Detailed explanation of how to use the private keyword in java

private keyword in Chinese is a private keyword, so how to use it?

1. It can only be accessed in the same category

class A {
    private String msg="Try to access the private variable outside the class"; // 用 private 修饰，无法别的类调用，只能在这个类中被调用
}
 
public class PrivateExample {
    public static void main(String[] args) {
        A a=new A();
        System.out.println(a.msg);
    }
}

Recommended tutorial: java quick start

运行结果：（报错）
    Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
	The field A.msg is not visible

2. Cannot be assigned to external classes and interfaces

private class PrivateExample {  // private 是不能用来修饰类的
    void display(){
        System.out.println("Try to access outer private class");
    }
    public static void main(String[] args) {
        PrivateExample3 p=new PrivateExample();
        p.display();
    }
}

运行结果：（报错）
    Exception in thread "main" java.lang.Error: Unresolved compilation problem:

3. Create a fully encapsulated class

The best use of the private keyword is by making the class All data members are made private to create a fully encapsulated class in Java.

import java.lang.reflect.Method;
 
class A {
    private void display() {
        System.out.println("private method is invoked");
    }
}      
		   
public class PrivateExample {
    public static void main(String[] args)throws Exception {
 
        Class c = Class.forName("A");  // 加载 A 类
 
        Object o= c.newInstance();  // 实例化对象
 
        Method m =c.getDeclaredMethod("display", null);  // getDeclaredMethod方法返回指定方法，"display" 就是指定方法，null 表示该方法的参数
 
        m.setAccessible(true);  // setAccessible 方法能在运行时压制 Java 语言访问控制检查，从而能任意调用被私有化保护的方法 ( Method )，域 ( Field )、构造方法
        
        m.invoke(o, null);  //  invoke方法来执行对象的某个方法，括号中的 o 是方法，null 是参数
    }    
}

运行结果：
	private method is invoked

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