Java – How to inherit static field and change it’s value

inheritancejavastatic

I'm working on program/game where I have static utility class with params.

class ParamsGeneral {
   public static final int H_FACTOR = 100;
   public static int MAX_SCORE = 1000;
   ...
}

then I need to override this values in some specific cases, for example playing on map with limited score. So I did following:

class ParamsLimited extends ParamsGeneral {
   public static int MAX_SCORE = 500;
   // other params stay same
}

And the intended usage is following:

class Player {
   ParamsGeneral par;
   public Player() {
      if(onLimitedMap()){
          par = new ParamLimited();
      }
   }

   public boolean isWinner() {
      if(this.score == par.MAX_SCORE) {
          return true;
      }
      return false;
   }
}

I haven't actually tested this code, because IDE is complaining about calling static field through instance and also about field hiding. I clearly see that this code is stinks, so is there a way to achieve this or do I have to write each param class separately?

PS: I know I shoud make the default class abstract and use getters, I'm just curious if there is a way to make the values accesible statically.

Best Answer

You cannot override static members - in Java, neither methods nor fields could be overriden. However, in this case it does not look like you need to do any of that: since you have an instance of ParamsGeneral in the par variable, a non-static method would do what you need with the regular override.

class ParamsGeneral {
    public int getMaxScore() {
        return 1000;
    }
}
class ParamsLimited extends ParamsGeneral {
    @Override public int getMaxScore() {
        return 500;
    }
}

...

public boolean isWinner() {
    // You do not need an "if" statement, because
    // the == operator already gives you a boolean:
    return this.score == par.getMaxScore();
}

Related Solutions

Java inner class and static nested class

From the Java Tutorial:

Nested classes are divided into two categories: static and non-static. Nested classes that are declared static are simply called static nested classes. Non-static nested classes are called inner classes.

Static nested classes are accessed using the enclosing class name:

OuterClass.StaticNestedClass

For example, to create an object for the static nested class, use this syntax:

OuterClass.StaticNestedClass nestedObject = new OuterClass.StaticNestedClass();

Objects that are instances of an inner class exist within an instance of the outer class. Consider the following classes:

class OuterClass {
    ...
    class InnerClass {
        ...
    }
}

An instance of InnerClass can exist only within an instance of OuterClass and has direct access to the methods and fields of its enclosing instance.

To instantiate an inner class, you must first instantiate the outer class. Then, create the inner object within the outer object with this syntax:

OuterClass outerObject = new OuterClass()
OuterClass.InnerClass innerObject = outerObject.new InnerClass();

see: Java Tutorial - Nested Classes

For completeness note that there is also such a thing as an inner class without an enclosing instance:

class A {
  int t() { return 1; }
  static A a =  new A() { int t() { return 2; } };
}

Here, new A() { ... } is an inner class defined in a static context and does not have an enclosing instance.

Java – How to initialise a static Map

The instance initialiser is just syntactic sugar in this case, right? I don't see why you need an extra anonymous class just to initialize. And it won't work if the class being created is final.

You can create an immutable map using a static initialiser too:

public class Test {
    private static final Map<Integer, String> myMap;
    static {
        Map<Integer, String> aMap = ....;
        aMap.put(1, "one");
        aMap.put(2, "two");
        myMap = Collections.unmodifiableMap(aMap);
    }
}

Best Answer

Related Solutions

Java inner class and static nested class

Java – How to initialise a static Map

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