Demonstrate the difference between mutating a value and reassigning a different value.

DemonstrateMutableVsImmutable.java

class MutableInt {
  private int value;

  MutableInt(int value) {
    this.value = value;
  }

  void mutate() {
    value++;
  }

  MutableInt add(MutableInt other) {
    return new MutableInt(value + other.value);
  }

  public String toString() {
   return new Integer(value).toString();
  }
}

public class DemonstrateMutableVsImmutable {
  public static void main(String... args) {    
    demonstrateMutable();
    demonstrateImmutable();
  }

  static void demonstrateMutable() {
    MutableInt one = new MutableInt(1);
    System.out.println(one.add(one)); // 2

    MutableInt two = one;
    two.mutate();
    System.out.println(one.add(one)); // 4
  }

  static void demonstrateImmutable() {
    int one = 1;
    System.out.println(one + one);    // 2

    int two = one;
    two++;
    System.out.println(one + one);    // 2
  }
}