ch7

• Interface


  Similar to a class but with restrictions: all methods are implicitly abstract and public, cannot have instance variables (but can have constants), can extend other interfaces, and is implemented by a class using the 'implements' keyword

  
• Interfaces


  • All methods are implicitly abstract and public
  • Cannot have instance variables (but can have constants)
  • Can extend other interfaces
  • Implemented by a class using the 'implements' keyword

  
• Java does not allow for multiple inheritance, but interfaces can be used to achieve similar advantages
• An interface is a type that groups together a number of different classes that all include method definitions for a common set of method headings
• An interface specifies the headings for methods that must be defined for any class that implements the interface
• An interface contains method headings and constant definitions only, no instance variables or complete method definitions
• Interfaces are implicitly abstract, and all methods in an interface are implicitly abstract and public
• Implementing an Interface Type

  Include 'implements Interface_Name' in class definition

  2. Implement all method headings listed in the interface definitions
• Converting Between Class and Interface Types

  Can convert from a class type to an interface type if the class implements the interface, but cannot convert between unrelated types
• Narrowing the scope of the overridden methods in the implementing class is a common error
• Inner (or Nested) Classes


  Classes defined/declared within other classes or interfaces, where the class that includes the inner class is called the outer class

  
• Within the definition of a method of an inner class, it is legal to reference a private instance variable of the outer class or invoke a private method of the outer class
• Within the definition of a method of the outer class, it is legal to reference a private instance variable of the inner class on an object of the inner class or invoke a non-static method of the inner class
• Four categories of Java inner classes

  • Non-static inner classes: Member inner classes, Local classes, Anonymous classes

  Static nested classes

  
• Member Inner Class


  A non-static class that is created inside a class but outside a method
• Within the definition of a method of the outer class, it is legal to reference a private instance variable of the inner class on an object of the inner class
• Within the definition of a method of the outer class, it is legal to invoke a (non-static) method of the inner class as long as an object of the inner class is used as a calling object
• Within the definition of the inner or outer classes, the modifiers public and private are equivalent
• Four categories of Java inner classes

  • Non-static inner classes
  • Member inner classes
  • Local classes (defined inside a block of Java code)
  • Anonymous classes (defined inside a block of Java code)
  • Static nested classes

  
• Member Inner Class


  A class created within class and outside method
• Local Inner Class


  A class created within method
• Anonymous Inner Class


  A class created for implementing interface or extending class. Its name is decided by the Java compiler

  
• Static Nested Class


  A static class created within class

  
• Nested Interface


  An interface created within class or interface
• static is used for a constant, variable or a method that is same for every instance of a class
• Member Inner Class


  • A non-static class that is created inside a class but outside a method
  • If an inner class is marked with the public modifier instead of private, then it can be used outside of the outer class
  • To create an object of the non-static inner class, you must start with an object of the outer class
  • For a static inner class, you can create objects of a public static inner class and do so outside of the inner class—in fact, even outside of the outer class
  • If both the inner and outer classes have a method with the same name, and the intent is to invoke the method in the outer class, then the following invocation must be used: OuterClassName.this.methodName()

  
• Nesting inner classes within inner classes

  • The rules are the same as before, but the names get longer
• Inner Classes and Inheritance

  • Given an OuterClass that has an InnerClass. If you derive DerivedClass from OuterClass, then DerivedClass automatically has InnerClass as an inner class just as if it were defined within DerivedClass

  
• Local Inner Class


  • A class that is created inside a method
  • Local inner class cannot be invoked from outside the method
  • If you want to invoke the methods of local inner class, you must instantiate this class inside the method
  • Local classes are completely hidden in their containing block
  • A local class can access instance variables of the outer class and any local variable [only the final local variables (till jdk 1.7 only)] of the enclosing block

  
• Anonymous Inner Class


  • A class that has no name
  • It should be used if you have to override method of class or interface
  • It can be created by two ways: 1. Class (may be abstract or concrete), 2. Interface
  • The class definition is embedded inside the expression with the new operator
  • Anonymous class has no constructors
  • It is either derived from a class or implements an interface
• Static Nested Class


  • A static class created inside a class
  • It cannot access non-static data members and methods
  • It can be accessed by outer class name
  • It can access static data members of outer class including private
  • Static nested class cannot access non-static (instance) data member or method
  • If you have the static member inside static nested class, you don't need to create instance of static nested class

  
• Compiling any class in Java produces a (.class) file named: ClassName.class

  
• Compiling a class with one or more inner classes causes both (or more) classes to be compiled and produces two or more (.class) files such as: ClassName.class and ClassName$InnerClassName.class

  
• Often, instead of having one action listener object deal with all the action events in a GUI, a separate ActionListener class is created for each button or menu item
• Each button or menu item has its own unique action listener
• When this approach is used, each class is usually made a private inner class