Java: Using same instance of a class in various other classes

Question

it might be stupid question, but I dont know the answer to it and I dont know where to search the answer, so it would be nice if someone could help me.

I've a class (lets name it A) with different members and methods. I use the methods of this class in another class (lets name it B).

For every B-Object created I want to use the SAME instance of A. Is that possible? Actually I have a constructor in B where I call A a = new A(); Of course I always get different instances of this class.

How can I now change this? I know it could be possible to solve it with spring framework (inject always the same object into the instances of B), but I cant use it. How else could this problem be solved?

Thank you very much for your help! :-)

Answer 1

Yes its possible. You need to define a singleton instance of classA that is static, and use it wherever you want.

So there are multiple ways of doing this:

public class ClassA {
   public static ClassA classAInstance = new ClassA();  

}

then anywhere you can do

ClassA.classAInstance.whatever();

This is simple, but it might be enough for you.

If you really want to use the singleton pattern, look here for a Java example. The advantage of this approach is that it makes sure that you only have 1 classA instance.

Answer 2

To elaborate on other answers:

public class A
{
private static A instance;
private A()
{
    ...
}

public static A getInstance()
{
    if (instance == null)
        instance = new A();

    return instance;
}
...
public void doSomething()
{
    ... 
}
}

And then call in B like this:

A.getInstance().doSomething();