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Thursday, 20 December 2012

Does Java pass by value or pass by reference in java

Does Java is pass by value or pass by reference is one of the tricky Java question mostly asked on fresher level interviews. Before debating whether Java is pass by value or pass by reference lets first clear what is pass by value and what is pass by reference. This question has its origin on C and C++ where you can pass function parameter either value or memory address, where value is stored (pointer). As per Java specificationeverything in Java is pass by value whether its primitive value or objects and it does make sense because Java doesn't support pointers or pointer arithmetic, Similarly multiple inheritance and operator overloading is also not supported in Java. This question becomes confusing when interviewer ask about how object is passed in Java ? Answer to this question is simple whenever a method parameter expect object, reference of that object is passed. Many programmer confuses reference with pointers here which is not correct, reference is a kind of handle which is used to locate object or change object, but it doesn’t allows any pointer arithmetic i.e. you can not increase or decrease memory address and locate a different object using reference in Java.

Pass by Value and Pass by Reference Example in Java

Let’s see two example of calling method and passing parameter this will clear any doubt whether Java is pass by value or pass by reference. consider following example:

public class PassByValueExample {
public static void main(String args[]) {
int number = 3;
System.out.println("number Inside main(): "+number);
public static void printNext(int number){
System.out.println("number Inside printNext(): "+number);
number Inside printNext
number Inside main

Above example clearly shows that primitives are passed as pass by value to method parameters, had Java pass by reference both main method and printNext() would have printed same value. Now look at another example of passing object as method parameter which will confuse you that Java is pass by reference, which Java is not.

public class PassByReferenceConfusion {
public static void main(String args[]) {
       Car car = 
new Car("BMW");
System.out.println("Brand of Car Inside main() before: "+ car.brand);
System.out.println("Brand of Car Inside main()after: "+ car.brand);
public static void printBrand(Car car){
brand = "Maruti";
System.out.println("Brand of Car Inside printBrand(): "+car.brand);
private static class Car{
private String brand;
public Car(String brand){
this.brand = brand;

Brand of Car Inside main
() before: BMW
Brand of Car Inside printBrand
(): Maruti
Brand of Car Inside main
()after: Maruti

If you see change made in method parameter is reflected globally i.e. brand of car is changed in all places it means one object is used in both method. Well in reality if you pass object as method parameter in Java  it passes "value of reference" or in simple term object reference or handle to Object in Java. Here reference term is entirely different than reference term used in C and C+ which directly points to memory address of variable and subject to pointer arithmetic. in Java object can only be accessed by its reference as you can not get memory address where object is stored or more precisely there is no method to get value of object by passing memory address.

To conclude everything in Java including primitive and objects are pass by value. In case of object value of reference is passed.