Java Modifiers: Access and Non-Access Explained with Examples

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질문:

Introduce Java Modifiers and give examples

답변:

In Java, modifiers are keywords that you can use to change the way classes, methods, and variables behave. They can be broadly categorized into two types: access modifiers and non-access modifiers.

1. Access Modifiers

Access modifiers control the visibility of classes, methods, and variables. There are four main access modifiers in Java:

  • public: The member is accessible from any other class.
  • protected: The member is accessible within its own package and by subclasses.
  • default (no modifier): The member is accessible only within its own package.
  • private: The member is accessible only within its own class.

Example of Access Modifiers:

class Example {
    public int publicVar;      // Accessible from anywhere
    protected int protectedVar; // Accessible within the package and subclasses
    int defaultVar;            // Accessible only within the package
    private int privateVar;    // Accessible only within this class

    public void display() {
        System.out.println("Public: " + publicVar);
        System.out.println("Protected: " + protectedVar);
        System.out.println("Default: " + defaultVar);
        System.out.println("Private: " + privateVar);
    }
}

2. Non-Access Modifiers

Non-access modifiers provide additional functionality to classes, methods, and variables. Some common non-access modifiers include:

  • static: Indicates that a member belongs to the class rather than instances of the class.
  • final: Indicates that a variable's value cannot be changed, a method cannot be overridden, or a class cannot be subclassed.
  • abstract: Indicates that a class cannot be instantiated or a method must be implemented by subclasses.
  • synchronized: Used in multi-threading to ensure that a method or block of code is accessed by only one thread at a time.
  • volatile: Indicates that a variable's value will be modified by different threads.

Example of Non-Access Modifiers:

class NonAccessModifiersExample {
    static int staticVar = 0; // Belongs to the class

    final int finalVar; // Cannot be changed once initialized

    // Constructor to initialize final variable
    NonAccessModifiersExample(int value) {
        this.finalVar = value;
    }

    static void staticMethod() {
        System.out.println("Static method called. Static variable: " + staticVar);
    }

    final void finalMethod() {
        System.out.println("This is a final method.");
    }
}

// Abstract class example
abstract class AbstractClass {
    abstract void abstractMethod(); // Must be implemented by subclasses
}

// Subclass implementing the abstract method
class ConcreteClass extends AbstractClass {
    void abstractMethod() {
        System.out.println("Abstract method implemented.");
    }
}

Summary

Modifiers in Java are essential for controlling access and behavior of classes, methods, and variables. Understanding how to use them effectively is crucial for writing robust and maintainable Java code.