用Java创建数组对象？

Question

I'm quite new to Java and had a little problem concerning arrays. In an exercise given one had to create a class named "Exam" with a method "addExam". Now I tried to solve the problem but only got to the definition of the cass, the instance variables and the method heading. The rest I looked up in the solution and it looked like:

public class Exam{
private int numberOfExams=15;
private Exam[]exams = new Exam[numberOfExams];

public void addExam(Exam exam){
exams[numberOfExams] = exam;
numberOfExams++;

So now to my question: When I assign "exam" to the array exams in the method addExam, is a new object created here? And when yes, why aren't bracktes and the "new operator" used? This question leads to a more general one: I know that arrays are of a class type, do they now create array objects or variables of that array type?

I really need some help!

Answer 1

public void addExam(Exam exam){
exams[numberOfExams] = exam;

No. No new object created, what ever the object passed here assigned to the 0th element of your array.

The actual place of creating new object is the place where addExam method called. Something like

Exam exam = new Exam();
// do something
addExam(exam);

Answer 2

When I assign "exam" to the array exams in the method addExam, is a new object created here?

No . A reference to the existing Exam instance will be added to the array. So, you will have 2 references to the same instance of Exam

Answer 3

When you assign an "exam" to the array, it doesn't create a new object. It merely stores the reference of an existing object in the array.

If I understand your second question, when you create an array with a call such as Exam[] exams = new Exam[numberOfExams]; you create an object that contains enough space for numberOfExams references to Exam objects.

BTW, exams[numberOfExams] = exam; would give you an ArrayIndexOutOfBoundsException , since your array has numberOfExams elements, so the valid indices are from 0 to numberOfExams-1 .