PassByValueEx.java

// Java is just like c, everything pass by value!
// C++ is different, parameters can pass by reference!
// These are good interview questions and answers!
// Don't be confused by the word of "reference" and the expression "pass by reference"!

// The following is Java specific!

// For primitive types, value means the value of int, double, boolean, char, etc.
// You can change the value of primitive type inside a method
// but when the method returns, the value of passed in variable will stay 
// the same as before the methods call.

// For Objects, the value is the reference of the Object!
// When you pass an Object as parameter to a method
// You can change the reference of the object inside of a method
// but when the method returns, the reference of the object will stay  
// the same as before the methods call.

// !!Attention!!
// However, the values of data members of the Object can be changed permanently
// Since the copy of reference still refer to the same object.
// In other words, it is a shallow copy.
// See more discussion on this topic here


class MyObject{ 
   public int n = 10; 
} 

public class PassByValueEx{ 
   public static void main(String argv[]){ 
      PassByValueEx pbve = new PassByValueEx(); 
      pbve.aMethod(); 
   } 
   public void aMethod(){ 
      int      i = 99; 
      MyObject v = new MyObject(); 
      
      v.n = 30; 
      
      // v and i are both pass by value
      bMethod(v, i);
      
      // after bMethod call, value of v and i are not changed
      // but the data member v.n is changed
      System.out.println(v.n + ", " + i); // 20, 99
   }

   public void bMethod(MyObject v, int i){ 
      i = 0;               // i value changed inside, but not outside bMethod
      v.n = 20;            // The value of v.n is permanently changed
      v = new MyObject();  // reference of v changed inside, but not outside of 
      System.out.println(v.n + ", " + i); // 10, 0
   } 
} 

// output
// 10, 0
// 20, 99