ch8

• Java (5.0+) allows class definitions with parameters for types, are called as parameterized classes, generic definitions, or simply generics

  
• Parameterized classes


  Class definitions with parameters for types
• Generic definitions


  Class definitions with parameters for types
• Generics


  Class definitions with parameters for types
• The class ArrayList is a parameterized class. It has a parameter, denoted by Base_Type, that can be replaced by any reference type to obtain a class for ArrayLists with the specified base type

  
• A class definition with a type parameter is stored in a file, and compiled and used just like any other class. However, the class type plugged in for the type parameter must be specified before it can be used in a program. Doing this is said to instantiate the generic class
• Type parameter


  Included in angular brackets after the class name in the class definition heading
• Any non-keyword identifier can be used for the type parameter, but by convention, the parameter starts with an uppercase letter
• The type parameter can be used like other types in class definition
• Type parameter is not used in heading of default constructor
• Methods can use the type parameter without angular brackets
• Instantiating a generic class

  Using angular brackets
• The type parameter cannot be a primitive type such as int, double, or char
• We cannot make an array of objects from a parameterized class
• A generic class cannot extend Exception, Error, Throwable (or its descendants)
• A generic class can extend an ordinary class or a generic class
• Many pitfalls can be encountered when using type parameters. Compiling with -Xlint option (to enable all warnings) will provide more informative diagnostics of any problems in the code
• ArrayList is a class in the standard Java libraries

  
• ArrayList


  Serves the same purpose as an array, except that an ArrayList can change length while the program is running

  
• The class ArrayList is implemented using an array as a private instance variable
• When the hidden array is full, a new larger hidden array is created and the data is transferred to this new array
• Disadvantages of ArrayList Class

  • An ArrayList is less efficient than an array
  • It does not have the convenient square bracket notation
  • The base type of an ArrayList must be a class type (or other reference type), it cannot be a primitive type

  
• The definition of class ArrayList is in the package java.util, and any code that uses it must import it, normally at the start of the file

  
• Creating an ArrayList


  ArrayList<BaseType> aList = new ArrayList<BaseType>(size);

  
• Specifying an initial capacity does not limit the size to which an ArrayList can eventually grow
• add method


  Used to set an element for the first time in an ArrayList
• set method


  Used to replace any existing element
• get method


  Used to access the value of any existing element
• size method


  Returns the number of elements stored in the ArrayList
• When looking at the methods available in the ArrayList class, there appears to be some inconsistency in the parameter types

  
• Invoking clone on an ArrayList object produces a shallow copy, not a deep copy
• The Java standard libraries have a class named Vector that behaves almost exactly the same as the class ArrayList

  
• The ArrayList class is newer, and is becoming the preferred class over Vector

  
• There are also ArrayList and Vector classes with no parameter whose base type is Object. These classes are left over from earlier versions of Java