Edit: Answered - error was method wasn't static
I'm used the Singleton Design Pattern
public class Singleton {
private static final Singleton INSTANCE = new Singleton();
// Private constructor prevents instantiation from other classes
private Singleton() {}
public static Singleton getInstance() {
return INSTANCE;
}
}
My question is how do I create an object of class Singleton in another class?
I've tried:
Singleton singleton = new Singleton();
// error - constructor is private
Singleton singleton = Singleton.getInstance();
// error - non-static method cannot be referenced from a static context
What is the correct code?
Thanks, Spencer
Singleton singleton = Singleton.getInstance();
is the correct way. Make sure your getInstance()
method is indeed static
.
Since your Singleton
implementation is far from being safe - your object can be instantiated via reflection, you may want to create a singleton based on enum
Singleton singleton = Singleton.getInstance();
should work -- that error doesn't make sense, given your code; are you sure you're reporting it correctly? (It would make sense if you had forgotten to make the getInstance
method static, which you've done in your code above.)
The code you've given us for the class is correct.
Lastly, one conceptual note: First, you aren't "creating an object of class Singleton" -- that's the whole point of a Singleton. :) You're just getting a reference to the existing object.
Since we doesn't want to allow more than one copy to be accessed. So We need to manually instantiate an object, but we need to keep a reference to the singleton so that subsequent calls to the accessor method can return the singleton (rather than creating a new one). Thats why is
Singleton singleton = Singleton.getInstance();
Correct way to access any singletonObject.
This one:
Singleton singleton = Singleton.getInstance();
should work. This is how you call static methods in Java. And the getInstance()
method is declared as static
. Are you sure you are using the very same Singleton
class? Or maybe you have imported a class called the same in some other package.
you should be using public static Singleton getInstance()
, but the implementation is not very correct.
if (instance == null) {
instance = new Singleton();
}
return instance;
This is how you should be doing it. This ensure that it creates the instance if it does not exist, or simply returns the existing instance. Your code would also do the same thing, but this add to the readability.
仍然可以创建多个类的实例,如下所示:
Singleton.getInstance().clone()
There is nothing wrong in using
Singleton singleton = Singleton.getInstance();
// error - non-static method cannot be referenced from a static context
This is the way to get the singleton object form the class. There must me something else. Please post some more details
since getInstance() method is "static" and instance field too, yo can use Singleton.getInstance(); Without creating new exeple of class. Thihs is the poit of singletone
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