More on Classes

This section covers more aspects of classes that depend on using object references and the dot operator that you learned about in the preceding sections on objects:

• Returning values from methods.
• The this keyword.
• Class vs. instance members.
• Access control.
•  

• Nested Classes

  The Java programming language allows you to define a class within another class. Such a class is called a nested class and is illustrated here:
  

  class OuterClass {
      ...
      class NestedClass {
          ...
      }
  }
  

  ---

  Terminology: Nested classes are divided into two categories: static and non-static. Nested classes that are declared static are simply called static nested classes. Non-static nested classes are called inner classes.

  ---

  class OuterClass {
      ...
      static class StaticNestedClass {
          ...
      }
      class InnerClass {
          ...
      }
  }
  

  A nested class is a member of its enclosing class. Non-static nested classes (inner classes) have access to other members of the enclosing class, even if they are declared private. Static nested classes do not have access to other members of the enclosing class. As a member of the OuterClass, a nested class can be declared private, public, protected, or package private. (Recall that outer classes can only be declared public or package private.)
  
  

  Why Use Nested Classes?

  There are several compelling reasons for using nested classes, among them:
  
• It is a way of logically grouping classes that are only used in one place.
• It increases encapsulation.
• Nested classes can lead to more readable and maintainable code.

Logical grouping of classes—If a class is useful to only one other class, then it is logical to embed it in that class and keep the two together. Nesting such "helper classes" makes their package more streamlined.
Increased encapsulation—Consider two top-level classes, A and B, where B needs access to members of A that would otherwise be declared private. By hiding class B within class A, A's members can be declared private and B can access them. In addition, B itself can be hidden from the outside world.
More readable, maintainable code—Nesting small classes within top-level classes places the code closer to where it is used.

Static Nested Classes

As with class methods and variables, a static nested class is associated with its outer class. And like static class methods, a static nested class cannot refer directly to instance variables or methods defined in its enclosing class — it can use them only through an object reference.

---

Note: A static nested class interacts with the instance members of its outer class (and other classes) just like any other top-level class. In effect, a static nested class is behaviorally a top-level class that has been nested in another top-level class for packaging convenience.

---

Static nested classes are accessed using the enclosing class name:

OuterClass.StaticNestedClass

For example, to create an object for the static nested class, use this syntax:

OuterClass.StaticNestedClass nestedObject =
     new OuterClass.StaticNestedClass();

Inner Classes

As with instance methods and variables, an inner class is associated with an instance of its enclosing class and has direct access to that object's methods and fields. Also, because an inner class is associated with an instance, it cannot define any static members itself.
Objects that are instances of an inner class exist within an instance of the outer class. Consider the following classes:

class OuterClass {
    ...
    class InnerClass {
        ...
    }
}

An instance of InnerClass can exist only within an instance of OuterClass and has direct access to the methods and fields of its enclosing instance. The next figure illustrates this idea.

An Instance of InnerClass Exists Within an Instance of OuterClass

To instantiate an inner class, you must first instantiate the outer class. Then, create the inner object within the outer object with this syntax:

OuterClass.InnerClass innerObject = outerObject.new InnerClass();

Additionally, there are two special kinds of inner classes: local classes and anonymous classes.

---

Note: If you want more information on the taxonomy of the different kinds of classes in the Java programming language (which can be tricky to describe concisely, clearly, and correctly), you might want to read Joseph Darcy's blog: Nested, Inner, Member and Top-Level Classes.

