The abstract keyword in Java is utilized to designate a class or a method as abstract. When applied to a class, it implies that the class itself or some of its methods do not possess a concrete implementation and need to be implemented by its subclasses. In the case of a method, it signifies that the method lacks a concrete body and must conclude with a semicolon. To illustrate, consider the following abstract class, Car:

public abstract class Car {
    public abstract void drive();
}

Here, the method drive() is declared as abstract.

1. Guidelines for Abstract Classes and Abstract Methods:

Abstract Class:

• An abstract class cannot be instantiated using the new keyword. Its primary purpose is to be inherited by derived classes.
• It can incorporate both abstract and non-abstract (concrete) methods.

Abstract Method:

• Lacking a body, an abstract method must end with a semicolon.
• Concrete subclasses are obligated to implement abstract methods.
• The enclosing class must also be declared as abstract if it contains one or more abstract methods.

2. Example of Java Abstract Class and Method:

Consider a class, SuperCar, which extends the abstract class Car and implements its abstract method:

class SuperCar extends Car {
    public void drive() {
        // Implementing drive for a super car, driving faster than a normal car.
    }
}

Here, SuperCar provides a concrete implementation for the abstract drive() method.

Another example involves an abstract class, Airplane, featuring both abstract and non-abstract methods:

public abstract class Airplane {
    public abstract void takeOff();

    public void landing() {
        // Implementing a smooth landing.
    }
}

In this case, Airplane contains an abstract method takeOff() and a non-abstract method landing() with a concrete implementation for a smooth landing.