Java的Modifier Types修饰符类型-笔记

Java Modifier Types修饰符类型

本文参考这里

• Access Modifiers
• Non Access Modifiers

Access Control Modifiers 访问控制

• default：Visible to the package, the default. No modifiers are needed.

• private：Visible to the class only.

  • Class 和 interfaces 不能是 private

• public：Visible to the world.

  • 接口：The fields in an interface are implicitly public static final and the methods in an interface are by default public

• protected：Visible to the package and all subclasses.

  • class 和 interfaces 不能是 protected
  • interface 中的 methods 和 fields 不能是 protected

访问控制的继承规则

规则：不降级

• Methods declared public in a superclass also must be public in all subclasses.
• Methods declared protected in a superclass must either be protected or public in subclasses; they cannot be private.
• Methods declared without access control (no modifier was used) can be declared more private in subclasses.
• Methods declared private are not inherited at all, so there is no rule for them.

Non Access Modifiers

• static

  • 变量：Local变量不能 static
  • 方法

  private static int cnt = 0;
  public static int getCount()
  {
      return cnt;
  }

• final

  • 变量：只能显式初始化一次，不能再赋值，final int var = 10;
  • 方法：不能被重写（A final method cannot be overridden by any subclasses）

  public class Test{
      public final void changeName(){
          // body of method
      }
  }

  • 类：不能被继承

  public final class Test
  {
      // body of class
  }

• abstract

  • 类：抽象类不能实例化出对象。一个类不能同时 既abstract 又 final。一个类若含有abstract methods，则类必须申明为abstract

  • 方法：没有实现体（implementation，methods body），而将在子类中进行实现；只要子类不是 abstract，就须对从父类继承来的方法进行实现。抽象方法不能 final 或 strict。

  public abstract class SuperClass
  {
      abstract void m();  //abstract method
  }
  
  class SubClass extends SuperClass
  {
      // implements the abstract method
      void m()
      {
          ...
      }
  }

• synchronized 和 volatile，用于threads

  • synchronized

  // method can be accessed by only one thread at a time
  public synchronized void showDetails()
  {
      ...
  }

  • transient

  public transient int limit = 55;  // will not persist
  public int b;  // will persist

  • volatile

  private volatile boolean active;
  
  // 线程1
  public void run()
  {
      active = true;
      while (active)
      {
          ...
      }
  }
  
  // 线程2
  public void stop()
  {
      active = false;
  }

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