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classes in JAVA

Elements of a class

Name of class

A class has a name. In the following example, the name of the class is name_exp. The name of the class must follow the rules of valid identifiers. 
class name_exp {

   // more elements, such as
   // constructors
   // methods
   // variable
}

Constructor

The constructor is used when a new object is created. It's purpose is to initialize the newly created object.
If a class does not explicitly specify a constructor, the compiler supplies a default constructor that looks like: 
class_name() {}

Methods

class class_name {
  public        void do_something     () { /* here is the code that does something */ }
  public static void some_class_method() { /* here is the code that does something */ }
}
A method must follow the rules of valid identifiers.
A static method is also called a class method

Variables

class class_name {
  private int[]   some_array;
  private boolean is_valid;
  private static int counter;
}
A variable must follow the rules of valid identifiers.

Nested classes vs top level classes

A class is either a top level class (also known as top-level package member classes) or a nested class.

Top level package member classes (or interfaces)

package SomePackage;

class ATopLevelClass {
  // The class is not nested in another class
}

Top level nested classes (or interfaces)


class FooBar {
  static TopLevelNestedClass {
    // Top level nested class because within a top level class and static

    static AnotherTopLevelNestedClass {
      // Top level nested class because within a top level class and static 
      // top level nested classes are considered top level although they're nested
    }
  }
}
Although a nested class, it is also considered a top level class.

Non-static inner classes

A non-static inner class cannot have static members. 
class FooBar {
  public class NonStaticInnerClass {
    private int i_;
    public  void do_something() { }
    // static float f_;    // ILLEGAL
  }
}
A non-static inner class must be associated with an existing object of the enclosing class.

Local classes

Local classes are defined in a block.

Anonymous classes

interface IFoo {
  void msg();
}

class Foo implements IFoo {
  public void msg() {System.out.println("The message is: " + s() );}
  protected String s() {return "Bar";}
}

public class A {
  public static void main(String[] args) {
    Foo foo = new Foo() {
      protected String s() {return "anonymous";}
    };

    foo.msg();
  }
}