Differences Between Final and Abstract Methods in Java

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質問:

in Java, Difference between the final method and the abstract method

回答:

In Java, final methods and abstract methods serve different purposes and have distinct characteristics. Here’s a breakdown of the differences between them:

Final Method

  1. Definition: A final method is a method that cannot be overridden by subclasses. Once a method is declared as final, its implementation is fixed and cannot be changed in any subclass.

  2. Usage: The final keyword is used to prevent method overriding. This is useful when you want to ensure that the behavior of a method remains consistent across all subclasses.

  3. Implementation: A final method must have a complete implementation. It cannot be abstract; it must provide a body.

  4. Example:

    class Parent {
        final void display() {
            System.out.println("This is a final method.");
        }
    }
    
    class Child extends Parent {
        // This will cause a compile-time error
        // void display() {
        //     System.out.println("Trying to override final method.");
        // }
    }
    

Abstract Method

  1. Definition: An abstract method is a method that is declared without an implementation (i.e., it does not have a body). It is meant to be overridden in subclasses.

  2. Usage: The abstract keyword is used to define a method that must be implemented by any non-abstract subclass. This is useful for defining a contract that subclasses must adhere to.

  3. Implementation: An abstract method cannot have a body and must be declared in an abstract class. Any class that contains an abstract method must also be declared as abstract.

  4. Example:

    abstract class Animal {
        abstract void makeSound(); // Abstract method
    }
    
    class Dog extends Animal {
        void makeSound() {
            System.out.println("Bark");
        }
    }
    

Summary of Differences

FeatureFinal MethodAbstract Method
DefinitionCannot be overriddenMust be overridden
ImplementationMust have a bodyCannot have a body
Class RequirementCan be in a regular classMust be in an abstract class
PurposeTo prevent modification of behaviorTo define a contract for subclasses

In summary, use final methods when you want to lock down the implementation, and use abstract methods when you want to enforce a certain behavior that subclasses must implement.