• Frequently:
  • We need to swap the contents of two variables
• A Context:
  • A fantasy role playing game in which players commonly acquire items of various kinds (e.g., weapons, spells, gold, food)
• An Example of a Swap:
  • As the game progresses, two players meet and realize that it would benefit both of them to swap their swords

• The Assignment Operator:
  • Stores the right side operand in the memory location identified by the left side operand
• Variables:
  • Can only hold one "thing"

javaexamples/programmingpatterns/Swapping.java (Fragment: assignment)

        char swordOfAlice, swordOfBetty;
        
        swordOfAlice = 'C'; // Caladbolg
        swordOfBetty = 'D'; // Durendal
        

javaexamples/programmingpatterns/Swapping.java (Fragment: badswap)

        swordOfAlice = swordOfBetty;
        swordOfBetty = swordOfAlice;
        

• Suppose:
  • You have Caladbolg in your right hand and Durendal in your left hand, and you want to swap them
• To Make Progress:
  • You need a place to temporarily store one of the swords

• The Idea:
  • Use another variable to temporarily hold the contents of one variable
• javaexamples/programmingpatterns/Swapping.java (Fragment: swap)

          temp = a;
          a = b;
          b = temp;
          

javaexamples/programmingpatterns/Swapping.java (Fragment: swordswap)

        temp = swordOfAlice;        
        swordOfAlice = swordOfBetty;
        swordOfBetty = temp;
        

• The Idea:
  • Write a method that swaps the contents of its two parameters
• The Issue:
  • To understand what is and isn't possible one must understand parameter passing mechanisms