Java Abstract Class

In Java, an abstract class is a class that cannot be instantiated (i.e., you cannot create an object of an abstract class). It is meant to be subclassed (inherited) by other classes. Abstract classes can have abstract and non-abstract methods.

In this tutorial, we'll cover the following topics related to abstract classes in Java:

Creating an Abstract Class
Abstract Methods
Inheriting from an Abstract Class
Abstract Classes vs. Interfaces
Creating an Abstract Class:

To create an abstract class, use the abstract keyword before the class keyword:

abstract class Animal {
    // Class body
}

Abstract Methods:

An abstract method is a method that has no implementation in the abstract class and must be implemented by any non-abstract subclass of the class. To create an abstract method, use the abstract keyword before the method signature and end the method declaration with a semicolon:

abstract class Animal {
    abstract void sound(); // Abstract method
}

Inheriting from an Abstract Class:

When a class inherits from an abstract class, it must implement all the abstract methods of the abstract class, unless the subclass is also an abstract class. To inherit from an abstract class, use the extends keyword:

class Dog extends Animal {
    @Override
    void sound() {
        System.out.println("Woof!");
    }
}

Abstract Classes vs. Interfaces:

While both abstract classes and interfaces can contain abstract methods, there are some differences between the two:

Abstract classes can have non-abstract methods with implementation, while interfaces can only have abstract methods (Java 8 introduced default and static methods in interfaces with implementation, but they are not the same as non-abstract methods in abstract classes).
A class can inherit from only one abstract class, but it can implement multiple interfaces.
Abstract classes can have constructors, instance variables, and can maintain state, while interfaces cannot.
Abstract classes can have access modifiers (like private, protected, or public) for their methods and variables, while interface methods are implicitly public, and their fields are implicitly public, static, and final.

Here's an example to illustrate the differences:

abstract class Mammal {
    protected int age;

    public Mammal(int age) {
        this.age = age;
    }

    abstract void sound();

    public void sleep() {
        System.out.println("The mammal is sleeping.");
    }
}

interface Pet {
    void play();
}

class Cat extends Mammal implements Pet {
    public Cat(int age) {
        super(age);
    }

    @Override
    void sound() {
        System.out.println("Meow!");
    }

    @Override
    public void play() {
        System.out.println("The cat is playing.");
    }
}

In the example above, Mammal is an abstract class with an abstract method sound() and a non-abstract method sleep(). Pet is an interface with an abstract method play(). Cat is a concrete class that extends Mammal and implements Pet, providing implementations for both sound() and play() methods.

This tutorial should give you a basic understanding of abstract classes in Java. With this knowledge, you can effectively use abstract classes to create reusable and organized code in your Java programs.

Java Abstract Class Definition:
- Description: An abstract class in Java is a class that cannot be instantiated on its own and may contain abstract methods, which are declared but not implemented in the abstract class.
- Example Code:
```
// Abstract class definition
abstract class Shape {
    // Abstract method declaration
    abstract void draw();
}
```
Java Abstract Class vs. Interface:
- Description: Abstract classes and interfaces are both used for abstraction, but abstract classes can have instance variables and constructors, while interfaces cannot.
- Example Code:
```
// Abstract class example
abstract class Animal {
    abstract void makeSound();
}

// Interface example
interface Movable {
    void move();
}
```

Abstract Class in Java Example:

Description: An example of an abstract class with abstract and concrete methods.

Example Code:

abstract class Animal {
    abstract void makeSound();
    
    void sleep() {
        System.out.println("Zzz...");
    }
}

How to Declare Abstract Methods in Java:
- Description: Abstract methods are declared with the abstract keyword and have no implementation in the abstract class.
- Example Code:
```
abstract class Vehicle {
    abstract void start();
}
```

Extending an Abstract Class in Java:

Description: Subclasses extend abstract classes using the extends keyword.

Example Code:

// Concrete class extending an abstract class
class Circle extends Shape {
    void draw() {
        System.out.println("Drawing a circle");
    }
}

Implementing Abstract Methods in Java:

Description: Subclasses must provide implementations for all abstract methods of the abstract class.

Example Code:

// Concrete class implementing abstract methods
class Car extends Vehicle {
    void start() {
        System.out.println("Car starting...");
    }
}

Java Abstract Class Constructor:
- Description: Abstract classes can have constructors, and they are invoked when a subclass object is instantiated.
- Example Code:
```
abstract class Bird {
    Bird() {
        System.out.println("A bird is born");
    }
    
    abstract void fly();
}
```

Abstract Class and Inheritance in Java:

Description: Abstract classes support inheritance, allowing subclasses to inherit both abstract and concrete methods.

Example Code:

abstract class Vehicle {
    void start() {
        System.out.println("Vehicle starting...");
    }
}

class Car extends Vehicle {
    // Inherits the start method
}

Abstract Class and Polymorphism in Java:
- Description: Abstract classes can be used in polymorphism, where a reference of the abstract class type can refer to objects of its subclasses.
- Example Code:
```
// Polymorphism with abstract class
Vehicle myCar = new Car();
myCar.start(); // Calls the start method of Car
```
Abstract Class and Encapsulation in Java:
- Description: Abstract classes can encapsulate behavior and data through a combination of abstract and concrete methods.
- Example Code:
```
abstract class Shape {
    private String color;
    
    Shape(String color) {
        this.color = color;
    }
    
    abstract void draw();
}
```
Abstract Class vs. Concrete Class in Java:
- Description: Abstract classes can have a mix of abstract and concrete methods, while concrete classes have only concrete methods.
- Example Code:
```
// Abstract class with concrete and abstract methods
abstract class Shape {
    abstract void draw();
    
    void resize() {
        System.out.println("Resizing...");
    }
}
```
Abstract Class and Final Keyword in Java:
- Description: Abstract classes can be marked as final to prevent further subclassing.
- Example Code:
```
final abstract class Shape {
    abstract void draw();
}
```
Abstract Class and Access Modifiers in Java:
- Description: Abstract classes can have access modifiers on their methods and variables.
- Example Code:
```
abstract class Shape {
    private int sides;
    
    protected abstract void draw();
}
```
Abstract Class and Static Methods in Java:
- Description: Abstract classes can have static methods, but they cannot be abstract.
- Example Code:
```
abstract class Utility {
    static void doSomething() {
        System.out.println("Doing something...");
    }
}
```
Abstract Class and Instance Variables in Java:
- Description: Abstract classes can have instance variables along with abstract and concrete methods.
- Example Code:
```
abstract class Animal {
    String name;
    
    Animal(String name) {
        this.name = name;
    }
    
    abstract void makeSound();
}
```
When to Use Abstract Classes in Java:
- Description: Use abstract classes when you want to provide a common base for multiple related classes, and you have shared implementation details.
- Example Code:
```
abstract class Shape {
    abstract void draw();
    
    void resize() {
        System.out.println("Resizing...");
    }
}
```
Abstract Class Design Patterns in Java:
- Description: Abstract classes are often used in design patterns like Template Method, where the skeleton of an algorithm is defined in an abstract class.
- Example Code:
```
abstract class Algorithm {
    void execute() {
        step1();
        step2();
        step3();
    }
    
    abstract void step1();
    abstract void step2();
    abstract void step3();
}
```
Common Pitfalls with Java Abstract Classes:
- Description: Pitfalls may include improper use of abstract methods, neglecting to provide implementations in subclasses, or misunderstanding the purpose of abstract classes.
- Example Code:
```
abstract class Vehicle {
    abstract void start();
}

// Incorrect: Missing implementation for start in subclass
class InvalidCar extends Vehicle {
    // Compilation error
}
```