Question

Before I ask the question I like to provide the code for clarity. Below is my code for singleton class.

public class CoreData {

    private boolean VarA;

    private static CoreData instance = null;

    protected CoreData() {
          // Exists only to defeat instantiation.
    }

    public static CoreData getInstance() {
          if(instance == null) {
             instance = new CoreData();
          }
          return instance;
    }

    public  boolean getVarA(){
        return VarA;
    }

    public  void setFirstTime(boolean B){
        VarA = B;
    }
}

Now I have few questions to ask

1. What will be difference if make the member variable VarA as static?
2. Can I initialize the member variable in the method getInstance()?
3. What is the best practice in initializing member variables in Singleton class?
4. What is the meaning of making this class as final?
5. What is the meaning of making the member variable final.

I am very new to java and OOPS. I am learning it now. I would be grateful if someone answer my queries to make my knowledge better.

Answer 1

What will be difference if make the member variable VarA as static?

It will be harder to make it non singleton later

Can I initialize the member variable in the method getInstance()?

Yeah. Why not. But actually constructors are done for this.

What is the best practice in initializing member variables in Singleton class?

By the best practice you should use some IoC and don’t put any code about scope in your logic code.

What is the meaning of making this class as final?

You should use private constructor instead of protected one or make it final to prevent creating several instances by extending. Like new CoreData(){};

What is the meaning of making the member variable final?

I believe all variables should be final by default. Also it can help you with multi threading issues.