Understanding Java Polymorphism with Examples

回答：

Java polymorphism is a core concept in object-oriented programming that allows methods to do different things based on the object that it is acting upon. It enables a single interface to represent different underlying forms (data types). Polymorphism can be achieved in Java through method overriding and method overloading.

Types of Polymorphism in Java

Compile-time Polymorphism (Method Overloading): This occurs when multiple methods have the same name but different parameters (different type or number of parameters) within the same class.
Runtime Polymorphism (Method Overriding): This occurs when a subclass provides a specific implementation of a method that is already defined in its superclass. The method that gets executed is determined at runtime based on the object being referred to.

Examples

1. Compile-time Polymorphism (Method Overloading)

class MathOperations {
    // Method to add two integers
    int add(int a, int b) {
        return a + b;
    }

    // Method to add three integers
    int add(int a, int b, int c) {
        return a + b + c;
    }

    // Method to add two double values
    double add(double a, double b) {
        return a + b;
    }
}

public class Main {
    public static void main(String[] args) {
        MathOperations math = new MathOperations();
        
        System.out.println("Sum of 2 and 3: " + math.add(2, 3)); // Calls the first add method
        System.out.println("Sum of 2, 3, and 4: " + math.add(2, 3, 4)); // Calls the second add method
        System.out.println("Sum of 2.5 and 3.5: " + math.add(2.5, 3.5)); // Calls the third add method
    }
}

2. Runtime Polymorphism (Method Overriding)

class Animal {
    void sound() {
        System.out.println("Animal makes a sound");
    }
}

class Dog extends Animal {
    void sound() {
        System.out.println("Dog barks");
    }
}

class Cat extends Animal {
    void sound() {
        System.out.println("Cat meows");
    }
}

public class Main {
    public static void main(String[] args) {
        Animal myAnimal; // Declare an Animal reference
        myAnimal = new Dog(); // Assign a Dog object
        myAnimal.sound(); // Calls Dog's sound method

        myAnimal = new Cat(); // Assign a Cat object
        myAnimal.sound(); // Calls Cat's sound method
    }
}

Explanation

In the first example, we demonstrate compile-time polymorphism through method overloading. The add method is defined multiple times with different parameter lists, allowing it to handle different types and numbers of inputs.
In the second example, we demonstrate runtime polymorphism through method overriding. The sound method is defined in the Animal class and overridden in the Dog and Cat subclasses. The method that gets executed is determined at runtime based on the actual object type (Dog or Cat) that the Animal reference points to.

Conclusion

Polymorphism enhances flexibility and maintainability in code, allowing developers to write more generic and reusable code. It is a fundamental principle that supports the dynamic behavior of objects in Java.