Strings and Operators

• Java does not support operator overloading, you must use methods instead.
• In general, operators only work with primitive types.
• Equality (= = and != ) is an exception, but they do not work the way you might expect!
  
  
• When used with class instances, equality checks if the two objects are the same reference (point to the same memory) and not if the two objects have the same value.
  
  
• For instance, like all classes, the String class uses the equals() method to compare the values of two strings.
  
  
• Be careful with string literals. Java optimizes strings by storing the value of each string literal once and giving you pointers to this value wherever the string is used.
• This is called interning.
• Remember to use new when you want to have distinct objects:

      String s1 = new String("Cathy");
      String s2;
  
      s2 = s1;        // same spot in memory
  
      if (s1 == s2)       // true
          ...
  
      s2 = new String(s1);    // each have their own memory (probably!)
  
      if (s1 == s2)       // false, and probably WRONG!
          ...
  
      if (s1.equals(s2))  // true, and probably what you really want!
          ... 
  

• Also, Strings are what is called immutable, meaning they cannot be changed.
• If you read the documentation, you will notice that no method in the String class will change the String object; instead each one will return a new String.
• Note that operators work the same for all objects, not just Strings.

• Let's draw this code to see how objects of class Point work:

  
  Point p1 = new Point();
  Point p2 = new Point(p1);
  
  if (p1 == p2)
      System.out.println("YES 1");
  else
      System.out.println("NO 1");
  
  if (p1.equals(p2))
      System.out.println("YES 2");
  else
      System.out.println("NO 2");
  
  p1.x = 18;
  
  if (p1.equals(p2))
      System.out.println("YES 3");
  else
      System.out.println("NO 3");
  
  p1 = p2;
  if (p1 == p2)
      System.out.println("YES 4");
  else
      System.out.println("NO 4");
  
  if (p1.equals(p2))
      System.out.println("YES 5");
  else
      System.out.println("NO 5");
  
  p1.x = 28;