When do you use instance initializer?

An instance initializer is the code block which will be executed before executing any constructors when object of a class type is create.

It is important to realize that the instance initializer(s) are executed first no matter which constructor is executed. The instance initializer can be used for common initialization code.

Here is one example to use the instance initializer. The Program class does not have a constructor (compiler will add a default constructor), but you want to initialize the instance variable s to refer an instance of Sample class at the beginning.

class Sample {
   Sample() throws Exception { }
}
public class Program {
   //Sample s = new Sample(); //Compile time error
   Sample s;
   {
       try {
          s = new Sample();
       } 
       catch(Exception e) {
          ;
       }
   }
}