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Create Singleton class and restrict to two objects in java?(Like double )

I have created one Singleton with private constructor and public static method. Now my requirement is, need to create two objects for same class.

First of all, if you are gonna retain more than one instance of the class then you are actually not using Singleton pattern (as pointed out by ANS).
Anyway your question sounds like you need multiple yet limited number of instances of a class. In that case, Object Pool pattern can be useful for you. Please find the wiki link below for your reference (sample java code snippet also found in the same wiki page):
https://en.wikipedia.org/wiki/Object_pool_pattern

If you want to create two instances of a class, then what's the point of creating a singleton class?
Singleton pattern restricts the instantiation of a class and ensures that only one instance of the class exists in the java virtual machine.
If you want only two objects to be created, have a static counter to check the number of objects created.. But your class will cease to be singleton.

Is there a point to having a class with all non-static methods but no non-static fields? (or all static fields and methods along with a constructor)

I am looking at other peoples' code.
I see a class with no non-static fields but in which most of the methods are non-static, requiring you to make an object to access methods that effectively operate statically.
Is there a possible reason for this, that I am just not understanding?
EDIT
Someone asked for examples. Here is some more info.
For instance there is a file manager class. The only fields are static and are Comparators. There are some methods to do things like sort files in a list, count files, copy files, move files to an archive folder, delete files older than a certain time, or create files (basically take a base name as string, and return a File with given base name and date/time tacked on the end.)
9 non-static methods
5 static methods
I don't see a particular rhyme reason for the ones that are static vs non.
One particularly odd thing is that there are two methods for removing files. One that removes a file no matter what, and one that only removes it if it is empty. The former is a static method while the latter is not. They contain the same exact code except the later first checks if the file.length is 0.
Another odd one is a class that does encryption - all fields and methods are static but it has a constructor that does nothing. And an init() method that checks if a static variable contains an object of itself and if not instantiates an object of itself into that field that is then never actually used. (It seems this is done with a lot of classes - init methods that check for an object of itself in a static variable and if not instantiate itself)
private static File keyfile;
private static String KEYFILE = "enc.key";
private static Scrambler sc;
It has methods to encrypt and decrypt and some methods for dealing with key and file.
Does this make sense to anyone? Am I just not understanding the purpose for this stuff? Or does it seem weird?

Objects don't have to have state. It's a legitimate use case to create an instance of a class with only behaviour.
Why bother to create an instance ? So you can create one and pass it around e.g. imagine some form of calculator which adheres to a particular interface but each instance performs a calculation differently. Different implements of the interface would perform calculations differently.
I quite often create classes with non-static methods and no members. It allows me to encapsulate behaviour, and I can often add members later as the implementation may demand in the future (including non-functionality related stuff such as instrumentation) I don't normally make these methods static since that restricts my future flexibility.

You can certainly do it that way. You should look carefully at what the instance methods are doing. It's perfectly okay if they're all operating only on parameters passed in and static final static class constants.
If that's the case, it's possible to make all those methods static. That's just a choice. I don't know how the original developers would justify either one. Maybe you should ask them.

Let me rephrase this question a bit,
Even though methods are non-static why would one declare fields as static?
I have taken below quoting from Java Docs,
Sometimes, you want to have variables that are common to all objects. This is
accomplished with the static modifier. Fields that have the static modifier in their declaration are called static fields or class variables. They are associated with the class, rather than with any object. Every instance of the class shares a class variable, which is in one fixed location in memory. Any object can change the value of a class variable, but class variables can also be manipulated without creating an instance of the class.
For example, suppose you want to create a number of Bicycle objects and assign each a serial number, beginning with 1 for the first object. This ID number is unique to each object and is therefore an instance variable. At the same time, you need a field to keep track of how many Bicycle objects have been created so that you know what ID to assign to the next one. Such a field is not related to any individual object, but to the class as a whole.
For Bicycle example, kindly refer the Java Docs.

Making all methods non-static allows you to override them. This makes it a lot easier to use this class in testing, because instead of the actual implementation you can use a mock that behaves as you want it for the tests. Static methods are, in my book, a code smell and should be avoided unless there's a good reason (e.g. quite trivial utility methods).
Also, at some point in the future you might want to change the behaviour of the methods in some situation, e.g. in the form of a strategy.
In the case of your encryption class, you might want to hand your class an instance of the encryption class to handle encrypting/decrypting, but be able to configure the details in some other place. That would allow you to change the algorithm and much more easily test your own code without also having to test the encryption.

Why is it better to use class name instead of objects to access class methods or variables in Java?

I was reading Code Conventions for Java from http://www.oracle.com/technetwork/java/javase/documentation/codeconventions-137265.html#587.
In that, they have mentioned that we should avoid the use of objects to access a class variable or a method and should we use the class name instead.
Avoid using an object to access a class (static) variable or method.
Use a class name instead. For example:
classMethod(); //OK
AClass.classMethod(); //OK
anObject.classMethod(); //AVOID!
Is there a particular reason for this in terms or performance or something else?

By class variables I assume you mean static variables.
The use of static variables/methods through instance variables should be avoided because it's confusing to the reader. Since you can only use instances to access instance variables, reading a code that calls static methods through an instance can confuse the reader about what's going on.
Image this case, with Thread.sleep, which is a static method:
Thread.sleep(1000);
Since the method is static and we are calling it through the class name, it's intuitive to the reader to deduce that the effect is to put the current thread to sleep.
Now if we did this:
Thread t = new Thread(...);
t.sleep(1000);
Now which thread is put to sleep? The current one "obviously". Someone not knowing about how sleep works might think that the child thread is somehow put to sleep.

