Handling java types

Question

Does java really handle all primitive types differently than custom Objects? I pose this question as I examined and tried to "interpret" the results of this simple experimental program:

public class RandomObject {

 String name;
 int value;

public RandomObject(String s, int i){
setName(s);
setValue(i);
}

public int getValue() {
return value;
}

public void setValue(int value) {
this.value = value;
}

public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
 }

}


public static void main(String[] args) {

int x = 2;
int y = x;
System.out.println(y);
x = 4;
System.out.println(y);


RandomObject obj1 = new RandomObject("object1", 4);
RandomObject obj2;

obj2 = obj1;

System.out.println(obj2.getValue());
obj1.setValue(17);
System.out.println(obj2.getValue());

 }

The results are: 2 2 4 17

Although x has changed, the value of y remains immutable, whereas in objects the change affects both of them. Does the same happen in all primitive types (other than integers) and why?

Edd · Accepted Answer · 2012-08-29 11:07:28Z

2

Java does treat the values stored in the y and obj2 variables at the point that they are assigned (y = x and obj2 =obj1` respectively) similarly, but the key difference is that with Objects the variable only contains a reference to an actual object, not the data-values themselves.

int x,y;
x = 2;
y = x; //x = 2, y = 2
x = 3; //x = 3, y = 2 - Changing x does not change y

Object a,b;
a = new MyObject("foo"); //Create Object O on the heap, a = [Address of Object O]
b = a; //a = [Address of Object O], b = [Address of Object O]
a.updateValue("barr"); //Update a property of Object O
//a and b still contain [Address of Object O], but Object O has a new value.
a = new MyObject("Bazz"); //Create Object P, a = [Address of Object P]
//a = [Address of Object P], b = [Address of Object O]

The key thing to note is that the variables only get updated when there is an assignment (=) statement.

edited Aug 29, 2012 at 11:07

answered Aug 29, 2012 at 10:54

Edd

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1 Comment

arjacsoh Over a year ago

I have to admit, this is a quite illustrative explanation.

rubixibuc · Accepted Answer · 2012-08-29 11:15:47Z

2

In Java, variables which are any type of object, only contain a reference (pointer), to the data they represent. When you assign obj2 to obj1 you are saying:

take the address in obj2 and put it in obj1.

when you use the dot operator "." you are saying

The object at that address, give me that object.

Variables which are any type of primitive actually store the data themselves. So when you say

int i = 2;
int j = 3;

j = i;

Store the value 2 in j

edited Aug 29, 2012 at 11:15

answered Aug 29, 2012 at 10:59

rubixibuc

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Comments

Amit Deshpande · Accepted Answer · 2012-08-29 11:00:19Z

0

You are just assigning Obj1 to Obj2 Assignment will be same for both. The concept of Immutable comes in picture when you try to change the value.

Example below:

    String one = "String";
    String two = one;
    System.out.println("equal: " + one.equals(two));
    System.out.println("same:  " + (one == two));
    // Change one
    one = one.concat("concat");
    System.out.println("equal: " + one.equals(two));
    System.out.println("same:  " + (one == two));

It outputs

equal: true
same:  true
equal: false
same:  false

Because String is immutable every operation on String creates new String.

edited Aug 29, 2012 at 11:00

answered Aug 29, 2012 at 10:53

Amit Deshpande

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Comments

perilbrain · Accepted Answer · 2012-08-29 11:00:55Z

0

RandomObject obj1 = new RandomObject("object1", 4);

This assignment first creates an object of RandomObject and assigns its reference to obj1.

obj2 = obj1;

Now obj1 and obj2 both reference the same object.Any operation run on obj1 will reflect to the created object, which simultaneously being refered by obj2.Hence obj2 will also show the side effect of any operation on obj1 that directly or indirectly tampers the object created.

However removing reference of obj1 like obj1=null wont affect obj2 because the reference has been changed and only obj2 is now pointing to the created object.

More Info Here

answered Aug 29, 2012 at 11:00

perilbrain

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