Overriding methods with different access modifiers and exceptions

[Sergej Smoljanov]

package b;
import java.io.IOException;

public class B {
    protected int a;
    public void pMethWE() throws IOException{
        System.out.println("public method with exception");
    }
     protected void prMethWE() throws IOException{
        System.out.println("public method with exception");
    }
      void defMethWE() throws IOException{
        System.out.println("default method with exception");
    }
}

package a;
import b.B;
import java.io.IOException;

public class A extends B{
    
     public void pMethWE() {
        System.out.println("public method without exception");
    }
     protected void prMethWE() {
        System.out.println("public method without exception");
    }
    int defMethWE(){
//note changing in return type!
        System.out.println("default method without exception");
        return 10;
    }
    public static void main(String[] args) {
        A aA = new A();
        aA.pMethWE();
        aA.prMethWE();
        aA.defMethWE();
        
        System.out.println(aA.a);
        
        
        B bA = new A();
        bA.pMethWE();//unreported exception
        bA.prMethWE();//protected access
        bA.defMethWE();//default access
        System.out.println(bA.a);//protected acces
        
        B bB = new B();
        bB.pMethWE();//unreported exception
        bB.prMethWE();//protected acces
        bB.defMethWE();//default access
        System.out.println(bB.a);//protected acces
    }    
}

when i work with access modifier i use this code. and i have a question about polymorphism:
bA.prMethWE(); - i can`t use it but i cant change it type because it will fail overriding rules
bA.defMethWE(); - i can`t use it because it has default access modifier, but i can change it signature in derived class
i assume that if you use reference type of basic class you can`t use overridden method if in basic class it has protected and derived class that override this method place in separate package. (but it still must follow overriding rules)
and about default method from base class it will not considered like overridden if subclass place in separate package.
is my assumption right?

[Roel De Nijs]

Any overridden method MUST meet (adhere to) all the overriding rules. One important rule to remember: you can only override a method if this method is inherited (by the subclass). So the visibility/accessibility of a default method is limited to the package of the class which defines this method. So a subclass of this class, defined in another package, will NOT inherit this method, so the overriding rules don't apply (and you can change e.g. return type and/or checked exceptions in throws-clause)

A protected method can be used/accessed by a subclass in another package, but ONLY through inheritance (NOT using the dot operator on a reference variable).

[Sergej Smoljanov]

package food;
public class Apple {

    protected String a() {
        return "Proteced";
    }
}

package test;
import food.Apple;
public class Test extends Apple{
    void meth(){
         System.out.println(a());
         System.out.println(super.a());  
         Test t = new Test();
         System.out.println(t.a());
         SubClassOfTest bd = new SubClassOfTest();
         System.out.println(bd.a());
         Test b = new SubClassOfTest();
         System.out.println(b.a());
         Apple aple  = new Test();
     //    System.out.println(aple.a());//Only this line is problem 
    }
    public static void main(String[] args) {
     new Test().meth();
    }
}
class SubClassOfTest extends Test{
}

A protected method can be used/accessed by a subclass in another package, but ONLY through inheritance (NOT using the dot operator on a reference variable).

i assume that you mean NOT using the dot operator on a reference variable of base class (one class up on hierarchy) (if inside subclass)

[Roel De Nijs]

Sergej Smoljanov wrote:i assume that you mean NOT using the dot operator on a reference variable of base class (one class up on hierarchy) (if inside subclass)

The protected access modifier is the most tricky one. When a class inherits a protected method from its parent class, the method is treated as a "private" method, but a subclass of the subclass can still inherit this method too. So that means if you are in (a method of) the subclass, you can use a subclass reference variable to invoke this protected method. But if you use a superclass reference variable to invoke this protected method, you'll get a compiler error. If you are in (a method of) the subsubclass (the subclass of the subclass) you can use a subsubclass reference variable to invoke this protected method. But if you use a reference variable of any of its superclasses to invoke this protected method, you'll get a compiler error.

This example illustrates all these possible scenarios:

package food;
public class Fruit {
    protected String a() {
        return "Fruit";
    }

    void fruit() {
        System.out.println(a());
        System.out.println(this.a());
        // System.out.println(super.a()); // no Object.a() method
        Fruit fruit = new Fruit();
        System.out.println(fruit.a());
    }
}

package food.fruit;
import food.Fruit;
public class Apple extends Fruit {
    void apple() {
        System.out.println(a());
        System.out.println(this.a());
        System.out.println(super.a());
        Fruit fruit = new Fruit();
        // System.out.println(fruit.a()); // Fruit.a() not visible
        Apple apple = new Apple();
        System.out.println(apple.a());
    }
}

package test;
import food.Fruit;
import food.fruit.Apple;
class DeliciousApple extends Apple {
    void deliciousApple() {
        System.out.println(a());
        System.out.println(this.a());
        System.out.println(super.a());
        Fruit fruit = new Fruit();
        // System.out.println(fruit.a()); // Fruit.a() not visible
        Apple apple = new Apple();
        // System.out.println(apple.a()); // Fruit.a() not visible
        DeliciousApple deliciousApple = new DeliciousApple();
        System.out.println(deliciousApple.a());
    }
}

class SuperDeliciousApple extends DeliciousApple {
    void superDeliciousApple() {
        System.out.println(a());
        System.out.println(this.a());
        System.out.println(super.a());
        Fruit fruit = new Fruit();
        // System.out.println(fruit.a()); // Fruit.a() not visible
        Apple apple = new Apple();
        // System.out.println(apple.a()); // Fruit.a() not visible
        DeliciousApple deliciousApple = new DeliciousApple();
        // System.out.println(deliciousApple.a()); // Fruit.a() not visible
        SuperDeliciousApple superDeliciousApple = new SuperDeliciousApple();
        System.out.println(superDeliciousApple.a());
    }
}

Hope it helps!