按值传递引用意味着什么？

Question

I read in this question that Java is always pass-by-value . And so even references are passed by value.

I don't understand what this means, can somebody clarify this for me please?

Answer 1

Given this

Object ref = new Object();

ref is actually storing a value, some address to an object. Let's say 1234 .

When you pass ref around

public void method(Object passed) {...}
...
method(ref);

Java actually copies the value of the reference and assigns it to the parameter. So, passed will also have the value 1234 .

Similarly, if you had

Object otherRef = ref;

the value 1234 would be copied and assigned to otherRef .

If you then re-assign otherRef , like

otherRef = new Object();

that would assign a new value to otherRef , but ref would still have the same value as before. That's what pass by value is.

When you call a method

ref.toString();

Java uses the value of the reference to find the referenced object and invoke the method. This is called dereferencing.

---

You might want to go through the JPDA javadoc, starting with StackFrame . Go through the fields and types and you'll start to understand how everything is mapped. For example, it has a getValues(..) method which returns a Map<LocalVariable, Value> . That should tell you that a variable doesn't actually store anything. Instead, it is mapped to a value, where that value may be all sorts of things .

Answer 2

int a = 5;

public void foo(int num) {
num = num + 5;
System.out.println(num);
}

foo(a);

System.out.println(a);

In the above code, a is passed into foo() by value. That means that a new variable is created with the scope of foo() that has the same initial value as a. When the value of num is changed, the value of a is not changed. The first println() will print a 10, the second will print a 5. In c++ you could pass a in by reference, which means that the value of a would be changed too.

Answer 3

I can try, in some other languages you have the option of passing something by a pointer (eg a reference to a memory region). Java calls every function with a value (not a pointer), but the value of an Object is a reference to a memory region. See also the default toString() in your own classes.

Answer 4

I always find this a good example:

Dog foo = new Dog("Rocky");
modifyDog(foo);
System.out.println(foo.name); //Alice

public void modifyDog(Dog aDog)
{
   aDog.name = "Alice"; //here you change the field 'name' of the object located in memory currently referenced by foo and aDog variables. 
   aDog =  new Dog();   //since aDog is a copy of foo then this won't recreate foo object
}