How do getter methods work on arrays?

Question

public class foo {

private int a[];
private int b;

public foo(){
    a = new int[] {1,2};
    b= 3;
}

public int[] getA(){
    return this.a;
}

public int getB(){
    return this.b;
}

I noticed that it's possible to change a value of A by accessing the object like this:

foo f = new foo();
f.getA()[0] = 5; // f.a[0] changes to 5

but it isn't possible to do something like:

f.getB = 5; // gives error
f.getA() = new int[]{2,3};  //gives error

can someone explain me how this works, and how to prevent the user from changing the value of an array cell?

Thanks in advance.

Answer 1

In Java, array is a reference type, which means that the value of an array expression is a reference to the actual array.

The return value of getA() is, therefore, a reference to the private array inside your object. This breaks encapsulation: you give access to your object's internals.

You can avoid this by either returning a reference to a copy of your internal array, or by providing a different API which only returns individual elements, say a method getA(int index).

Answer 2

f.get(A) returns a reference to an array. You can access that array the way you access any array, and assign values to its elements with f.get(A)[i]=... (though it makes more sense to store the returned array in a variable, which would let you access that array multiple times, without having to call f.get(A) each time).

You can't, however, assign anything f.get(A) via f.get(A)=.., since a method call is not a valid left side of an assignment operator. For all you know, a call to f.get(A) may generate a new array that is not referred to by a member of the foo class, so assigning f.get(A)= new int[5]; would make no sense, since there would be no variable in which to store the new array. The same explanation applies to f.getB() = 5;.