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If I instantiate an object and pass it to an function, in this function I assign this object to null. It seems when return from the function, the object still there.

I just want to know when I assign null, what happens.

shirley
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4 Answers4

9

You can never assign to an object. All you ever have are primitives and references. A reference is either null or a pointer to an object of suitable class.

Java arguments are passed by value. Your called method got a copy of a reference. It made that reference null. The calling method has its own reference which was unaffected by any assignments to the passed copy. That reference still points to the object.

Patricia Shanahan
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1

Arguments to methods in Java are 'pass-by-value', which means you are passing a copy of the object reference into the method. Assigning this reference a value of null will change its value within the method call, but does nothing to the reference outside the method, since its a copy. Illustrated with code:

void doSomething(final String input) {
    input = null;
    System.out.println("Input is: " + input); // prints null
    return;
}

final String name = "Bob";
doSomething(name);
System.out.println("Name is: " + name); // prints 'Bob'
Perception
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    I think the `final` hinders the understanding of your answer. Also: you cannot reassign to a `final` variable (line 2). – Kijewski Feb 09 '13 at 02:59
  • Yes, that is indeed the point. In the rare case you find you need to reassign a variable, simply don't declare it final. – Perception Feb 09 '13 at 05:10
1

when you instantiate an object and pass it to a function, and inside the function you reassign that to null or whatever, at the calling side it is not reflected as arguments are pass by value (copy of reference in case of objects), at calling side it'll still point to the old object. If you want to restrict reassigning in a method, you can use final keyword in method parameter

vishal_aim
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0

When you pass the object reference to a function(Java always call it method),in the method scope,a new reference is created on stack memory,but they point to the same object in heap memory.So if you assign null to the new reference,Only this reference's link to that object is break,It does not affect the prevous one.

BlackJoker
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  • when a parameter is an Object,it's passed by reference;when a parameter is a primitive type,it's passed by value. And reference is a 2 byte variable that point to the actual object. – BlackJoker Dec 18 '12 at 06:02
  • In java, it is **never** passed by reference – vishal_aim Dec 18 '12 at 06:29