manipulating collections as/from parameter/arguments

**chronoz13** · October 1st, 2011, 08:56 AM

the title of the post sounds a little bit confusing and i dont even know what title should i give on this critical thing i just encountered ,

this 1st code

public class ManipulateData {
 
    public static void main(String[] args) {
 
        int d = 1;
        change(d); // this will change a value 
        print(d);    // im expecting that this method will print the copy value of 1
    }
 
    public static void change(int d) {
 
        d = 10;
    }
 
    public static void print(int d) {
 
        System.out.println(d);
    }
}

prints 1 because the passing of the value came from a separate call of the methods, even though it has been changed

but. when it comes to collection

public class ManipulateArrays {
 
    public static void main(String[] args) {
 
        int[] a = {1};
 
        // 2 methods being called, and 2 separate passes of the array value
 
        change(a);  
 
        // i was expecting that this method will still print out the copy value of 1,but it prints the changed value
        // from a separate call of a method from which it was passed
        print(a);
    }
 
    public static void change(int[] a) {
 
        a[0] = 99;
    }
 
    public static void print(int[] a) {
 
        System.out.println(a[0]);
    }
}

it prints the value even though it has been passed on a separate method call, i was thinking at first, that maybe its because a static call, that it has to do something with class data, but i tried to make everything as an instance call, the flow is still the same..

simple question is,... why does the array is being changed even on a separate passes to different methods?(sorry for bad english)

**Norm** · October 1st, 2011, 10:02 AM

Java passes arguments by value. The value of the variable is copied to a new location and that is given to the method.The original value of the variable is not reachable via the passed value.

**chronoz13** · October 1st, 2011, 10:12 AM

The original value of the variable is not reachable via the passed value.

but why does the values of the array is being changed even on separate calls, just like the sample codes above?

**Norm** · October 1st, 2011, 10:20 AM

why does the values of the array is being changed

A copy of the reference to the array is passed. It's that reference can't be changed.

**chronoz13** · October 1st, 2011, 10:27 AM

it still makes me confuse >.<, i do understand somehow about the passing / copy of values , primitive or reference. but its still a big ? to me about the values/value of that array

**chronoz13** · October 1st, 2011, 10:45 AM

 
int[] array = {1}; // here the value of array is 1
 
change(array); // here the value of the array is being changed(separate call of method)
print(array);    // it prints the changed value of the array(separate call)

it differs this one, this one below, is easy to follow the flow of the calling of the methods..and passing of the value

 
int aNumber = 1;
 
change(aNumber); // it does some changes here(separate call)
print(aNumber);    // it prints a value, which is 1 (A Separate call again)

**Norm** · October 1st, 2011, 10:46 AM

Do you understand how the compiler assigns memory locations to hold the value of a variable?
If you have this concept then I can explain what happens when the you call a method with some arguments.

**Sean4u** · October 1st, 2011, 10:56 AM

Arrays are not part of Java Collections, and neither are they Primitives:
Primitive Data Types (The Java™ Tutorials > Learning the Java Language > Language Basics)
Arrays are a 'special' data type somewhere between a primitive and an object.
Arrays (The Java™ Tutorials > Learning the Java Language > Language Basics)

**chronoz13** · October 1st, 2011, 08:52 PM

Do you understand how the compiler assigns memory locations to hold the value of a variable?

somehow, i do understand that for example (int x = 1), the compiler will allocate a memory space as an integer type and assign a value of 1, in case of objects
like String as reference for example (String word = "Hello"), compiler allocates 2 memory , 1 for the variable 1 for the value of the String, and refers the value to the variable, as for passing arguments to parameters, just like what you have said, and what i read before, values are not exactly being passed, its their copies that is being sent to parameters.

Arrays are not part of Java Collections

thank you Sean for enlightening me.. i do know that arrays are not primitive nor an object, but instead a reference.. that is being reffered to an identifier of same type,

**chronoz13** · October 1st, 2011, 08:57 PM

i just found an article, explaining that , the reference is being passed, so all the methods that receives that array argument, will refer to the ORGININAL array location?. so no matter how many separate calls a program will make to an array, all of those calls are reffering to the original one?

Beginning Java - Unit 6 Arrays - Arrays and Methods

**Norm** · October 1st, 2011, 08:59 PM

A copy of the reference to the array is passed to each method so they can manipulate the contents of the array.

**chronoz13** · October 1st, 2011, 09:04 PM

so somehow we can say that arrays although as a reference.. is mutable?

**chronoz13** · October 1st, 2011, 09:05 PM

thank you norm.

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