after list add some object and set object null

Question

class TestEntity {
    public int x;
    public int y;

    public TestEntity(int x, int y) {
        this.x = x;
        this.y = y;
    }

    public String toString() {
        return super.toString() + ", x -> " + x + ", y -> " + y;
    }
}

TestEntity t = new TestEntity(666, 777);
List<TestEntity> list = new ArrayList<>();
list.add(t);
t = null;
System.out.println(list.get(0));

why correct print list.get(0) with

@xxxxx,x -> 666, y -> 777

if I remove t = null; and do t.x = 888, the print looks correct.

`t = null;` just dereference the variable `t`, but doesn't change anything on the `TestEntity` object that it referenced previously. You should learn/search about "java references" — Joel, Sep 01 '17 at 08:13
you cahnge the reference of t when yyou do t=null, that doesnt mean the TestEntity in the list is modified thoe — ΦXocę 웃 Пepeúpa ツ, Sep 01 '17 at 08:14
Java is pass-by-value https://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value. This is a duplicate — Jack Flamp, Sep 01 '17 at 08:14
What logical should be applied here to think that `t=null` would affect the object added to the list? You __obviously__ don't add `t` to the list, but rather the reference to an object currently hold by `t`. If you alter `t` to refer to something else (a new object or `null`) then it obviously doesn't matter much to `list`. — Tom, Sep 01 '17 at 08:15

score 0 · Accepted Answer · answered Sep 01 '17 at 08:17

When you add t in list, t and the first element of list refer to the same object.

But as you write t = null, it assigns null to the t variable.
So, now t and the first element of list doesn't refer any longer to the same object.
So printing the first or the second will give distinct result.

As you write t.x = 888, t and the first element of list refer still to the same object.
You change only the value of a field of the object.
So printing the first one or the second give exactly the same result.

score 0 · Answer 2 · answered Sep 01 '17 at 08:18

You're not setting an object to null. You're setting a reference to an object to null.

t is a reference to an object. When calling list.add(t), you're storing a copy of this reference (let's call it t2) inside the list. So you end up with

t -----> theObject
           ^
           |           
ArrayList [t2]

Two references both pointing at the same object.

Now, when you execute t = null, you just change what t points to. The copy, stored in the list, doesn't care about that. Only t now points to no object at all (null). So you end up with

t--> nothing    theObject
                   ^
                   |           
        ArrayList [t2]

So, printing the first element of the list still prints the object that is referenced by t2, the first reference in the list internal array.

after list add some object and set object null

2 Answers2