Singleton

class DBC{
    static int a = 10;
    private static DBC dbc;
    private DBC(){}
    public static DBC getInstance(){ //not thread safe
        if(dbc == null){
            dbc = new DBC();
        }
        return dbc;
    }
}

public class Client{
    public static void main(String[] args){
        DBC obj = DBC.getInstance();
        System.out.println(obj.a); //10
    }
}


//Synchronized
class DBC{
    static int a = 10;
    private static DBC dbc;
    private DBC(){}
    public synchronized static DBC getInstance(){ //now thread safe, but its blocking code
        if(dbc == null){
            dbc = new DBC();
        }
        return dbc;
    }
}

//Double check locking
class DBC{
    static int a = 10;
    private static DBC dbc;
    private DBC(){}
    public static DBC getInstance(){ //now thread safe, double check locking
        if(dbc == null){
            synchronized(DBC.class){ //making critical section synchronized
                if(dbc == null){
                    dbc = new DBC();
                }
            }
        }
        return dbc;
    }
}

//Best implementation
class DBC{
    static int a = 10;
    private static DBC dbc = new DBC(); //only one instance
    private DBC(){}
    public static DBC getInstance(){ //now thread safe
        return dbc;
    }
}