Java path by value or reference ?

Pass-by-value vs Pass-by-reference

The terms “pass-by-value” and “pass-by-reference” are talking about variables :

  • Pass-by-value means that the value of a variable is passed to a function/method.

  • Pass-by-reference means that a reference to that variable is passed to the function. The latter gives the function a way to change the contents of the variable.

But I pass an reference in Java ?

Java always passes arguments by value, but what you are passing by value is a reference to an object, not a copy of the object

Java can give the illusion of pass by reference. When we deal with variables holding objects we are really dealing with object-handles called references which are passed-by-value as well.

Understand with exemple

public class Main {
     public static void main(String[] args) {
        Foo f = new Foo("f");
        changeReference(f); // It won't change the reference!
        modifyReference(f); // It will modify the object that the reference variable "f" refers to!
     }

     public static void changeReference(Foo a) {
        Foo b = new Foo("b");
        a = b;
     }

     public static void modifyReference(Foo c) {
        c.setAttribute("c");
     }
}

  • Foo f declares a new pointer to a Foo object. Suppose the Foo object resides at memory address 42.

  • changeReference(f) means you pass 42 to the method

    • the parameter a is set with the value 42

    • Foo b declares a new pointer to a Foo object at the address 84

    • a = b makes a new assignment to the reference a, not f, of the object whose its address is 84.

    • Conclusion, f address is always 42

  • modifyReference(f) means you pass 42 to the method

    • c.setAttribute("c") will change the attribute of the object that reference c points to it (address 42), and it’s the same object that reference f points to it (address 42).

What say the Java documentation

https://docs.oracle.com/javase/specs/jls/se11/html/jls-8.html#jls-8.4.1:

When the method or constructor is invoked (§15.12), the values of the actual argument expressions initialize newly created parameter variables, each of the declared type, before execution of the body of the method or constructor. The Identifier that appears in the FormalParameter may be used as a simple name in the body of the method or constructor to refer to the formal parameter.

https://docs.oracle.com/javase/specs/jls/se11/html/jls-15.html#jls-15.12.4.2

The argument expressions (possibly rewritten as described above) are now evaluated to yield argument values. Each argument value corresponds to exactly one of the method’s n formal parameters.