Java对对象的引用

Question

Let's say I have a Node class. It has a single field, Node parentNode. It's got setters and getters too.

I have 2 nodes: Node nodeA and Node nodeB .

Here's what I want to do: set nodeB 's parent to nodeA 's parent, and then set nodeA 's parent to null.

1. nodeB.setParent(nodeA.getParent());
2. nodeA.setParent(null); // bad since nodeB.getParent() will = null

To achieve the above, must I clone nodeA, and then do nodeB.setParent(nodeAClone.getParent()) ?

Answer 1

Because nodeX.parent is a reference to an object:

• When you call nodeB.setParent(nodeA.getParent()) you are saying to nodeB "here is the address of nodeA's parent."
• When you then say nodeA.setParent(null); you are saying to nodeA "Forget where your parent lives. Your parent is now nothing."
• You said nothing to nodeB in the second statement, which still has the address to what is now its parent.

Answer 2

nodeA.setParent(null); // bad since nodeB.getParent() will = null

No, nodeB.parent will not be set to null . Java always uses Pass by Value and not pass by reference . Repeat it 10 times.

And in case you pass references, you pass them by value of references.

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Let's understand in more detail.

When you do: -

nodeB.setParent(nodeA.getParent());

you simply create a copy of reference to nodeA parent , and store it in nodeB parent . So, you have now two references referring to nodeA parent object.Now, when you set nodeA parent to null , it is detached from that parent , but nodeB parent reference is still there.