There are two ways of passing arguments to a method (function/subroutine):
1. Call by Value
This method copies the value of an argument into the formal parameter of the method. Therefore, changes made to the parameter of the method are limited to that method only and there is no impact of changes on the argument. That is, when control returns back to the caller method, earlier values of the arguments will be seen.
2. Call by Reference
In this method, a reference to an argument (not the value of the argument) is passed to the parameter. Inside the method, this reference is used to access the actual argument specified in the call. This means that changes made to the parameter will affect the actual argument also. When the control returns back to the caller method, the changes made in the method can be seen in the arguments in the caller method too.
In Java, when we pass a simple type to a method, it is passed by value. When we create a variable of a class, we are only creating a reference to an object. Thus, when we pass this reference method, the parameter that receives it will refer to the same object as that referred by the argument. This means that objects are passed to methods by reference.
Demonstration of Pass by Reference
class Passby
{
int a,b;
void setdata(int x, int y)
{
a = x;
b = y;
}
void display(Passby m)
{
m.a =m.a+10;
m.b=m.b-5;
}
}
class PassbyRef
{
public static void main(String args[])
{
Passby Ref =new Passby();
Ref.setdata(50,100);
System.out.println(“Before Function Call: a=” + Ref.a +” b= ” + Ref.b) ;
Ref.display(Ref);
System.out.println(“After Function Call: a=” + Ref.a +” b= ” + Ref.b) ;
}
}
Output
Before Function Call: a = 50 b= 100
After Function Call: a = 60 b= 95
In above program, first an instance of class Passbyby name Ref is made. The value of its data members a and b are set to 50 and 100 respectively by invoking setdata() method. After that by invoking display (), the instance Ref itself is sent as argument to parameter m. Since, we know that objects are passed to methods by reference, any changes made in the data members of parameter m will also effect the argument Ref. In the display (), the value of data member a is incremented by 10 and the value of data member is decremented by 5. The same effects will be seen in the argument Ref when the control returns back to the main() method.
Demonstration of Pass by Value
class PassbyValue
{
public static void main(String args[])
{
int a =50, b=100;
System.out.println(“Before Function Call : a = ” +a+ ” b = “+b);
chng(a,b);
System.out.println(“After Function Call : a = ” +a+ ” b = “+b);
}
static void chng(int x, int y)
{
x=x+10;
y=y-5;
}
}
Output:
Before Function Call: a= 50 b= 100
After Function call: a= 50 b= 100
When we pass a simple type to a method, it is passed by value. So, any changes made to the data in the method are limited to that method only and the data retains its older values when returns back to the caller.
Dinesh Thakur holds an B.C.A, MCDBA, MCSD certifications. Dinesh authors the hugely popular