Chapter 4
Application Program Interfaces

4.1 The Purpose of Interfaces

• An interface is a place where two entities meet and interact with each other.
• In programming, an interface specifies a contract between a service provider and a user.
• An interface specifies behavior, but not how it may be realized.
• Java has rules on how interfaces should be specified.
• We already considered interfaces of fields, methods, and constructors.
• An API shows the interfaces of a class.

4.2 Interfaces in Class Definitions

Strip:

• Braced body following the interfaces of methods and constructors.
• Initialization parts ‘=...’ following the interfaces of fields.

4.3 Inspecting Interfaces through Web Pages

• The web pages are created from the class files through commands of the form ‘javadoc <options> filename’
• The ‘javadoc’ command list the legal options when issued without arguments.

4.4 Introspecting Class Implemetations

• Requested through commands of the form ‘javap classname’.

  javap java.lang.String:

  Compiled from String.java 
  public final class java.lang.String 
           extends    java.lang.Object 
           implements java.io.Serializable, 
                      java.lang.Comparable { 
    public static final java.util.Comparator 
                     CASE_INSENSITIVE_ORDER; 
    public java.lang.String(); 
    public java.lang.String(java.lang.String); 
    public java.lang.String(char[]); 
    .... 
    java.lang.String(int,int,char[]); 
    public int length(); 
    public char charAt(int); 
    public void getChars(int, int, char[], int); 
    public void getBytes(int, int, byte[], int); 
    .... 
    public boolean equals(java.lang.Object); 
    public boolean 
             equalsIgnoreCase(java.lang.String); 
    ..... 
    public static java.lang.String valueOf(float); 
    public static java.lang.String valueOf(double); 
    public native java.lang.String intern(); 
    static {}; 
  } 
  
• General format: javap <options> <classes>...

  where options include:

  -public	Show only public classes and members
  -protected	Show protected/public classes and members
  -package	Show package/protected/public classes and members (default)
  -private	Show all classes and members

4.5 Interface Classes and Abstract Classes

• The Java language allows to define classes with members that consist just of interfaces of methods and (finazlized) fields.
  interface a { 
    int readInt(); 
    char readCh(); 
  } 
  
• Classes of the above kind are referred to as interfaces.

  Interface Summary (java.util)
  Collection	The root interface in the collection hierarchy.
  Iterator	An iterator over a collection.
  List	An ordered collection (also known as a sequence).

• An abstract class is a cross between a concrete class and an interface. Some methods, but not necessarily all of them, may be expressed as interfaces.
  absract class A { 
    void xx(); 
    void yy(){ ; } 
  } 
  

4.6 Assignment #4: Syntax of Interfaces

Due: We, Apr 9, at class

Issue the command ‘javap -private java.lang.String’ and inspect its outcome. Provide the following information.

1. Identify two instance fields of class ‘String’.
2. Identify two class (static) fields of class ‘String’.
3. Identify two constructors of class ‘String’.
4. Identify two class (static) methods of class ‘String’.
5. Identify two instance methods of class ‘String’.
6. Identify all the modifiers in use within the interfaces.
7. Identify all the listed primitive types.
8. Identify all the listed class types.

Q&A