How to access an specific object from other object without passing pointer to that object in java?

Question

I have an object say A that loads some data from disk and it takes a bit long time to load. Many of other objects need its methods and data so I don't want neither any time I need it create a new one nor passing it through class constructor. Is there any way to create an instance of the class A only once at the beginning of the running project and all the other objects have access to the object A?

I apologize if my question is duplicated but I don't have any idea about what keywords relate to this question to find related questions.

Answer 1

In that case you are dealing with the Singleton Design Pattern you should declare youre class like this:

public class SingleObject {

   //create an object of SingleObject
   private static SingleObject instance = new SingleObject();

   //make the constructor private so that this class cannot be
   //instantiated
   private SingleObject(){}

   //Get the only object available
   public static SingleObject getInstance(){
      return instance;
   }

   public void showMessage(){
      System.out.println("Hello World!");
   }
}

And then you can use it as intended.

---

In fact the approach here is to use static members like this:

public class Vehicle {

   private static String vehicleType;

    public static String getVehicleType(){
        return vehicleType;
    }

}

The static modifier allows us to access the variable vehicleType and the method getVehicleType() using the class name itself, as follows:

Vehicle.vehicleType
Vehicle.getVehicleType()

Take a look at Java static class Example for further information.

Answer 2

Sure. The design pattern is called a singleton. It could look like this:

public class Singleton {
    private static Singleton instance;
    private Singleton () {}

    /*
     * Returns the single object instance to every
     * caller. This is how you can access the singleton
     * object in your whole application
     */
    public static Singleton getInstance () {
        if (Singleton.instance == null) {
            Singleton.instance = new Singleton();
        }
        return Singleton.instance;
    }
}

All objects can use the singleton by calling Singleton.getInstance()