I guess it depends on what you want to do.I for one always use the class name to acces a static variable.Being static it doesn matter the way you do it, but i does save some memory usage.
As for nonstatic variables, always use objects.

It is not any different in terms of the bytecode that's produced, so performance is not the issue.
What is the issue is that using a variable to access static members looks as if the reference held by the variable actually matters to what is being accessed. But it does not! In fact even if anObject where null, you would not get a NPE, but it would just call classMethod.
So the principle is simple: when accessing static members, avoid mentioning information (in this case the variable name) that's actually not relevant to the access being made.

I don't know about performance, but clearly another developper looking your code will know at first sight its a static variable/method if you use it with the classname.
I don't think compiler will give different code using a variable though.

it's because.
1.It tells that the variable or method is a static actually.
2.Also, its checked that the object refers to which class. that incurs extra cost.

Access singleton's fields via a static method

I have a singleton class.
When accessing the methods of the class I have the choice of two possibilities.
Create those methods as instance specific and then get the instance and invoke them
Create those methods as static and invoke them and they will get the instance
For example:
Class Test{
private int field1;
Test instance;
private Test(){};
private Test getInstance(){
if (instance == null)
instance = new Test();
return instance;
}
public int method1() { return field1;}
public static int method2() {return getInstance().field1;}
}
Now, elsewhere I can write
int x = Test.getInstance().method1();
int y = Test.method2();
Which is better?
I can think of a 3rd alternative where I use "instance" directly in the static method and then capture the exception if it is null and instantiate it and then re-invoke itself.
I could, in theory, just make the whole lot static.
However, this will create me problems when saving the state at activity close since the serialization doesn't save static.

I think the first one is cleaner.
However, keep in mind that under some extreme cases, Android may kill your static instances. See this for example: http://code.google.com/p/acra/ .
A workaround I've found somewhere for this, is to keep a reference to your singleton from the Application class, as well. I don't know how problem-proof this is, though.

You should avoid making everything static. Some people would even say that a singleton is not done.

The whole point of the singleton pattern is that you can change the implementation. In most cases you use it to keep the possibility open to "hook" in some other implementations of this functionality later.
Read: when deciding in favor of singleton plan for a setInstance method too, not just for a getInstance. - If this does not make sense, just use a plain static class.
In the other hand singletons are out of season, if you want to be hip and all that. Do a search for "eliminating global state". There are some Google-sponsored talks about it too. In short: your code will be more testable and helps you avoid some dependency chaos. (Besides being hip and all, it is definitely a step into the right direction).

In my personal opinion having static methods is bad design in the first place. It, of course, depends on the program itself, but allowing a class to have static method will have impact on the whole design. Some reasoning behind my statement:
If static method can easily change state of some object, sooner or later bugs will emerge
If you publish static method with your program, every client that will use it will have a very strong dependency on your code. If you decide to remove or change this method someday - you will break every single client that used your class.
So, if you can - avoid it.
If, from any reason, you will insist on having static method, I guess the first solution is better. That's how singleton should work. You should obtain a reference to a SINGLETON OBJECT via static method, but this object should be then used according to all principles from Object Oriented Programming.

java singleton pattern, should all variables be class variables?

If a class implements a singleton pattern, should all the variables be declared static?
Is there any reason they shouldn't be declared static? Does it make a difference?

No. The singleton pattern just means that a single instance is the only instance -- it does not mean "make everything statically accessible".
The singleton pattern gives you all the benefits of a "single instance", without sacrificing the ability to test and refactor your code.
Edit:
The point I'm trying to make is that there is a difference between how functionality should be consumed (which depends on context), and how functionality should be initialized.
It may be appropriate that in most cases your object will only ever have a single instance (for example, in your final production system). But there are also other contexts (like testing) that are made much more difficult if you force it to be the only choice.
Also, making something static has more significant implications than just "only one instance of my class should be accessible" -- which is usually the intention.
Further, in software I've worked on, the initialization and lifecycle of objects is often controlled by someone else (I'm talking about DI here) -- and making something static really doesn't help here.

In one common singleton pattern, you do not use statics. You code the class to use ordinary fields, you initialize in the constructor, and then you arrange to execute new MyClass() once, storing the results in some static place.

No, the only thing that is usually static is the reference to the singleton itself (and there are other ways to store that reference, too, such as JNDI or dependency injection containers).
The reason for not declaring fields as static (even though in a singleton pattern you will need only one instance of them) is that this gives you the flexibility to create another, slightly different instance of the normally singleton class. You may want to do that in special situations, such as for testing.
Even if you do not (think you) need that flexibility, there is no reason to give it up. Declaring a field as static has no benefits that you would lose.

You can do this (not necessarily should). But, even for a singleton, I tend to make all the variables object-level rather than class-level because:
I may at some point decide a singleton was a bad idea for that class and having class-level variables will make refactoring harder.
With object-level variables, they only come into existence when you instantiate the singleton. With class-level, they're always there.
Bottom line: I've never been able to think of a disadvantage to having them as object-level so that's how I do it. The above two disadvantages to class-level may be minuscule but they're there. It probably comes down to personal preference in the end.

You can read up on how (one possible way) to create a singleton in Java here:
Wikibooks Design Patterns: Java Singleton
Basically you don't need (nor should) make all things in the class static just because you intend to use something as a singleton. There are several reasons
Check answers from paxdiablo and Thilo
Also don't forget make it all static doesn't make it a singleton you would also need to remove every constructor (and make the default constructor private)