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Java 访问控制符

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访问控制符指定了类之间的访问范围。

分类

  • public
    表示对外公开，所有类都可以访问。
  • default
    没有修饰符，表示同一个包内公开，也可以理解为包内私有。即同一个包内的类可以访问。
  • protected
    不管是包内还是包外的子类，以及同一个包内的类公开。即同一个包内和所有子类能访问。
  • private
    只有当前类内部可以访问。

访问范围

Modifier Class Package Subclass World
public Y Y Y Y
protected Y Y Y N
no modifier Y Y N N
private Y N N N

0047-java-accsee-control.png

示例

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// 1. BaseBean
package com.***.inner;
public class BaseBean {

    private class InnerClass{
        private String innerPrivateStr;
        String innerDefaultStr;
        protected String innerProtectedStr;
        public String innerPublicStr;

        public InnerClass(){
            innerPrivateStr = "innerPrivateStr";
            innerDefaultStr = "innerDefaultStr";
            innerProtectedStr = "innerProtectedStr";
            innerPublicStr = "innerPublicStr";
        }
    }

    private String privateStr;
    String defaultStr;
    protected String protectedStr;
    public String publicStr;

    public BaseBean(){
        privateStr = "privateStr";
        defaultStr = "defaultStr";
        protectedStr = "protectedStr";
        publicStr = "publicStr";
    }

    public void show(){
        InnerClass innerClass = new InnerClass();
        System.out.println("BaseBean:InnerClass: " + innerClass.innerPrivateStr);
        System.out.println("BaseBean:InnerClass: " + innerClass.innerDefaultStr);
        System.out.println("BaseBean:InnerClass: " + innerClass.innerProtectedStr);
        System.out.println("BaseBean:InnerClass: " + innerClass.innerPublicStr);

        System.out.println("BaseBean: " + privateStr);
        System.out.println("BaseBean: " + defaultStr);
        System.out.println("BaseBean: " + protectedStr);
        System.out.println("BaseBean: " + publicStr);
    }
}

// 2. SubBean in the same package.
package com.***.inner;
public class SubBean extends BaseBean {

    public void show(){
        System.out.println("SubBean: " + defaultStr);
        System.out.println("SubBean: " + protectedStr);
        System.out.println("SubBean: " + publicStr);
    }

}

// 3. SubBean in the other package.
package com.***.outer;
public class OuterPackageSubBean extends BaseBean{

    public void show(){
        System.out.println("OuterPackageSubBean: " + protectedStr);
        System.out.println("OuterPackageSubBean: " + publicStr);
    }

}

// 4. Test. 
package com.***.test;
public class TestAccessControl {
    BaseBean baseBean;
    SubBean subBean;
    OuterPackageSubBean outerPackageSubBean;

    public TestAccessControl(){
        baseBean = new BaseBean();
        subBean = new SubBean();
        outerPackageSubBean = new OuterPackageSubBean();
    }

    public void show(){
        System.out.println("********The Same Class.########");
        baseBean.show();
        System.out.println("********The Sub Class in the same package.########");
        subBean.show();
        System.out.println("********The Sub Class in other packages.########");
        outerPackageSubBean.show();
        System.out.println("********The World.########");
        System.out.println("TestAccessControl: baseBean " + baseBean.publicStr);
        System.out.println("TestAccessControl: subBean " + subBean.publicStr);
        System.out.println("TestAccessControl: outerPackageSubBean " 
            + outerPackageSubBean.publicStr);
    }

    public static void main(String[] args) {
        TestAccessControl testAccessControl = new TestAccessControl();
        testAccessControl.show();
    }
}

特点

  • 方法内部的局部变量是不存在访问控制符的概念的，不能使用访问控制符修饰
  • 在外部类整个类内部这个范围内，是不受访问控制符限制的，即 private 变量外部类和内部类是可以相互直接访问到的。所以内部类中通常会省略访问控制符。
  • 接口会默认将所有成员设置为 public

参考文档