Java Basics: Abstract and Nested Classes

Modeling Business Problems with Classes

Class Inheritance

When designing an object-oriented solution, you should attempt to avoid code duplication. One technique to avoid duplication is to create library methods and classes. Libraries function as a central point to contain often reused code. Another technique to avoid code duplication is to use class inheritance. When there is a shared base type identified between two classes, any shared code may be placed in a parent class.

When possible, use object references of the most generic base type possible. In Java, generalization and specialization enable reuse through method inheritance and virtual method invocation (VMI). VMI, sometimes called “late-binding,” enables a caller to dynamically call a method as long as the method has been declared in a generic base type.

Inheritance (or subclassing) is an essential feature of the Java programming language. Inheritance provides code reuse through:

• Method inheritance: Subclasses avoid code duplication by inheriting method implementations.
• Generalization: Code that is designed to rely on the most generic type possible is easier to maintain.

Enabling Generalization

Coding to a common base type allows for the introduction of new subclasses with little or no modification of any code that depends on the more generic base type.

ElectronicDevice dev = new Television();
dev.turnOn(); // all ElectronicDevices can be turned on

Always use the most generic reference type possible.

Coding for Generalization

Always use the most generic reference type possible. Java IDEs may contain refactoring tools that assist in changing existing references to a more generic base type.

Identifying the Need for Abstract Classes

Subclasses may not need to inherit a method implementation if the method is specialized.

public class Television extends ElectronicDevice {
   public void turnOn() {
      changeChannel(1);
      initializeScreen();
   }
   public void turnOff() {}

   public void changeChannel(int channel) {}
   public void initializeScreen() {}

}

Method Implementations

When sibling classes have a common method, it is typically placed in a parent class. Under some circumstances, however, the parent class’s implementation will always need to be overridden with a specialized implementation.

In these cases, inclusion of the method in a parent class has both advantages and disadvantages. It allows the use of generic reference types, but developers can easily forget to supply the specialized implementation in the subclasses.

Defining Abstract Classes

Declaring a class as abstract prevents any instances of that class from being created. It is a compile-time error to instantiate an abstract class. An abstract class is typically extended by a child class and may be used as a reference type.

– A class can be declared as abstract by using the abstract class-level modifier.

public abstract class ElectronicDevice { }

– An abstract class can be subclassed.

public class Television extends ElectronicDevice { }

– An abstract class cannot be instantiated.

ElectronicDevice dev = new ElectronicDevice(); // error

Defining Abstract Methods

A method can be declared as abstract by using the abstract method-level modifier.

public abstract class ElectronicDevice {

  public abstract void turnOn();
  public abstract void turnOff();
}

An abstract method:

• Cannot have a method body
• Must be declared in an abstract class
• Is overridden in subclasses

Inheriting Abstract Methods

When a child class inherits an abstract method, it is inheriting a method signature but no implementation. For this reason, no braces are allowed when defining an abstract method. An abstract method is a way to guarantee that any child class will contain a method with a matching signature.

Note: An abstract method can take arguments and return values. For example:

abstract double calculateArea(double dim1, double dim2);

Validating Abstract Classes

The following additional rules apply when you use abstract classes and methods:

• An abstract class may have any number of abstract and nonabstract methods.
• When inheriting from an abstract class, you must do either of the following:
  • Declare the child class as abstract.
  • Override all abstract methods inherited from the parent class.Failure to do so will result in a compile-time error

error: Television is not abstract and does not override abstract method turnOn() in ElectronicDevice

Making Use of Abstract Classes

While it is possible to avoid implementing an abstract method by declaring child classes as abstract, this only serves to delay the inevitable. Applications require nonabstract methods to create objects. Use abstract methods to outline functionality required in child classes.

Final Methods

A method can be declared final. Final methods may not be overridden.

public class MethodParentClass {
      public final void printMessage() {
          System.out.println("This is a final method");
     }
 }

public class MethodChildClass extends MethodParentClass {
   // compile-time error
   public void printMessage() {
       System.out.println("Cannot override method");
   }
}

Performance Myths

There is little to no performance benefit when you declare a method as final. Methods should be declared as final only to disable method overriding.

