Question

I am trying to implement Lazy Singleton with Generics and Inheritance. I have created a abstract super-class and declared a Map which will store all the instances of the child class of this class.

Here it is:

public abstract class AbstractXMLParser<T> {
    @SuppressWarnings("rawtypes")
    private static final Map<Class<? extends AbstractXMLParser>, AbstractXMLParser> INSTANCES = new HashMap<>();

    public AbstractXMLParser() {
        throw new UnsupportedOperationException("Cannot instantiate");
    }

    private static class SingletonHolder<T> {       
        @SuppressWarnings({ "unchecked"})
        private static <T> T getInstance() throws InstantiationException, IllegalAccessException {
            Class<T> clazz = (Class<T>) ((ParameterizedType) SingletonHolder.class.getClass().getGenericSuperclass()).getActualTypeArguments()[0];
            return clazz.newInstance();
        }
    }

    @SuppressWarnings({ "unchecked", "rawtypes" })
    public static <T extends AbstractXMLParser> T getInstance() throws InstantiationException, IllegalAccessException {
        Class<T> clazz = (Class<T>) ((ParameterizedType) AbstractXMLParser.class.getClass().getGenericSuperclass()).getActualTypeArguments()[0];
        if(INSTANCES.containsKey(clazz)) {
            return (T) INSTANCES.get(clazz);
        } else {
            T instance = SingletonHolder.getInstance();
            INSTANCES.put(clazz, instance);
            return instance;
        }
    }
}

And one of the child class is:

public class ActivityTypeXMLParser extends AbstractXMLParser<ActivityTypeXMLParser>{
    private ActivityTypesXMLModel activityTypes;

    private ActivityTypeXMLParser() {

    }

    public static void main(String... strings) throws InstantiationException, IllegalAccessException {
        ActivityTypeXMLParser.getInstance();

    }
}

The main method is for testing purpose. But I am getting exception:

Exception in thread "main" java.lang.ClassCastException: java.lang.Class cannot be cast to java.lang.reflect.ParameterizedType
    at com.edfx.adb.xml.parser.AbstractXMLParser.getInstance(AbstractXMLParser.java:25)
    at com.edfx.adb.xml.parser.ActivityTypeXMLParser.main(ActivityTypeXMLParser.java:30)

Now after debugging I found that

Class<T> clazz = (Class<T>) ((ParameterizedType) AbstractXMLParser.class.getClass().getGenericSuperclass()).getActualTypeArguments()[0];

is the reason for the exception, because AbstractXMLParser.class.getClass().getGenericSuperclass() is returning class java.lang.Object. I need to get the Class<T> here. How can I extract it in the class AbstractXMLParser and in SingletonHolder of AbstractXMLParser?

Answer 1

There are multiple issues with this code:

• static methods are not inherited. AbstractXMLParser.getInstance() doesn’t know that it’s invoked in context of ActivityTypeXMLParser when called as ActivityTypeXMLParser.getInstance().

• AbstractXMLParser.class.getClass() returns Class' ofClass, notClass` of your class.

• It’s not clear why do you need AbstractXMLParser<T> to be generic at all. Note that Ts in AbstractXMLParser, SingletonHolder and getInstance() are different type parameters.

You need to pass the class in question to getInstance() implicitly:

getInstance(ActivityTypeXMLParser.class);

.

@SuppressWarnings({ "unchecked", "rawtypes" })
public static <T extends AbstractXMLParser> T getInstance(Class<T> clazz) 
    throws InstantiationException, 
    ...
}