为什么我没有收到NullPointerException

Question

Why is it, that when I add an object, to my ArrayList, which Should be passing it a Reference to the object, then I make the object equal to null, it outputs perfectly fine.

    ArrayList<Test> testList = new ArrayList<>();

    Test test = new Test();
    System.out.println(test);

    testList.add(test);

    for(Test t : testList)
    {
        System.out.println(t);
    }

    test = null;

    for(Test t : testList)
    {
        System.out.println(t);
    }

The test constructor goes as:

int x = 0;

Test()
{
    x = 50;
}

public String toString()
{
    return x + "";
}

Yet, the output is 50, 50, 50 instead of 50, 50, null

Answer 1

When you call add , the ArrayList now has its own reference to the same object.

test ----> Test() object
             ^
testList ----|

That is unaffected by setting your reference test to null .

test ----> null

testList ----> Test() object

Answer 2

Because there is a difference between Reference Types and Value Types in Java.

You add a reference to the value (aka an object) to the list, then you delete the reference. However the value is still there in memory, and the list still has a reference to that value, despite your original reference being nullified

Answer 3

When you are passing reference to object to ArrayList you actually create yet another reference that is used by ArrayList to refer to the object.

When you assign null to variable test you actually "cancel" your reference. Nothing happens with reference of array list.

Even simpler.

If you have 2 variables: one and two:

Test one = new Test()
Test two = one;

At this point both references point to the same object.

one = null;

Now one is null, but two still refers to that object.