Q.Define inheritance. Write a Java program in which a class Student inherits from a class Person, reusing a field and a method of Person.

Inheritance lets one class reuse another's members instead of rewriting them, using the extends keyword.

class Person { String name; void introduce(){ System.out.println("I am "+name); } }
class Student extends Person { int rollNo; }

Definition. Inheritance is the mechanism by which one class — the child (subclass) — acquires the fields and methods of another class — the parent (superclass). The child gets the parent's members automatically and may add its own. It models an 'is-a' relationship: a Student is a Person. The keyword is extends.

Program:

class Person {                       // parent / superclass
    String name;
    void introduce() {
        System.out.println("I am " + name);
    }
}

class Student extends Person {       // child inherits from Person
    int rollNo;                      // Student's own field
    void showRoll() {
        System.out.println(name + " has roll " + rollNo);  // 'name' reused
    }
}

public class Demo {
    public static void main(String[] args) {
        Student s = new Student();
        s.name = "Amit";             // field inherited from Person
        s.rollNo = 21;               // field of Student
        s.introduce();               // I am Amit  (method inherited)
        s.showRoll();                // Amit has roll 21
    }
}

What is reused. Student never declares name or introduce(), yet the object s uses both — that is inheritance at work. Student also adds rollNo and showRoll() for its own needs, showing that a child can extend the parent.

Why it is useful. Common code (here, name and introduce) is written once in Person and shared by every class that extends it, avoiding duplication and keeping related classes organised.

Writing implements (used for interfaces) instead of extends for class inheritance. Trying to access a private field of the parent from the child — a child inherits protected/public/default members, but not private ones directly.