如何读写抽象类的变量

Question

Put simply, I have an abstract class containing several variables and methods. Other classes extend this abstract class, yet when I try to read the private variable in the abstract class by calling getter methods inside the abstract class, it returns null as the value of the variable.

public class JavaApplication {

    public static void main(String[] args) {
        NewClass1 n1 = new NewClass1();
        NewClass2 n2 = new NewClass2();

        n1.setVar("hello");
        n2.print();

    }
}



public class NewClass1 {

    public String firstWord;

    public void setVar(String var) {
        firstWord = var;
    }

    public String getVar () {
        return firstWord;
    }

}



public class NewClass2 extends NewClass1{

    public void print() {
        System.out.println(makeCall());
    }

    public String makeCall() {
        return getVar();
    }

}

Still prints out null.

Answer 1

Until the String is initialized, it will be null. You should probably have a constructor in the abstract class to set it.

public abstract class Command
{
     String firstWord; // = null 

     protected Command(){}

     protected Command( String w )
     {
         firstWord = w;
     }
     //...
}

public class Open extends Command
{
     public Open()
     {
         this( "your text" );
     }

     public Open( String w )
     {
         super( w );
     }

     // ...
}

---

If you need to modify the firstWord string everytime execute() is called then it may not be necessary to use a constructor with a String parameter (I added a default constructor above). However, if you do it this way then either

• You must make sure setFirstWord() is called before getFirstWord() , or,
• Handle the case when getFirstWord() returns null . This could be by simply using a default value (maybe determined by each subclass) or something else, like failing to execute.

As I do not know all the details of your implementation I cannot tell you further information.