Mystery sum: 40
sum: 99

Two Objects

Colors in the following show how this works. this is used in the constructor where the instance variable, not the parameter, should be used.

class Mystery
{
  private int sum;
  
  public Mystery( int sum )
  {
    this.sum = sum;
  }
  
  public void increment( int inc )
  {
    sum = sum + inc;
    System.out.println("Mystery sum: " + sum );
  }
}

public class Tester
{
  public static void main ( String[] args)
  {
    int sum = 99;
    Mystery myst = new Mystery( 34 );
    myst.increment( 6 );
    System.out.println("sum: " + sum );
  } 
}

Now look at this modified version:

class Mystery
{
  private int sum;
  
  public Mystery( int x )
  {
    sum = x;
  }
  
  public void increment( int inc )
  {
    sum = sum + inc;
    System.out.println("Mystery sum: " + sum );
  }
}

public class Tester
{
  public static void main ( String[] args)
  {
    Mystery mystA = new Mystery( 34 );
    Mystery mystB = new Mystery( 13 );

    mystA.increment( 6 );
    mystB.increment( 7 );
  } 
}