---

Shadowing

If a declaration of a type (such as a member variable or a parameter name) in a particular scope (such as an inner class or a method definition) has the same name as another declaration in the enclosing scope, then the declaration shadows the declaration of the enclosing scope. You cannot refer to a shadowed declaration by its name alone. The following example, ShadowTest, demonstrates this:

 
public class ShadowTest {

    public int x = 0;

    class FirstLevel {

        public int x = 1;

        void methodInFirstLevel(int x) {
            System.out.println("x = " + x);
            System.out.println("this.x = " + this.x);
            System.out.println("ShadowTest.this.x = " + ShadowTest.this.x);
        }
    }

    public static void main(String... args) {
        ShadowTest st = new ShadowTest();
        ShadowTest.FirstLevel fl = st.new FirstLevel();
        fl.methodInFirstLevel(23);
    }
}

The following is the output of this example:

x = 23
this.x = 1
ShadowTest.this.x = 0

This example defines three variables named x: The member variable of the class ShadowTest, the member variable of the inner class FirstLevel, and the parameter in the method methodInFirstLevel. The variable x defined as a parameter of the method methodInFirstLevel shadows the variable of the inner class FirstLevel. Consequently, when you use the variable x in the method methodInFirstLevel, it refers to the method parameter. To refer to the member variable of the inner class FirstLevel, use the keyword this to represent the enclosing scope:

System.out.println("this.x = " + this.x);

Refer to member variables that enclose larger scopes by the class name to which they belong. For example, the following statement accesses the member variable of the class ShadowTest from the method methodInFirstLevel:

System.out.println("ShadowTest.this.x = " + ShadowTest.this.x);

• 
  

  Local Classes

  Local classes are classes that are defined in a block, which is a group of zero or more statements between balanced braces. You typically find local classes defined in the body of a method.
  This section covers the following topics:
  

  • Declaring Local Classes
  • Accessing Members of Enclosing Class
    • Shadowing
  • Local Classes Are Non-Static

  Declaring Local Classes

  You can define a local class inside any block (which is a group of zero or more statements between balanced braces; see Expressions, Statements, and Blocks for more information). For example, you can define a local class in a method body, a for loop, or an if clause.
  The following example, LocalClassExample, validates two phone numbers. It defines the local class PhoneNumber in the method validatePhoneNumber:
  

   
  public class LocalClassExample {
    
      static String regularExpression = "[^0-9]";
    
      public static void validatePhoneNumber(
          String phoneNumber1, String phoneNumber2) {
        
          final int numberLength = 10;
          
          // Valid in Java SE 8 and later:
         
          // int numberLength = 10;
         
          class PhoneNumber {
              
              String formattedPhoneNumber = null;
  
              PhoneNumber(String phoneNumber) {
                  // numberLength = 7;
                  String currentNumber = phoneNumber.replaceAll(
                    regularExpression, "");
                  if (currentNumber.length() == numberLength)
                      formattedPhoneNumber = currentNumber;
                  else
                      formattedPhoneNumber = null;
              }
  
              public String getNumber() {
                  return formattedPhoneNumber;
              }
              
              // Valid in Java SE 8 and later:
  
  //            public void printOriginalNumbers() {
  //                System.out.println("Original numbers are " + phoneNumber1 +
  //                    " and " + phoneNumber2);
  //            }
          }
  
          PhoneNumber myNumber1 = new PhoneNumber(phoneNumber1);
          PhoneNumber myNumber2 = new PhoneNumber(phoneNumber2);
          
          // Valid in Java SE 8 and later:
  
  //        myNumber1.printOriginalNumbers();
  
          if (myNumber1.getNumber() == null) 
              System.out.println("First number is invalid");
          else
              System.out.println("First number is " + myNumber1.getNumber());
          if (myNumber2.getNumber() == null)
              System.out.println("Second number is invalid");
          else
              System.out.println("Second number is " + myNumber2.getNumber());
  
      }
  
      public static void main(String... args) {
          validatePhoneNumber("123-456-7890", "456-7890");
      }
  }
  
  

  The example validates a phone number by first removing all characters from the phone number except the digits 0 through 9. After, it checks whether the phone number contains exactly ten digits (the length of a phone number in North America). This example prints the following:
  

