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abstract class Animal { abstract void sound(); } class Dog extends Animal { void sound() { System.out.println("Bark"); } } public class Main { public static void main(String[] args) { Dog d = new Dog(); d.sound(); } }
| Step | Action | Evaluation | Result |
|---|---|---|---|
| 1 | Define abstract class Animal with abstract method sound() | No method body | Animal cannot be instantiated |
| 2 | Define subclass Dog extending Animal | Dog must override sound() | Dog provides sound() method with body |
| 3 | Create Dog object d | Dog is concrete, so instantiation allowed | Object d created |
| 4 | Call d.sound() | Calls Dog's overridden sound() | Prints 'Bark' |
| 5 | End of execution | No more statements | Program ends |
| Variable | Start | After 1 | After 2 | After 3 | Final |
|---|---|---|---|---|---|
| d (Dog object) | null | null | null | Dog instance created | Dog instance exists |
abstract class ClassName {
abstract returnType methodName(); // no body
}
Subclass must override abstract methods.
Cannot instantiate abstract class.
Calling overridden method runs subclass code.abstract methods in Java is correct?abstract, have no body, and specify the return type and method name.abstract after void. abstract public void display() {} incorrectly provides a method body ({}). void abstract display(); misses the access modifier and places abstract incorrectly.abstract class Animal {
abstract void sound();
}
class Dog extends Animal {
void sound() {
System.out.println("Bark");
}
}
public class Test {
public static void main(String[] args) {
Animal a = new Dog();
a.sound();
}
}Animal is abstract with an abstract method sound(). Class Dog extends Animal and provides implementation for sound() that prints "Bark".main, an Animal reference points to a Dog object. Calling sound() invokes Dog's implementation, printing "Bark".abstract class Shape {
abstract void draw();
}
class Circle extends Shape {
void draw() {
System.out.println("Drawing Circle");
}
}
class Square extends Shape {
}
public class Test {
public static void main(String[] args) {
Shape s = new Square();
s.draw();
}
}Shape has abstract method draw(). Circle correctly implements it. Square does not implement draw().Square does not implement the abstract method, it must be declared abstract or implement the method. Otherwise, compilation error occurs.startEngine() behavior. Which approach using abstract methods is best to ensure this?startEngine() behavior, so subclasses must be forced to implement this method.startEngine(). Create a concrete class Vehicle with a startEngine() method that prints a generic message, and override it in subclasses. provides a generic method that can be overridden but does not force implementation. Create an interface Vehicle with a default startEngine() method, and override it in subclasses if needed. uses an interface with a default method, which subclasses may skip overriding. Create a class Vehicle with a private startEngine() method and call it from subclasses. uses a private method, which is not accessible to subclasses.