Overview

• Generics
  • Provides increased type-safety for collections
  • Eliminates most downcasting
• Autoboxing
  • Eliminates much of the drudgery of working with wrapper classes

  Generics

  • Prior to 1.5:

    Vector v = new Vector();
    while (...) {			// adding String's to the vector 
    	String s = ...;
    	v.add(s);
    }
    ...
    Iterator iter = v.iterator();
    while (iter.hasNext()) {		// Extracting elements of the vector
    	String s = (String)iter.next();	 	// downcast needed
    	...
    }
    		

  • Collections can now be declared with their element types:

    Vector<String> v = new Vector<String>():
    		

  • The compiler enforces this declaration

    Vector<String> v = new Vector<String>():
    v.add(new Integer(5));		// compiler error
    		

  • Collection is then guaranteed to be homogeneous
    • Above vector can only contain Strings
  • Downcasting can then be done automatically by the compiler

    Vector<String> v = new Vector<String>():
    String s = ...;
    v.add(s);
    ...
    String s2 = v.elementAt(0);		// no need for downcast-- compiler takes care of it
    		

  • Collection processing is now much simpler

    Vector<String> v = new Vector<String();
    while (...) {			// adding String's to the vector 
    	String s = ...;
    	v.add(s);
    }
    ...
    Iterator<String> iter = v.iterator();
    while (iter.hasNext()) {		// Extracting elements of the vector
    	String s = iter.next();	 	// no downcast needed
    	...
    }
    		

  • Users can create their own generic class definitions as well
    • But we won't

  Autoboxing

  • As of 1.5,
    • any time a primitive type appears in a context requiring an object, the type is wrapped (by the compiler) into its wrapper class object

      					Vector<Integer> v = new Vector<Integer>();
      					...
      					v.add(3);		// compiler automatically insert a 'new Integer(3)'
      				

    • any time an object appears in a context requiring a primitive type, the object is unwrapped into its corresponding primtive type

      					Vector<Integer> v = new Vector<Integer>();
      					...
      					int i = v.elementAt(0);		// compiler automatically insert a '.intValue()'