Copy objects from one array over to another

[Kd Martin]

Im trying to search an array of the Students object type by their student ID (UFID). TAs2 is an array of GradStudent objects. GradStudent is a subclass of student. When I find the student in the Student array that has the same ID as the TA object, I need to add the matching Student object to the TAs array. When I try to run the program, it says TAs2 and uStudents are of incompatible types. How can I get around this error? Thank you for any help!

//search array of students by UFID to find the TAS and fill array of TAs
		for(int i=0; i< numTAs; i++) {
			int TaID = in3.nextInt();
			for(int j=0; j<uStudents.length; j++) {
				if(uStudents[j].getUFID() == TaID) {
					TAs2[i] = uStudents[j];
				}
			}
			
		}

[Manoj Kumar Jain]

As you said TAs2 is the array of GradStudent , what is the type of uSudents, they must be different types that why compiler is complaining..
Can you post the complete code. ?

[Kd Martin]

My code for this project is very extensive. I have about 9 different classes, but I will attach the more important ones below. uStudents is of the Student type. Student is a super class for GradStudent.

Student:

// Abstract class which represents a student

public abstract class Student extends Person {
	// properties
	protected Course[] courses;
	protected final int maxCourses = 4;
	protected int numCoursesEnrolled;
	protected double gpa;
	
	// Constructor
	public Student(String name, int UFID, String dob, double gpa) {
		super(name, UFID, dob);
		
		this.gpa = gpa;
		courses = new Course[maxCourses];
		numCoursesEnrolled = 0;
	}
	
	public Student(String name, int UFID, String dob, double gpa, Course[] courses) {
		super(name, UFID, dob);
		
		this.gpa = gpa;
		this.courses = courses;
		// numCoursesEnrolled = courses.length; // wrong!
		
		numCoursesEnrolled = 0;
		for (Course i : courses) {
			if (i != null)
				numCoursesEnrolled++;
		}
	}
	
	// abstract methods
	public abstract boolean addCourse(Course course);
	
	
	// Drops a course (if it exists)
	public boolean dropCourse(Course course) {
		// check if you can remove the student from the course
		boolean removeCheck = course.removeStudent(this);
		
		// remove the course from the array list and update variables
		if (removeCheck) {
			// remove course to the student's list of courses
			for (int i = 0; i < courses.length; i++) {
				if (courses[i] == course) { // found course (why is reference comparison enough?) 
					courses[i] = null; 		// empty the space
					numCoursesEnrolled--; 	// reduce num courses enrolled
					break;
				}
			}
		}
		
		return removeCheck;
	}
	
	// toString
	public String toString() {
		String str = super.toString();
		str += "\nGPA: " + gpa;
		str += "\nCourses enrolled in:";
		for (int i = 0; i < courses.length; i++) {
				//if (courses[i] != null)
					str += "\nCourse " + (i+1) + ":\n" + courses[i];//.toString();
		}
		
		return str;
	}

	// get/set
	public Course[] getCourses() {
		return courses;
	}

	public void setCourses(Course[] courses) {
		this.courses = courses;
	}

	public int getNumCoursesEnrolled() {
		return numCoursesEnrolled;
	}

	public void setNumCoursesEnrolled(int numCoursesEnrolled) {
		this.numCoursesEnrolled = numCoursesEnrolled;
	}

	public double getGpa() {
		return gpa;
	}

	public void setGpa(double gpa) {
		this.gpa = gpa;
	}
	
	
}

GradStudent:

// Represents a graduate student

public class GradStudent extends Student {
	// Properties
	private Course courseTA;
	
	// Constructors
	public GradStudent(String name, int UFID, String dob, double gpa) {
		super(name, UFID, dob, gpa);
	}
	
	public GradStudent(String name, int UFID, String dob, double gpa, Course[] currentCourses) {
		super(name, UFID, dob, gpa, currentCourses);
	}

	public GradStudent(String name, int UFID, String dob, double gpa, Course[] currentCourses, Course courseTA) {
		super(name, UFID, dob, gpa, currentCourses);
		setCourseTA(courseTA);
	}

	// abstract methods
	public boolean addCourse(Course course) {
		// can only add courses numbered between 1000-4999
		if (course.getNumber() < 5000)
			return false;
		
		// check student's courses capacity
		if (numCoursesEnrolled < this.maxCourses) {
			// can enroll in more courses, so try to add the student into the course
			
