Q: Explain Polymorphism. What is the difference between Method Overloading and Overriding?

Answer:

Polymorphism = "many forms." It allows a single interface to represent different underlying types.

Compile-Time Polymorphism (Method Overloading)

Same method name, different parameter lists in the same class. Resolved at compile time by the compiler based on the method signature.

public class Calculator {
    public int add(int a, int b) { return a + b; }
    public double add(double a, double b) { return a + b; }
    public int add(int a, int b, int c) { return a + b + c; }
}

Rules:

• Must differ in parameter count, type, or order.
• Return type alone is NOT sufficient to overload.
• Access modifiers can differ.

Runtime Polymorphism (Method Overriding)

Subclass provides a specific implementation of a method already defined in its parent class. Resolved at runtime via dynamic dispatch based on the actual object type.

public class Shape {
    public double area() { return 0; }
}

public class Circle extends Shape {
    private double radius;

    @Override
    public double area() { return Math.PI * radius * radius; }
}

public class Rectangle extends Shape {
    private double width, height;

    @Override
    public double area() { return width * height; }
}

// Runtime polymorphism in action:
Shape shape = new Circle(5);   // Reference type: Shape, Object type: Circle
shape.area();                   // Calls Circle.area() — resolved at RUNTIME

Rules:

• Same method signature (name + parameters).
• Return type must be the same or a covariant (subclass) return type.
• Access modifier must be the same or less restrictive.
• Cannot override static, final, or private methods.
• Must use @Override annotation (not required but strongly recommended).

Overloading vs Overriding

[!IMPORTANT] The most common interview trick question: "Can you override a static method?" No. Static methods belong to the class, not the instance. You can hide a static method (by defining one with the same signature in a subclass), but this is method hiding, not overriding — there's no dynamic dispatch.