  First number is 1234567890
  Second number is invalid

  Accessing Members of Enclosing Class

  A local class has access to the members of its enclosing class. In the previous example, the PhoneNumber constructor accesses the member LocalClassExample.regularExpression.
  In addition, a local class has access to local variables. However, a local class can only access local variables that are declared final. For example, the PhoneNumber constructor can access the local variable numberLength because it is declared final.
  Starting in Java SE 8, if you declare the local class in a method, it can access the method's parameters. For example, you can define the following method in the PhoneNumber local class:
  

  public void printOriginalNumbers() {
      System.out.println("Original numbers are " + phoneNumber1 +
          " and " + phoneNumber2);
  }

  The method printOriginalNumbers accesses the method parameters phoneNumber1 and phoneNumber2.
  Starting in Java SE 8, a local class can access local variables and parameters of the enclosing block that are final or effectively final. A variable or parameter whose value is never changed after it is initialized is effectively final. For example, suppose the variable numberLength is not declared final, and you add the highlighted assignment statement in the PhoneNumber constructor:
  

  PhoneNumber(String phoneNumber) {
      numberLength = 7;
      String currentNumber = phoneNumber.replaceAll(
          regularExpression, "");
      if (currentNumber.length() == numberLength)
          formattedPhoneNumber = currentNumber;
      else
          formattedPhoneNumber = null;
  }

  Because of this assignment statement, the variable numberLength is not effectively final anymore. As a result, the Java compiler generates an error message similar to "local variables referenced from an inner class must be final or effectively final" where the inner class PhoneNumber tries to access the numberLength variable:
  

  if (currentNumber.length() == numberLength)

  Shadowing

  Declarations of a type (such as a variable) in a local class shadow declarations in the enclosing scope that have the same name. See Shadowing for more information.
  

  Local Classes Are Inner Classes

  Local classes are like inner classes because they cannot define or declare any static members. Local classes in static methods, like class PhoneNumber, which is defined in the static method vaidatePhoneNumber, can only refer to static members of the enclosing class. For example, if you do not define the member variable regularExpression as static, the Java compiler generates an error similar to "non-static variable regularExpression cannot be referenced from a static context."
  Local classes are non-static because they have access to instance members of the enclosing block. Consequently, they cannot contain most kinds of static declarations.
  You cannot declare an interface inside a block; interfaces are inherently static. For example, the following code excerpt does not compile because the interface HelloThere is defined inside the body of the method greetInEnglish:
  

      public void greetInEnglish() {
          interface HelloThere {
             public void greet();
          }
          class EnglishHelloThere implements HelloThere {
              public void greet() {
                  System.out.println("Hello " + name);
              }
          }
          HelloThere myGreeting = new EnglishHelloThere();
          myGreeting.greet();
      }

  You cannot declare static initializers or member interfaces in a local class. The following code excerpt does not compile because the method EnglishGoodbye.sayGoodbye is declared static. The compiler generates an error similar to "modifier 'static' is only allowed in constant variable declaration" when it encounters this method definition:
  

      public void sayGoodbyeInEnglish() {
          class EnglishGoodbye {
              public static void sayGoodbye() {
                  System.out.println("Bye bye");
              }
          }
          EnglishGoodbye.sayGoodbye();
      }

  A local class can have static members provided that they are constant variables. (A constant variable A variable is a variable of primitive type or type String that is declared final and initialized with a compile-time constant expression. A compile-time constant expression is typically a string or an arithmetical expression that can be evaluated at compile time. See Understanding Instance and Class Members for more information.) The following code excerpt compiles because the static member EnglishGoodbye.farewell is a constant variable:
  

      public void sayGoodbyeInEnglish() {
          class EnglishGoodbye {
              public static final String farewell = "Bye bye";
              public void sayGoodbye() {
                  System.out.println(farewell);
              }
          }
          EnglishGoodbye myEnglishGoodbye = new EnglishGoodbye();
          myEnglishGoodbye.sayGoodbye();
      }