			// try to add the student to the course
			boolean addCheck = course.addStudent(this);
			if (addCheck) {
				// add course to the student's list of courses
				for (int i = 0; i < courses.length; i++) {
					// find an empty spot in the array, now fill it
					if (courses[i] == null) {
						courses[i] = course;
						numCoursesEnrolled++;
						break;
					}
				}
			}
			
			return addCheck;
		}
		// max number of courses added
		return false;
	}

	// Get/set methods
	public Course getCourseTA() {
		return courseTA;
	}

	public void setCourseTA(Course courseTA) {
		this.courseTA = courseTA;
	}
	
	public String toString() {
		String str = "Grad Student:\n" + super.toString();
		str += "\nCourse TA for:\n" + courseTA;
		return str;
	}
}

Clip of the University Class: (it is very long so I only posted the important bit)

public static Course readCourse(Scanner in, Instructor[] uInstructors, Student[] uStudents) { 
		//create the instructor and array of TAs for the course
		Instructor instructor2;
		GradStudent[] TAs2;
		Course course;
		//search array of students by UFID to find the TAS and fill array of TAs
		for(int i=0; i< numTAs; i++) {
			int TaID = in3.nextInt();
			for(int j=0; j<uStudents.length; j++) {
				if(uStudents[j].getUFID() == TaID) {
					TAs2[i] = uStudents[j];
				}
			}
			
		}

[Manoj Kumar Jain]

As you can see that TAs2 is of type GradStudent that is the sub class of Students and uStudents is the array of Students.

You can't refer an object of super class to sub class reference type.
so while

Student std= new GradStudent();

is correct

GradStudent gradStd= new Student(); 

is incorrect
this is why you are getting error.
you are trying to store a super class type of object in sub class type reference.

[Kd Martin]

Is there any way around this? For instance, is there a way to cast an object of a subclass of Class A to another subclass of Class A? Or can you cast a super class to a subclass type? Students is an abstract method so I can't create objects of the students type :/ The students in the array uStudent of the Student type must be either GradStudent or UndergradStudent, but then I have to search through the array of uStudents for the TAs for each course and store them in an array of the GradStudent type. I'm just so confused how I should go about this

[Jeff Verdegan]

Kd Martin wrote:Is there any way around this? For instance, is there a way to cast an object of a subclass of Class A to another subclass of Class A?

No. As someone pointed out recently, Lawnmowers and Alligators are both objects, but you can't cast one to the other. If you want to "convert" one to the other, you'll have to create a new object of the target type based on the original type. Usually this is done by passing a reference to the original to a factory method or to a target constructor.

Or can you cast a super class to a subclass type?

You can cast a supertype reference to a subtype if the object that reference points to is actually of that subtype.

[Kd Martin]

Great! Ok I think I've got that figured out. Now I am coming across this error:
University.java:352: variable instructor2 might not have been initialized
course = new Course(cType, cNum, cTitle, cNumCredits, instructor2, TAs2, cap);

If two objects are of the same type, and object A is null, can't I set A equal to B so that the reference A points to object B as I did below?

Here is the bit of code it refers to.

	public static Course readCourse(Scanner in, Instructor[] uInstructors, GradStudent[] gStudents, Student[] uStudents) { 
		//create the instructor and array of TAs for the course
		Instructor instructor2;
		GradStudent[] TAs2;
		Course course;
		//course instructor's UFID
		str = in.nextLine();
		in2 = new Scanner(str).useDelimiter(":");
		in2.next(); 
		int instructorID = in2.nextInt();
		//find the instructor by searching instructor array by id
		for(int i=0; i<uInstructors.length; i++) {
			if(instructorID == uInstructors[i].getUFID()) {
				instructor2 =  uInstructors[i];
			}
		}
		//create Course object
		course = new Course(cType, cNum, cTitle, cNumCredits, instructor2, TAs2, cap);

[Jeff Verdegan]

Kd Martin wrote:Great! Ok I think I've got that figured out. Now I am coming across this error:
University.java:352: variable instructor2 might not have been initialized

The error message is telling you exactly what's wrong. On line 352 of University.java, where you're doing course = new Course(...), you're using a variable instructor2, but it's possible that you got there without giving that variable a value. In this case, if your for loop body never executes, or if the if condition is never true, you'll never have assigned a value to instructor2, so it will be undefined.

One approach would be to initialize instructor2 to null, and then check if it's still null after the loop. How to handle it really depends on what the requirements are if the instructor isn't found.

[Kd Martin]

Great thanks!