CODEWITHSUNDEEP
javaarraylist
Ryan Smith
Ryan Smith

Reputation: 709

How to create an instance variable only once?

I have a class with an ArrayList of pre-determined items, in this example fruits. In my code I have a constructor call a method that populates my list.

The problem is every time an instance of this class is instantiated, the list is repopulated. How can I create and populate the list once?

public class Test {    
    ArrayList<String> lst = new ArrayList<String>();

    public Test() {
        populate();
    }

    public void populate() {
        lst.add("Apple");
        lst.add("Banana");
    }
}

Upvotes: 2

Views: 2320

Answers (4)

tom3008
tom3008

Reputation: 100

have a look at the singleton pattern u get many simple informations form here: http://www.javaworld.com/article/2073352/core-java/simply-singleton.html

depending on this surce, i can try it once with your class:

public class TestClass {

    private static TestClass instance = null;

    ArrayList<String> lst = new ArrayList<String>();

    protected TestClass () {
        populate();
    }

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

        return instance;
    }

    public void populate() {
        lst.add("Apple");
        lst.add("Banana");
    }

}

so if u want to create OR get the class, run the getInstance() method and not the constructor (u even cant run the constructor cause it is protected) notice: i didnt test the code, but it looks fine, if there are any problems or further questions, i still can run it to fix them, just comment then or let me know if it works / solved your problem

Upvotes: 0

Elliott Frisch
Elliott Frisch

Reputation: 201447

You could make it static, and use a static initialization block. I would also prefer to program to the List interface, and I would use a diamond operator <>. Something like,

public class Test {
    private static List<String> lst = new ArrayList<>();
    static {
        lst.add("Apple");
        lst.add("Banana");
    }
    public Test() {
        // ...
    }
}

Or, use Arrays.asList(T...) and eliminate the initalization block like

public class Test {
    private static List<String> lst = Arrays.asList("Apple", "Banana");
    public Test() {
        // ...
    }
}

Upvotes: 4

Mohammad Hossein Gerami
Mohammad Hossein Gerami

Reputation: 1388

you can create singleton class

public class MyClass{

   private static MyClass instance;

   private ArrayList<String> lst;

   private MyClass(){
      lst = new ArrayList<String>();
      lst.add("Apple");
      lst.add("Banana");
   }

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

      return instance;
   }
}

Upvotes: 1

Siva Kumar
Siva Kumar

Reputation: 2006

You can use static with static block.

public class Test {

    static ArrayList<String> lst = new ArrayList<String>();

    static {
        populate();
    }

    public static  void populate() {
        lst.add("Apple");
        lst.add("Banana");
    }

}

It will call once while loading a class.

Upvotes: 1

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