Final Classes

A class can be declared final. Final classes may not be extended.

public final class FinalParentClass { }

// compile-time error
public class ChildClass extends FinalParentClass { }

Final Variables

The final modifier can be applied to variables. Final variables may not change their values after they are initialized. Final variables can be:

• Class fields: Final fields with compile-time constant expressions are constant variables. Static can be combined with final to create an always-available,never-changing variable.
• Method parameters
• Local variables

Benefits and Drawbacks of Final Variables

• Bug Prevention: Final variables can never have their values modified after they are initialized. This behavior functions as a bug-prevention mechanism.
• Thread Safety: The immutable nature of final variables eliminates any of the concerns that come with concurrent access by multiple threads.
• Final Reference to Objects: A final object reference only prevents a reference from pointing to another object. If you are designing immutable objects, you must prevent the object’s fields from being modified. Final references also prevent you from assigning a value of null to the reference. Maintaining an object’s references prevents that object from being available for garbage collection.

Declaring Final Variables

Final Fields

Initializing – Final fields (instance variables) must be either of the following:

• Assigned a value when declared.
• Assigned a value in every constructor.

Static and Final Combined: A field that is both static and final is considered a constant. By convention, constant fields use identifiers consisting of only uppercase letters and underscores.

public class VariableExampleClass {
   private final int field;
   public static final int JAVA_CONSTANT = 10;

   public VariableExampleClass() {
          field = 100;
   }

   public void changeValues(final int param) {
       param = 1; // compile-time error
       final int localVar;
       localVar = 42;
       localVar = 43; // compile-time error
  }
}

Nested Classes

A nested class is a class declared within the body of another class. Nested classes have multiple categories:
1. Inner classes:

• Member classes
• Local classes
• Anonymous classes

2. Static nested classes.

Nested classes are commonly used in applications with GUI elements. They Can limit utilization of a “helper class” to the enclosing top-level class.

An inner nested class is considered part of the outer class and inherits access to all the private members of the outer class.

A static nested class is not an inner class, but its declaration appears similar with an additional static modifier on the nested class. Static nested classes can be instantiated before the enclosing outer class and, therefore, are denied access to all nonstatic members of the enclosing class.

Reasons to Use Nested Classes

• Logical Grouping of Classes – If a class is useful to only one other class, then it is logical to embed it in that class an 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.

Example: Member Class

The example below demonstrates an inner class, EMICalculatorHelper, which is defined in the BankEMICalculator class.

public class BankEMICalculator {
     private String CustomerName;
     private String AccountNo;
     private double loanAmount;
     private double monthlypayment;
     private EMICalculatorHelper helper = new EMICalculatorHelper();

     /*Setters ad Getters*/     
    private class EMICalculatorHelper {
        int loanTerm = 60;
        double interestRate = 0.9;
        double interestpermonth=interestRate/loanTerm;

   protected double calcMonthlyPayment(double loanAmount)
    {
     double EMI= (loanAmount * interestpermonth) / ((1.0) - ((1.0) /
   Math.pow(1.0 + interestpermonth, loanTerm)));
     return(Math.round(EMI));
    }
  }
}

Enumerations

Java includes a typesafe enum to the language. Enumerations (enums): are created by using a variation of a Java class and provide a compile-time range check.

public enum PowerState {
    OFF,
    ON,
    SUSPEND;
}

These are references to the only three PowerState objects that can exist.

An enum can be used in the following way:

Computer comp = new Computer();
comp.setState(PowerState.SUSPEND);

Compile-Time Range Checking

In the example above, the compiler performs a compile-time check to ensure that only valid PowerState instances are passed to the setState method. No range checking overhead is incurred at runtime.

Enum Usage

Enums can be used as the expression in a switch statement.

public void setState(PowerState state) {
     switch(state) {
         case OFF:
         //...
    }
}

Complex Enums

Enums can have fields, methods, and private constructors. You can call a PowerState constructor to initialize the public static final OFF reference. The constructor may not be public or protected.

public enum PowerState {
   OFF("The power is off"),
   ON("The usage power is high"),
   SUSPEND("The power usage is low");

   private String description;
   private PowerState(String d) {
      description = d;
   }
   public String getDescription() {
      return description;
  }
}

Enum Constructors

You may not instantiate an enum instance with new.

Complex Enums

Here is the complex enum in action.

 
public class ComplexEnumsMain {

    public static void main(String[] args) {
      Computer comp = new Computer();
      comp.setState(PowerState.SUSPEND);
      System.out.println("Current state: " +
   comp.getState());
     System.out.println("Description: " +
   comp.getState().getDescription());
    }
}

Output:

Current state: SUSPEND
Description: The power usage is low

Enum Usage

When sent to be printed, the default behavior for an enumeration is to print the current value. Typically, an additional call to the enumation’s methods is necessary to get information stored in the enumeration.