Pass by Value and Pass by Reference

Let’s try to understand how pass by value and pass by reference works in Java.

It is very simple and read the next sentence twice☺️. In Java no matter what type of argument you pass the corresponding parameter (primitive variable or object reference) will get a copy of that data, which is exactly how pass-by-value (or copy-by-value) works.

Look at the example given below:

Here methodOne invokes methodTwo and passes an int type a whose value is 10. methodTwo is accepting this in a variable called c and incrementing its value by one inside the method. Also it returns the incremented value. A copy of the value is passed from methodOne to methodTwo and there is no change in the actual value of a in methodOne.

So what happens if an object reference is passed instead of primitive type int ?

Here methodOne invokes methodTwo and passes an object reference of Car type which is c which has an attribute called color whose values is black. methodTwo is accepting this in a variable called d and changing it’s color attribute to white. A copy of the object reference is passed from methodOne to methodTwo and a change in the attribute value in methodTwo will get reflected in methodOne too. Because it is actually modifying the original object through copied reference.

Conclusion:

In pass-by-value the caller(methodOne)  and callee(methodTwo)  have two independent variables with the same value. A change in callee’s parameter variable, will not get reflected in caller’s variable.

In pass-by-reference the caller(methodOne) and the callee(methodTwo) use the same variable for the parameter. A change in callee’s parameter variable, reflects in caller’s variable too.