意外的NullPointerException

Question

NullReferenceException如何在这里发生：

public class DataContextBinder extends Binder {
    private static final String DATA_CONTEXT = "DataContext";

    // Fields
    private final Listener<PropertyChangedEventArgs<Object>> dataContextListener = new Listener<PropertyChangedEventArgs<Object>>() {

        @Override
        public void onEvent(PropertyChangedEventArgs<Object> args) {
            setSource(args.getNewValue());
        }
    };

    // Constructors
    protected DataContextBinder(Object target, String targetPropertyName,
            Binding binding) {
        super(Property.getPropertyValue(target, DATA_CONTEXT), target,
            targetPropertyName, binding);
    }

    // Methods
    protected void createListeners(Object source) {
        Log.e("DataContextBinder", "DCL = " + dataContextListener.toString());
        // It throws NullPointerException on dataContextListener.toString()

        Property.addPropertyChangedListener(getTarget(), DATA_CONTEXT,
                dataContextListener);
        super.createListeners(source);
    }

    protected void removeListeners(Object source) {
        Property.removePropertyChangedListener(getTarget(), DATA_CONTEXT,
                dataContextListener);
        super.removeListeners(source);
    }
}

Answer 1

dataContextListener是null ..