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;
printNext(number);
System.out.println("number Inside main(): "+number);
}
public static void printNext(int number){
number++;
System.out.println("number Inside printNext(): "+number);
}
}
Output:
number Inside printNext(): 4
number Inside main(): 3
public static void main(String args[]) {
int number = 3;
printNext(number);
System.out.println("number Inside main(): "+number);
}
public static void printNext(int number){
number++;
System.out.println("number Inside printNext(): "+number);
}
}
Output:
number Inside printNext(): 4
number Inside main(): 3
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);
printBrand(car);
System.out.println("Brand of Car Inside main()after: "+ car.brand);
}
public static void printBrand(Car 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;
}
}
}
Output:
Brand of Car Inside main() before: BMW
Brand of Car Inside printBrand(): Maruti
Brand of Car Inside main()after: Maruti
public static void main(String args[]) {
Car car = new Car("BMW");
System.out.println("Brand of Car Inside main() before: "+ car.brand);
printBrand(car);
System.out.println("Brand of Car Inside main()after: "+ car.brand);
}
public static void printBrand(Car 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;
}
}
}
Output:
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.
