阅读原文小猫动图

Java 中的访问权限修饰符共有四种，分别是：

Default : 没有关键字修饰的
Private
Protected
Public

按照对于调用者来说可见性从小到大的次序排序是：

Private < Default < Protected < Public

在Java中访问修饰符可以作用在 class (类) ，field (属性) ，method （方法）上。

注意：作用在类上时，private ， protected 只能作用在内部类上，而不能作用在外部类上。这在语法上是不允许的会引起编译错误。

Private

用 private 修饰的部分只能在当前类中访问，它们的可见性范围就是当前类。在当前类之外无法通过硬编码的方式直接使用。这也是 Java 封装性的体现，以及安全性体现，将潜在的变化隐藏在自己内部不要对外界暴漏。

package org.blackhat.chapter2.privatemodifier;
/** * @author blackhat */public class A {
    private int num = 100;
    private void printNum() {        System.out.println("num = " + this.num);    }
    public static void main(String[] args) {        new A().printNum();    }}
class Simple {
    public static void main(String[] args) {
        A a = new A();//        a.printNum(); // Compile Time Error    }}

Default

访问修饰符为 default 的部分，允许本类中访问，允许在相同 package 下的非子类中访问，允许在相同 package 下的子类中访问。不允许在不同 package 下的类中访问，即使这个类是子类。

package org.blackhat.chapter2.defaultmodifier;/** * @author blackhat */class A {
    int num = 100;
    void printNum() {        System.out.println("num = " + this.num);    }
}

package org.blackhat.chapter2.defaultmodifier;
/** * @author blackhat */public class ASimple {
    public static void main(String[] args) {        A a = new A();        System.out.println(a.num);        a.printNum();    }}

package org.blackhat.chapter2.defaultmodifier;
/** * @author blackhat */public class ASub extends A {
    void invokeSuperPrintNum() {        super.printNum();    }
    int getNum() {       return super.num;    }}

package org.blackhat.chapter2.defaultmodifier;
/** * @author blackhat */public class ASubSimple {
    public static void main(String[] args) {        ASub aSub = new ASub();        aSub.invokeSuperPrintNum();        System.out.println(aSub.getNum());    }}

package org.blackhat.chapter2.defaultmodifier.otherpackage;

/** * @author blackhat */class ASimple {
    public static void main(String[] args) {//        new A(); // Compile Time Error    }}

package org.blackhat.chapter2.defaultmodifier.otherpackage;

/** * @author blackhat *///public class ASub extends A { // Compile Time Error//}

Protected

protected 修饰的部分，允许在本类中访问，允许在相同的 package 中访问，允许在子类中访问。不允许在不同的 package 的非子类中访问。

package org.blackhat.chapter2.protectedmodifier;
/** * @author blackhat */public class A {
    protected int num = 100;
    protected void printNum() {        System.out.println("num = " + this.num);    }
    public static void main(String[] args) {        new A().printNum();    }}

package org.blackhat.chapter2.protectedmodifier;
/** * @author blackhat */public class Simple {
    public static void main(String[] args) {        A a = new A();        a.printNum();        System.out.println(a.num);    }}

package org.blackhat.chapter2.protectedmodifier;
/** * @author blackhat */public class ASub extends A {
    void invokeSuperPrintNum() {        super.printNum();    }
    int getNum() {        return super.num;    }}

package org.blackhat.chapter2.protectedmodifier;
/** * @author blackhat */public class ASubSimple {
    public static void main(String[] args) {        ASub aSub = new ASub();        aSub.invokeSuperPrintNum();        System.out.println(aSub.getNum());    }}

package org.blackhat.chapter2.protectedmodifier.otherpackage;
import org.blackhat.chapter2.protectedmodifier.A;
/** * @author blackhat */public class OtherASub extends A {
    void invokeSuperPrintNum() {        super.printNum();    }
    int getNum() {        return super.num;    }}

package org.blackhat.chapter2.protectedmodifier.otherpackage;

import org.blackhat.chapter2.protectedmodifier.ASub;
/** * @author blackhat */public class ASubSimple {
    public static void main(String[] args) {        ASub aSub = new ASub();//        aSub.invokeSuperPrintNum(); // Compile Time Error//        System.out.println(aSub.getNum()); // Compile Time Error    }}

Public

public 也就是完全公开的，被它修饰的部分允许在任何位置访问。

内部类也同样遵守上面的规则

package org.blackhat.chapter2;
/** * @author blackhat */public class OuterSuper {
    int defaultNum = 1000;
    protected int protectedNum = 2000;}

package org.blackhat.chapter2;
/** * @author blackhat */public class Outer extends OuterSuper {
    private int a = 100;    int b = 200;    protected int c = 300;
    private class Inner1 {
        void fun() {            System.out.println("Outer.a = " + Outer.this.a                    + " , Outer.b = " + Outer.this.b                    + " ,  Outer.c = " + Outer.this.c                    + ", OuterSuper.defaultNum = " + Outer.super.defaultNum                    + ", OuterSuper.protectedNum = " + Outer.super.protectedNum);        }    }
    class Inner2 {
        void fun() {            System.out.println("Outer.a = " + Outer.this.a                    + " , Outer.b = " + Outer.this.b                    + " ,  Outer.c = " + Outer.this.c                    + ", OuterSuper.defaultNum = " + Outer.super.defaultNum                    + ", OuterSuper.protectedNum = " + Outer.super.protectedNum);        }    }
    protected class Inner3 {
        void fun() {            System.out.println("Outer.a = " + Outer.this.a                    + " , Outer.b = " + Outer.this.b                    + " ,  Outer.c = " + Outer.this.c                    + ", OuterSuper.defaultNum = " + Outer.super.defaultNum                    + ", OuterSuper.protectedNum = " + Outer.super.protectedNum);        }    }}

package org.blackhat.chapter2.otherpackage;
import org.blackhat.chapter2.OuterSuper;
/** * @author blackhat */public class OtherOuter extends OuterSuper {
    private int a = 100;    int b = 200;    protected int c = 300;
    private class Inner1 {
        void fun() {            System.out.println("Outer.a = " + OtherOuter.this.a                    + " , Outer.b = " + OtherOuter.this.b                    + " ,  Outer.c = " + OtherOuter.this.c//                    + ", OuterSuper.defaultNum = " + OtherOuter.super.defaultNum // Compile Time Error                    + ", OuterSuper.protectedNum = " + OtherOuter.super.protectedNum);        }    }
    class Inner2 {
        void fun() {            System.out.println("Outer.a = " + OtherOuter.this.a                    + " , Outer.b = " + OtherOuter.this.b                    + " ,  Outer.c = " + OtherOuter.this.c//                    + ", OuterSuper.defaultNum = " + OtherOuter.super.defaultNum // Compile Time Error                    + ", OuterSuper.protectedNum = " + OtherOuter.super.protectedNum);        }    }
    protected class Inner3 {
        void fun() {            System.out.println("Outer.a = " + OtherOuter.this.a                    + " , Outer.b = " + OtherOuter.this.b                    + " ,  Outer.c = " + OtherOuter.this.c//                    + ", OuterSuper.defaultNum = " + OtherOuter.super.defaultNum // Compile Time Error                    + ", OuterSuper.protectedNum = " + OtherOuter.super.protectedNum);        }    }}

本期内容代码整理在 JavaTalk 代码仓库：

https://github.com/GitHeP/JavaTalk

长按识别

你点的每个在看，我都认真当成了喜欢

ITPub博客

JavaTalk第二期:Java中的访问权限修饰符

最新文章