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.

All java parameters and results are passed by value. Including arrays. But the value that is passed (for a reference type) is the reference. — Stephen C, Feb 06 '15 at 12:38
http://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value — robert, Feb 06 '15 at 12:38
https://stackoverflow.com/questions/51426788/setters-and-getters-for-arrays/51427122 — Leponzo, Apr 01 '21 at 16:58

score 7 · Accepted Answer · answered Feb 06 '15 at 12:38

7

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).

answered Feb 06 '15 at 12:38

Marko Topolnik

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Or OP could *not* use an array, but an immutable collection instead, like Guava's `ImmutableList` for example. – Zoltán Feb 06 '15 at 12:39
1

Your suggestion amounts to using a 3rd-party implementation of the same idea (`ImmutableList` still uses an array on the inside). – Marko Topolnik Feb 06 '15 at 12:42
1

Yes. I was just trying to complement your answer, not dispute it. – Zoltán Feb 06 '15 at 13:30
If we were nitpicking, there would also be the issue of a sixfold increase in memory consumption when going from `int` to `Integer`. But perhaps Guava has primitive type specializations for its immutable containers? – Marko Topolnik Feb 06 '15 at 13:36
1

@MarkoTopolnik: Not for immutable containers, but `Collections.unmodifiableList(Ints.asList(array))` would do. – Louis Wasserman Feb 06 '15 at 17:12

Eran · Answer 2 · 2015-02-06T12:43:05.803

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;.

Good mentioning left side assignment which is in my eyes the important aspect ;-) — Stefan, Feb 06 '15 at 12:43

score 0 · Answer 3 · answered Feb 06 '15 at 12:47

Instead of giving away the array, to allow the caller to do what they like with it you can use an indexed getter

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

public void setA(int n, int x) {
    this.a[n] = x;
}

Now, the caller has no access to the array and cannot change it without you knowing.

How do getter methods work on arrays?

3 Answers3