动态初始化Java单例的通用版本时出错（单行）

Question

Scenario: Exactly I am translating a my own Objective-C library to enable the same functionality to Android development using Java which is, a Singleton contains a Hashtable<String, V> (NSMutableDictionary in Objective-C) to facilitate programmer to inject and pop values based on key-value coding . The code is as follows...

Singleton.java

import java.util.Hashtable;

public class Singleton<V> {

    private static volatile Singleton sharedInstance = null;

    private Hashtable<String, V> sharedHashtable;

    private Singleton() {

        sharedHashtable = new Hashtable<String, V>();
    }

    public static Singleton getSharedInstance() {

        if (sharedInstance == null) {

            sharedInstance = new Singleton();
        }

        return sharedInstance;
    }

    public void inject(String key, V value) {

        sharedHashtable.put(key, value);
    }

    public V popValue(String key) {

        return sharedHashtable.get(key);
    }
}

Main.java

public class Main {

    public static void main(String[] args) {

        // Part 1

        Singleton<String> stringVersion = Singleton.getSharedInstance();

        stringVersion.inject("RAMStringKey", "Hello I am Using SO");

        String string = stringVersion.popValue("RAMStringKey");

        // Part 2

        Singleton<Integer> integerVersion = Singleton.getSharedInstance();

        integerVersion.inject("RAMIntegerKey", 5);

        Integer integer = integerVersion.popValue("RAMIntegerKey");

        System.out.println(string);
        System.out.println(integer);

        // Part 3

        String anotherString = Singleton.getSharedInstance().popValue("RAMStringKey"); // Error:(27, 70) java: incompatible types: java.lang.Object cannot be converted to java.lang.String

    }
}

Problem: I have succeeded to achieve strong typing therefore I can inject any object (String or Integer) and pop any object. But in the Part 3 I am trying to do the same on the fly (in single line) but it is giving an error

Error:(27, 70) java: incompatible types: java.lang.Object cannot be converted to java.lang.String 

I am really trying to do something like:

String anotherString = Singleton<String>.getSharedInstance().popValue("RAMKey"); 

but it is not allowed.

How can I create a singleton in a single line exactly like in part 3? Can anyone help me to achieve that? Where should I make the change?

Tip: Part 3 is the single line version of Part 1

Answer 1

The type parameter is used at compile time for type checking. The Singleton class does not know about the type parameter, instances may know about type parameters. The first call to Singleton.getSharedInstance() creates a HashMap<String,Object> in part 1. Its legal to store String or Integer values in this map. In part 3 getSharedInstance() accesses a map HashMap<String,Object> , thus popValue() returns an Object which cannot be cast to a String as the error says.

Please note there is only one static instance sharedInstance , not one per used type parameter!

And since you mentioned Android: Be careful with singletons in android: If the application is exited and restarted it is not guaranteed that the VM has been restarted for your application. This means a singleton instance from a previous run might survive and its state might or might not be useful for the new run.