I have three classes-A,B,C in java. Conditions-

1. A can access members of B and C
2. B can access members of C
3. C cannot access members of A and B
4. B cannot access members of A

If I declare members of B,C as protected and put all the classes in a package, I won't able to enforce the 3rd and 4th condition. What should be the approach here?

CodePudding user response：

You should create proper interfaces for each usecase and instead of using classes directly accept interfaces

For example it could look like

interface AccessibleA {
    int getA();
    void setA(int a);
    // other methods declarations - non accessors
}

interface InaccessibleA {
    // other methods declarations - non accessors
}

class A implements AccessibleA, InaccessibleA {
    private int a;

    // getter and setter

    public A(AccessibleB b, AccessibleC c) {
        // you can assign these values or use them
    }

    // other methods
}

// the same interfaces for B and C

class B implements AccessibleB, InaccessibleB {
    private int b;

    // getter and setter

    public A(InaccessibleA a, AccessibleC c) {
        // you can assign these values or use them
    }

    // other methods
}

// etc

Unfortunately in Java there is not "friend function" implementation (there is something like this in C ) however there are some ways to mock this behaviour. For example read:

• Is there a way to simulate the C 'friend' concept in Java?

Just I'm not sure does it worth in your case

CodePudding user response：

Like this

package c;
class C { 
    protected String foo;
}

package b;
class B extends C { 
    protected String bar;
}

package a;
class A extends B { 
    protected String baz;
}

protected fields are visible to subclasses in a different package, default (package-private) fields are only visible to subclasses in the same package.

See the documentation

CodePudding user response：

You can use inheritence here. If C is the parent most class, B can inherit C, and A can inherit B. So in this scenerio, A can access his parent B and parent's parent C, B can access his parent C, While C cannot access A or B. and B cannot access A as B is the parent class of A.