Polymorphism is one of the core features of Java object-oriented programming. Its core lies in "one interface, multiple implementations". It implements a unified interface to handle the behavior of different objects through inheritance, method rewriting and upward transformation. 1. Polymorphism allows the parent class to refer to subclass objects, and the corresponding methods are called according to the actual object during runtime; 2. The implementation needs to meet the three conditions of inheritance relationship, method rewriting and upward transformation; 3. It is often used to uniformly handle different subclass objects, collection storage and framework design; 4. When used, only the methods defined by the parent class can be called. New methods added to subclasses need to be transformed downward and accessed, and pay attention to type safety.

Java polymorphism is one of the core concepts in object-oriented programming. Simply put, it is "one interface, multiple implementations". It allows us to process different types of objects in a unified way, making the code more flexible and easier to expand. The key to understanding polymorphism is to understand the points of inheritance, method rewriting and upward transformation.

What is polymorphism?

Polymorphism literally means "multiple forms". In Java, it means that the same method call can show different behaviors according to different objects. For example, you have an animal class (Animal), which contains a method of barking (makeSound), and dogs (Dog) and cats (Cat) all inherit from this class and each realizes their own barking.

When you use an Animal type variable to refer to a Dog or Cat instance, calling the makeSound() method will execute the corresponding version based on the actual object. This is the manifestation of runtime polymorphism.

How is polymorphism implemented?

To achieve polymorphism, the following conditions are usually required:

• Inheritance relationship : Subclasses must inherit the parent class.
• Method override : The subclass needs to redefine the methods of the parent class.
• Upcasting : Use parent class type to refer to child class objects.

Let's give a simple example:

 class Animal {
    void makeSound() {
        System.out.println("Some sound");
    }
}

class Dog extends Animal {
    @Override
    void makeSound() {
        System.out.println("Bark");
    }
}

class Cat extends Animal {
    @Override
    void makeSound() {
        System.out.println("Meow");
    }
}

Then you can call this:

 Animal a1 = new Dog();
Animal a2 = new Cat();
a1.makeSound(); // Output Bark
a2.makeSound(); // Output Meow

As you can see, although the variable type is Animal, the method of the subclass is actually called.

What are the application scenarios of polymorphism?

Polymorphism is most common in cases where multiple subclass objects need to be handled uniformly. for example:

• Write a common method to handle different types of objects. For example, in a drawing program, you can call draw() method uniformly without caring about whether it is a circle or a rectangle.
• Use collections to store objects of different subclasses, such as List<Animal> to store Dog and Cat instances at the same time.
• In the design of a framework or library, a unified entry is provided through interfaces or abstract classes, and the specific implementation is determined by the user.

This design method improves the maintainability and scalability of the code, and new subclasses do not require modification of existing logic.

What to pay attention to in polymorphism

Although polymorphism is powerful, there are some details that are prone to errors:

• Only methods that exist in the parent class can be called, and even if there are new methods added to the subclass, they cannot be accessed directly (unless they are transformed down).
• Polymorphism is only applicable to instance methods , and static methods, private methods, and constructor methods will not participate in polymorphism.
• Polymorphism occurs at runtime (dynamic binding), while method overloading is determined at compile time (static binding).

For example, the following code will report an error:

 Animal a = new Dog();
a.bark(); // An error occurred! There is no bark method in Animal class

At this time, you need to first transform downward:

 Dog d = (Dog) a;
d.bark(); // Normal output Bark

However, you should pay attention to the security of transformation. It is best to use instanceof to judge before transformation.

Basically that's it. The core of polymorphism is to handle different implementations through a unified interface to improve code flexibility and reusability. By mastering the three contents of inheritance, rewriting and transformation, you can understand and use polymorphism well.

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