Double Brace Initialization in Java!

by · June 26, 2009

java-double-brace-initializationFew days back I came to know about a different way of initializing collections and objects in Java. Although this method of initialization has been there in Java since quite a few years now, very few people actually knows about it or have used it in their code. The technique is called Double Brace Initialization technique in Java. Let us see first how we will initialize an ArrayList in Java by “normal” way:
List<String> countries = new ArrayList<String>();
countries.add("Switzerland");
countries.add("France");
countries.add("Germany");
countries.add("Italy");
countries.add("India");

Code language: Java (java)
Above code snippet first creates an object of ArrayList type String and then add one by one countries in it. Now check the Double Brace Initialization of the same code:
List<String> countries = new ArrayList<String>() {{
	add("India");
	add("Switzerland");
	add("Italy");
	add("France");
	add("Germany");
}};

Code language: Java (java)

How does this works?

Well, the double brace ( {{ .. }} ) initialization in Java works this way. The first brace creates a new AnonymousInnerClass, the second declares an instance initializer block that is run when the anonymous inner class is instantiated. This type of initializer block is formally called an “instance initializer”, because it is declared withing the instance scope of the class — “static initializers” are a related concept where the keyword static is placed before the brace that starts the block, and which is executed at the class level as soon as the classloader completes loading the class . The initializer block can use any methods, fields and final variables available in the containing scope, but one has to be wary of the fact that initializers are run before constructors. If you have not understood the concept, try executing following code and it will make things clear for you.
public class Test {
	public Test() {
		System.out.println("Constructor called");
	}
	static {
		System.out.println("Static block called");
	}
	
	{
		System.out.println("Instance initializer called");
	}
	
	public static void main(String[] args) {
		new Test();
		new Test();
	}
}

Code language: Java (java)

Output:

Static block called
Instance initializer called
Constructor called
Instance initializer called
Constructor called
Thus the instance initializer code will be called each time a new instance of object is created.
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13 Comments

  1. ruslan says:
    27 June, 2009, 2:03  

    Unfortunatelly that code creates anonymous class. Multiple using of such technique will create dozen of anonymous classes, and that could cause problems with perm gen space.

    Reply
  2. Viral Patel says:
    27 June, 2009, 14:30  

    I agree with you Ruslan. This technique is not efficient way of doing this, but I dint knew if such thing exists :-)

    Reply
  3. Karthik says:
    28 June, 2009, 19:00  

    I agree with Viral that many Java developers will not be knowing it unless they read the specification. Nice to know that such an option exists.

    Reply
  4. Prashanth says:
    6 July, 2009, 18:28  

    Interesting to know with clean explanation

    Reply
  5. Daniel says:
    28 February, 2011, 15:00  

    Javas Double Brace Instatiation makes it a lot easier to read your source code. For review reasons is that important to you and your colleagues. I think most people only see these advantages if nobody uses them: they would be glad to have them.

    A few tips and backgrounds, too: is is about the obstacles and the possibillities with Javas Double Brace Instatiation:
    http://bit.ly/endUIi

    I hope I could help a little.

    Reply
  6. Ravi says:
    4 April, 2012, 17:47  

    Awesome! Never tried such a thing so far. This could be useful in providing easy fixes.

    Thanks

    Reply
  7. Raghu says:
    13 June, 2013, 13:44  

    Hi,

    Anybody please help me , how to send /receive sms from/to java web application/java application.

    Reply
  8. Dharmendra says:
    7 August, 2013, 11:55  

    Nice explanation…Thanks a lot for this article.

    Reply
  9. Vinay says:
    18 March, 2014, 23:24  

    import java.util.ArrayList;
import java.util.List;
import java.lang.*;
 
public class MyBasicArrayList {
 
    public static void main(String[] a){

      long beginTime = System.currentTimeMillis();
        List<String> al = new ArrayList<String>(){{
          add("JAVA");
        add("C++");
        add("PERL");
        add("PHP");  
        }};
        long endTime = System.currentTimeMillis();
        long difference1 = endTime - beginTime;
                System.out.println(al);
        
        long beginTime2 = System.currentTimeMillis();
        List<String> a2 = new ArrayList<String>();
        a2.add("JAVA");
         a2.add("C++");
         a2.add("PERL");
         a2.add("PHP");  
         long endTime2 = System.currentTimeMillis();
        long difference2 = endTime2 - beginTime2;
        System.out.println(al);
        //add elements to the ArrayList
        if(difference2>difference1){
            System.out.println("{{}} Wins");}
            else{
                System.out.println("{{}} Looses");}           
    }
}

    I tried this program and Guess what {{}} Looses….. n times So I appreciate your effort but this is lil slower think..kindly revert If I am wrong.

    Reply
  10. ali says:
    4 January, 2015, 5:33  

    nice explanation and useful tutorial , thanks ;)

    Reply
  11. jawed says:
    1 December, 2015, 6:22  

    Any one can tell me how to add values to List which parameter of a constructor java

    Reply
  12. jawed says:
    1 December, 2015, 6:24  

    private static List ga ;how ????
    Any one can tell me how to add values to List which is parameter of a constructor java
    public test(List g) {
    //how to add values?????
    }

    Reply
  13. Dinesh Krishnan says:
    1 November, 2016, 16:35  

    Very good article thanks for sharing. Good work

    Reply

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