继承的子类中的Java公共/受保护的变量值为Null

Question

I want to access a variable declared protected in Superclass, from subclass.

The values are displayed null in subclass, though it displays correctly in superclass. If the superclass variable is static, it works, but the loop values are not taken correctly.

My code:

public class App {
    public String var ;

    public static void main(String[] args) {    
        App ap = new App();
        ap.fromt();
    }

    public void fromt() {
        Sub s = new Sub();
        for(int i = 0; i <= 5; i++) {
            var = "Test" + i;
            System.out.println("Hello World!" + var);
            s.testtt();
        }
    }
}

//**Inherited Class**//
public class Sub extends App {
    public void testtt() {
        String var2 = super.var;
        System.out.println("Hello World! Subclass" + var2);
    }
}

Output :

Hello World!Test0
Hello World! Subclassnull
Hello World!Test1
Hello World! Subclassnull
Hello World!Test2
Hello World! Subclassnull
Hello World!Test3
Hello World! Subclassnull
Hello World!Test4
Hello World! Subclassnull
Hello World!Test5
Hello World! Subclassnull

Answer 1

The classes Sub and App do get their own the copies of the member variable var defined in the superclass. They are 2 different copies. Any changes made to one of the copies will remain mutually exclusive to the other. The things change when static members are used. Sharing of member data in inheritance should not be confused while the classes are instantiated.

To answer your doubt, String var is not initialised anywhere so assumes a null value by default. If it is initialised while declaring or in a constructor of the super class App , the value will be reflected in the instance of Sub as well.

Answer 2

You're dealing with a matter of scope here. You have the instance of App that is running, and within that instance you are running your loop and setting the value of your variable within this specific instance. Then you are creating an instance of Sub that is currently maintaining its own instance of var within itself.

Now, if you want a singular instance of var shared across all instances of App/Sub then you would make it:

public static String var ;

I am assuming that this is for a homework assignment so I won't go into the pros and cons of this, but just know that static vars are one of those things you really have to think about for any sort of app.