Java内部类和HashMaps

Question

So I am working on a Java project, and one of my classes uses inner nested classes (the TypesOfQuestions) and when I try to add a question from the inner class to the HashMap, it doesn't let me, and I'm not sure why.

public abstract class TypesOfQuestions {

Map<String, Double> scores = new TreeMap<String, Double>();
String text;
double points;

public TypesOfQuestions(String text, double points) {

}


public static class TrueFalse extends TypesOfQuestions {
    public TrueFalse(String text, double points, boolean answer) {
        super(text, points);
    }
}    

And the other class is Exam, which

public class Exam {
private String x;
private Map<Integer, Questions> q;

public HoldExam(String x) {
    q = new HashMap<Integer, Questions>();
    this.x = x;
}

public void addTrueFalseQuestion(int questionNumber, String text, 
                                 double points, boolean answer){
    q.put(questionNumber, new TrueFalse (text, points, answer));

}
}

I've tried a bunch of different things, but I keep getting errors, for this one I got

No enclosing instance of type TypesOfQuestions is accessible. 
Must qualify the allocation with an enclosing instance of type TypesOfQuestions 
(e.g. x.new A() where x is an instance of TypesOfQuestions).

Anndddd I have no idea what to do!!

Answer 1

TrueFalse should probably be static , as it's not associated with an outer instance of TypesOfQuestions ; it is a TypesOfQuestions .

Answer 2

Change your nested class to be static, so that it doesn't require an implicit reference to the outer class.

    public static class TrueFalse extends TypesOfQuestions { ... }

Inner classes without the static qualifier are allowed to access the members of the outer class. This requires that they hold an implicit reference to the outer class.