		double principle;
		int lifeofloan;
		String s = "Interest Rate     Monthly Payment                      Total Payment\n";
		
		// this method will be called each time amount of money is paid
		private void incrementRate();
		{ if(rate >= 3 && rate <= 6) rate += 0.125; // rate is instance variable
		}
		
		textArea.setText(s);
		}
	}	
} // end class TestCalc

for some reason however I keep getting an error on line 81 saying illegal start of expression. how do I fix that?

[Amit Ghorpade]

mike saggese wrote:for some reason however I keep getting an error on line 81 saying illegal start of expression. how do I fix that?

Look at line 81 carefully, you are declaring the incrementRate() without ending actionPerformed().