类实例化成对象之后，可以通过对象加上"."操作符访问和操纵该对象的域和方法，但是这种访问是有限制的，通过public、protected、default（啥都不写）、private来控制。

 

先看一个实验的例子：（不注释表示可以访问，注释掉表示无法访问）

 

 

 

package packageA;

import packageB.SubB;

public class Base {
    public String publicStr = "publicString";
    protected String protectedStr = "protectedString";
    String defaultStr = "defaultString";
    private String privateStr = "privateString";

    public void print() {
        System.out.println("packageA.Base has access to");
        System.out.println("    " + publicStr);
        System.out.println("    " + protectedStr);
        System.out.println("    " + defaultStr);
        System.out.println("    " + privateStr);

        Base b = new Base(); // -- other Base instance
        System.out.println("    b." + b.publicStr);
        System.out.println("    b." + b.protectedStr);
        System.out.println("    b." + b.defaultStr);
        System.out.println("    b." + b.privateStr);
        
        
        SubA subA=new SubA();
        System.out.println("    subA." + subA.publicStr);
        System.out.println("    subA." + subA.protectedStr);
        System.out.println("    subA." + subA.defaultStr);
     //   System.out.println("    subA." + subA.privateStr);
        
        SubB subB=new SubB();
        System.out.println("    subB." + subB.publicStr);
        System.out.println("    subB." + subB.protectedStr);
     //   System.out.println("    subB." + subB.defaultStr);
     //   System.out.println("    subB." + subB.privateStr);
     
    }
}

 

 

 

package packageA;

import packageB.SubB;

public class SubA extends Base {

    public void print() {
        System.out.println("packageA.SubA has access to");
        System.out.println("    " + publicStr + " (inherited from Base)");
        System.out.println("    " + protectedStr + " (inherited from Base)");
        System.out.println("    " + defaultStr + " (inherited from Base)");
        // -- not accessible - private elements are even not inherited
        // System.out.println(privateStr);

        Base b = new Base(); // -- other Base instance
        System.out.println("    b." + b.publicStr);
        System.out.println("    b." + b.protectedStr);
        System.out.println("    b." + b.defaultStr);
        // -- not accessible
        // System.out.println(b.privateStr);
        
        
        SubA subA=new SubA();
        System.out.println("    subA." + subA.publicStr);
        System.out.println("    subA." + subA.protectedStr);
        System.out.println("    subA." + subA.defaultStr);

     //   System.out.println("    subA." + subA.privateStr);
        
        SubB subB=new SubB();
        System.out.println("    subB." + subB.publicStr);
        System.out.println("    subB." + subB.protectedStr);
     //   System.out.println("    subB." + subB.defaultStr);
     //   System.out.println("    subB." + subB.privateStr);
        
        
    }
}

 

 

 

package packageA;
import packageB.SubB;

public class AnotherA {
    public void print() {
        System.out.println("packageA.AnotherA has access to");
        Base b = new Base();
        System.out.println("    b." + b.publicStr);
        System.out.println("    b." + b.protectedStr);
        System.out.println("    b." + b.defaultStr);
        // System.out.println(b.privateStr);
        
        
        SubA subA=new SubA();
        System.out.println("    subA." + subA.publicStr);
        System.out.println("    subA." + subA.protectedStr);
        System.out.println("    subA." + subA.defaultStr);
     //   System.out.println("    subA." + subA.privateStr);
        
        SubB subB=new SubB();
        System.out.println("    subB." + subB.publicStr);
        System.out.println("    subB." + subB.protectedStr);
     //   System.out.println("    subB." + subB.defaultStr);
     //   System.out.println("    subB." + subB.privateStr);
        
        
    }
}

 

 

 

package packageB;
import packageA.Base;
import packageA.SubA;

public class SubB extends Base {
    public void print() {
        System.out.println("packageB.SubB has access to");
        System.out.println("    " + publicStr + " (inherited from Base)");
        // -- protectedStr is inherited element -> accessible
        System.out.println("    " + protectedStr + " (inherited from Base)");
        // -- not accessible
        // System.out.println(defaultStr);
        // System.out.println(privateStr);

        Base b = new Base(); // -- other Base instance
        System.out.println("    b." + b.publicStr);
        // -- protected element, which belongs to other object -> not accessible
        // System.out.println(b.protectedStr);

        // -- not accessible
        // System.out.println(b.defaultStr);
        // System.out.println(b.privateStr);
        
        
        SubA subA=new SubA();
        System.out.println("    subA." + subA.publicStr);
     //   System.out.println("    subA." + subA.protectedStr);
     //   System.out.println("    subA." + subA.defaultStr);
     //   System.out.println("    subA." + subA.privateStr);
        
        SubB subB=new SubB();
        System.out.println("    subB." + subB.publicStr);
        System.out.println("    subB." + subB.protectedStr);
     //   System.out.println("    subB." + subB.defaultStr);
     //   System.out.println("    subB." + subB.privateStr);
        
        
    }
}

 

 

package packageB;
import packageA.Base;
import packageA.SubA;

public class AnotherB{
    public void print() {
        System.out.println("packageB.AnotherB has access to");
        Base b = new Base();
        System.out.println("    b." + b.publicStr);
        // -- not accessible
        // System.out.println(b.protectedStr);
        // System.out.println(b.defaultStr);
        // System.out.println(b.privateStr);
        
        
        
        SubA subA=new SubA();
        System.out.println("    subA." + subA.publicStr);
     //   System.out.println("    subA." + subA.protectedStr);
     //   System.out.println("    subA." + subA.defaultStr);
     //   System.out.println("    subA." + subA.privateStr);
        
        SubB subB=new SubB();
        System.out.println("    subB." + subB.publicStr);
     //   System.out.println("    subB." + subB.protectedStr);
     //   System.out.println("    subB." + subB.defaultStr);
     //   System.out.println("    subB." + subB.privateStr);
        
        
    }
}

 

 

总结：

 

1).不考虑继承的情况下：

一个类在 ”某处“实例化并访问域和方法，或者直接访问类域和类方法：

a). 在类定义出：四种类型都可访问。（见Base中的b对象）

b). 与类定义在同一包中：default、protected、public可访问。（见AnotherA中b对象）

c). 与类定义在不同包中：仅可访问public类型。（见AnotherB中b对象）

 

 

2).有继承关系

一个子类在 ”某处“实例化并访问父类继承的域和方法（子类新定义的域和方法参加上面不考虑继承的情况），或者直接访问父类类域和类方法：

a).定义在包内的子类（如上例的SubA）

A).在子类定义处、同包内：default、protected、public可访问。

B).在包外：只可访问public属性

 

 

b).定义在包外的子类（如上例的SubB）

A).在子类定义处、父类同包内：protected及public可访问。

B).在子类包内（非子类定义处）：只可访问public属性