How do I use the double brace initialization?

courtesy: kodejava

The double brace initialization {{ ... }} is another way for initializing a collection objects in Java. It is offer a simple syntax for initializing a collection object.

import java.util.List;
import java.util.ArrayList;

public class DoubleBraceInitialization {
public static void main(String[] args) {
//
// Creates a list of colors and add three colors of
// Red, Green and Blue.
//
List colors1 = new ArrayList();
colors1.add("Red");
colors1.add("Green");
colors1.add("Blue");


//
// Creates another list of colors and add three colors
// using the double brace initialization.
//
List colors2 = new ArrayList() {{
add("Red");
add("Green");
add("Blue");
}};

for (String color : colors2) {
System.out.println("Color = " + color);
}
}
}

What's actually happened is: the first brace creates an anonymous inner class and the second brace is an initializer block. Due to the need for creating an inner class the use of double brace initialization is considered to be slower.

Because of this performance issue you it's better not to use this technique for you production code, but using it in your unit testing can make your test looks simpler.