Pass by value or Reference

[joseph gonzales]

void method1(){
String fruit = "apple";
transform(fruit);
System.out.println(fruit);//<----line 4
}

void transform( String fruit){
fruit = "orange"
}


from the code above why is it in line 4 fruit is still equal to "apple" but not "orange", isn't passing object to a method pass by reference? since String is an Object..directly extends Object.
[ November 06, 2008: Message edited by: joseph gonzales ]

[Pandey Gautam]

First thing fruit is method variable and local to method1. So it cannot be modify by other method i.e. transform (String fruit) without returning value to that method. Main region of not getting modify is the void return type of transform method.

[saiharsha peddinti]

The life of the fruit = "orange" ends after returning from the transform method i.e, it(fruit) is local to method.

Where as the fruit = "apple" is local to the method1() and as it is not a static variable the changes are unaffected

[Michael Tolenruc]

hi joseph,

Strings are immutable, in your code there are two strings in constant string pool now, "apple" and "orange". The method transform(fruit) will indeed pass a copy of the reference that refers to the same object, but the body of the transform() method, specifically the fruit = "orange" will Create a New String Object (not modify) on the String pool (remember Immutable) and now refers to a New String object "Orange".

Btw, are you from PH (kabayan)

[Steven Landers]

void method1(){
String fruit = "apple"; //1
transform(fruit); //2
System.out.println(fruit);//6
}

void transform( String fruit){ //3
fruit = "orange" //4
}//5

Within transform(), there are two items to consider: a -reference- and an "object that reference points to." The local reference, named -fruit- exists only within transform(), while "apple", pointed to by -fruit- still exists within scope of method1().

In line 8, the reference, -fruit- is reassigned to a new string literal, "orange." The original object, "apple" remains untouched. Additionally, Strings have no way of being altered, as they are "immutable" (unchangeable).

A walkthrough (see line numbers above):
1. The String literal "apple" is assigned to reference -fruit-
2. transform() is called with a COPY of the reference, -fruit-
3. the COPY of the reference to "apple" is created, also named -fruit-
4. A new string literal, "orange" is assigned to the new local reference -fruit-
5. transform() ends, and the local copy of the -fruit- reference is gone
6. The original reference, -fruit- remains untouched, as only a copy of the reference was manipulated within transform(). "apple" is printed.

If these were StringBuffer objects, and instead of variable reassignment we saw fruit.append("orange") - the object would be changed to "appleorange". But because this was only a local variable reassignment, the object remained untouched.

I hope that helps. Let me know - good luck!

[joseph gonzales]

Hey thanks to all!!

@Micheal
Yup taga Cebu ako..thanks

@Stevens
the walkthrough helps a lot..


Now I get it. Here my Understanding , I hope I get it right.

void method1(){
String fruit = "apple"; //1
transform(fruit); //2
System.out.println(fruit);//6
}

void transform( String fruit){ //3
fruit = "orange" //4
}//5

excutions steps
1. reference to an object created named fruit and assigned a value "apple"
2. a copy of reference is passed
3. a local reference also named fruit is now pointing to the same value "apple"
4. Now change the value it pointing to into "orange" (it is now pointing to the address in memory that contains the value "orange")
5. the local reference fruit is discarded. gc?
6. the orignal reference fruit is still pointing the same address in memory that holds the value "apple"

so in this case there is no way an original reference will be modified without returning the modified copycat reference value and assigned it into original reference..

[Jesper de Jong]

Originally posted by joseph gonzales:
... isn't passing object to a method pass by reference? since String is an Object..directly extends Object.

No. Java is always pass by value. Note that variables in Java do not directly represent objects; they are references to objects. So you cannot technically pass an object to a method - you are really passing a reference to an object to a method.

Note that, since everything is passed by value, references are also passed by value. Passing a reference by value is not the same as passing by reference.

Have a look at: Pass by Value, Please