Java generics, singletones and static methods

Question

So, I have several "Manager" classes, for example GroupManager. All these managers are singlets.

Using this method for instancing:

private static GroupManager groupManager = null; private GroupManager() { } public static GroupManager Instance() { if (groupManager == null) { groupManager = new GroupManager(); } return groupManager; } 

I think I should start using some inheritance, since they have many copied methods.

Instance () methods are the same for each Manager.

So, for inheritance, I can do this (obviously):

 GroupManager extends Manager 

Is it possible to use generics to use the same Instance method for all managers, for example:

 public class Manager<E> { private static E instance = null; public static E Instance() { if (instance == null) { instance = new E(); } return instance; } } 

I think that makes sense :)

So, you would do GroupManager.Instance () as usual.

Answer 1

You do not understand how generics and statics work. If you have a static field or method (such as an “instance” or an instance ()) that can be called without creating an instance of the class manager, how do you expect the JVM (and even the compiler) to know what type E is supposed to be?

Here is an example, as suggested by G_H:

GeneralManager and AreaManager expand the capabilities of the Manager

The Manager class is the only one that has a static getInstance () method:

  public class Manager { private static Map<Class<? extends Manager>,Manager> INSTANCES_MAP = new java.util.HashMap<Class<? extends Manager>, Manager>(); //Also, you will want to make this method synchronized if your application is multithreaded, //otherwise you mihgt have a race condition in which multiple threads will trick it into //creating multiple instances public static <E extends Manager> E getInstance(Class<E> instanceClass) throws InstantiationException, IllegalAccessException { if(INSTANCES_MAP.containsKey(instanceClass)) { return (E) INSTANCES_MAP.get(instanceClass); } else { E instance = instanceClass.newInstance(); INSTANCES_MAP.put(instanceClass, instance); return instance; } } } 

Answer 2

No, that will not work. Java uses compile-time generalizations to check types, but does not generate additional classes or store information about type parameters at run time.

When you declare Manager<E> with a parameter of this type E , this is what will only play a role in the actual instance. You may have a subclass of type GroupManager extends Manager<String> or something else, but this will not magically produce many static methods.

Static methods and members belong to a class, not an instance. Therefore, attempts to use generics there, which are designed to enter instances, are not going to fly.

Answer 3

If you create the group manager class as follows, you can call your instance method.

 public class GroupManager extends Manager<GroupManager>{} 

And in your manager class try this ...

 public class Manager<E> { private static E instance = null; public static E Instance() { try { return instance.newInstance(); } catch (InstantiationException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (IllegalAccessException e) { // TODO Auto-generated catch block e.printStackTrace(); } return null; } 

Or, if you know an object that needs an instance, just create a generic method

 public static <T> T getInstance(Class<T> t){ try { return t.newInstance(); } catch (InstantiationException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (IllegalAccessException e) { // TODO Auto-generated catch block e.printStackTrace(); } return null; } 

I have not tried any of this, so I'm not sure if this will work.

Answer 4

Entering a constructor in a general context. Cash is not thread safe, but is used only in a static context, so its fine if you do not miss its use

 public class Example { public static class MySingletonClass { } public interface Provider<T> { T get(); } static final Provider<MySingletonClass> myClassInstanceProvider = new Cash<MySingletonClass>(new Provider<MySingletonClass>() { @Override public MySingletonClass get() { return new MySingletonClass(); } }); public static class Cash<T> implements Provider<T> { private Provider<T> provider; public Cash(Provider<T> provider) { this.provider = provider; } @Override public T get() { final T t = provider.get(); provider = new Provider<T>() { @Override public T get() { return t; } }; return t; } } } 

Answer 5

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 public class Manager<E>{ private static Object instance = null; public static E Instance() { if (instance == null) { instance = new E(); } return (E)instance; } }