Java Access Control

tags: Java, Access Control
source: DT-On Java 8

Determine who can use the definitions that follow.

When comes to determine whether elements accessible, see whether the current scope meets the access requirement.

Default

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Also called package-private access.

Don’t need to import package-private classes in the same package.

Package-private members (fields and methods) in any cases, inherited or inside public classes, remains package-private.

If Java Class is package-private, it’s invisible outside of the package.

Files in the same directory are implicitly part of the “default package” for that directory.

Public

The element is available to everyone

Private

No one can access that element except the creator.

Anyone trying to access this kind of elements illegally gets compile time error.

Protected

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Like private, but inheriting classes (same package or not) may access protected members
Also gives package access - other classes in the same package can access protected elements
It’s best to define fields private to always preserve right to change the underlying implementation

package package1;

public class Class1 {
    public void tryMePublic() {
    }

    protected void tryMeProtected() {
    }

    public static void main(String[] args) {
        Class1 c = new Class1();
        c.tryMeProtected(); // no error
    }
}


package package2;

import package1.Class1;

public class Class2 extends Class1 {
    doNow() {
        Class1 c = new Class1();
        c.tryMeProtected(); // ERROR: tryMeProtected() has protected access in Class1
        tryMeProtected();  // No error
    }
}

Java Class Access Control

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#Java Class

Java Class can only be Default or Public

Private would make the class inaccessible to anyone, which make the Class meaningless
Java Inner Class is an exception

Public members in a package access class

for usage outside of the package e.g., Use package private class instances outside of the package

package pkg1;

public class A {
    public void publicPrint() {System.out.println("Public print from A");}
}

class AA extends A {
    @Override
    public void publicPrint() { System.out.println("Public print from AA"); }
}

public class B {
    public A makeAA() {return new AA();}
    // need upcasting here
    // otherwise if return AA, it cannot use outside of the package
    // e.g., in class C:
    // new B().makeAA().publicPrint() will cause error
}

package pkg2;

public class C {
    public static void main(String[] args){
        B b = new B();
        A aa = b.makeAA();
        aa.publicPrint();
    }
}

/* Output
Public print from AA
*/

for usage in Derived Class outside of the package

package pkg1;

class Base {
    void packagePrint() { System.out.println("Package print from Base"); }
    public void publicPrint() { System.out.println("Public print from Base"); }
}

public class A extends Base{ }

package pkg2;

import pkg1.A;

public class C extends A {
    public static void main(String[] args){
        C c = new C();
        c.publicPrint();
    }
}

/* Output
Public print from Base
*/

Gyh's Braindump