What are Abstract Classes in Java and How Are They Used?

Abstract Classes in Java

Abstract classes play a vital role in object-oriented programming in Java. This article explores what abstract classes are and how they are utilized in Java development.

What is an Abstract Class in Java?

An abstract class in Java is a class that cannot be instantiated directly. Instead, it serves as a blueprint for subclasses that can be instantiated. Abstract classes define methods that can be used by subclasses and specify abstract methods that must be implemented in subclasses.

Key Benefits of Abstract Classes:

• Define Common Methods: Abstract classes define common methods that subclasses can inherit and use.
• Specify Abstract Methods: They require subclasses to implement specific abstract methods.
• Provide a Common Interface: Subclasses share a common interface, allowing them to be used interchangeably.

Example of an Abstract Class:

Consider the following example of an abstract class AbstractClass:

abstract public class AbstractClass {
    abstract public void abstractMethod();
    public void implementedMethod() { System.out.print("implementedMethod()"); }
    final public void finalMethod() { System.out.print("finalMethod()"); }
}

In this example:

• abstractMethod() is an abstract method that must be implemented in subclasses.
• implementedMethod() is an implemented method that can be used by subclasses.
• finalMethod() is a final method that cannot be overridden in subclasses.

Implementing an Abstract Class:

To implement an abstract class, create a subclass that extends the abstract class and provides implementations for abstract methods. For example:

public class ImplementingClass extends AbstractClass {
    public void abstractMethod() { System.out.print("abstractMethod()"); }
}

Multiple Implementations:

Abstract classes can be implemented multiple times, allowing for different implementations of the same interface. Consider the following second implementing class SecondImplementingClass:

public class SecondImplementingClass extends AbstractClass {
    public void abstractMethod() { System.out.print("second abstractMethod()"); }
}

Casting Between Classes:

To access subclass-specific members from an abstract class reference, cast the reference to the subclass:

AbstractClass b = new ImplementingClass();
((ImplementingClass) b).uniqueMethod();

Extending and Implementing:

Only one class can be extended at a time. However, multiple interfaces can be implemented. For example:

public class ImplementingClass extends AbstractClass implements InterfaceA, InterfaceB {
    ... // implementation
}

Conclusion:

Abstract classes offer flexibility and code reuse in Java inheritance hierarchies. They define common structures and behaviors while allowing subclasses to provide specific implementations. By leveraging abstract classes, developers can effectively organize and extend object-oriented designs.

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