Skip to main content

What is type erasure in Java?

I had read about type erasure in Java a long time back. However, today when a friend asked me a question related to type erasure, I found myself not quite certain of the answer.

I read up on it again and here is what I learned.

Angelika Langer has an excellent FAQ, where she explains generics, and type erasure. According to the FAQ:

A process that maps a parameterized type (or method) to its unique byte code representation by eliding type parameters and arguments.

OK, let's understand what that means. Below is a simple class which uses the generified version of Java Lists.









package net.adaptivelearningonline.examples.generics;

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

public class GenericsErasure {
  public static void main(String args[]) {
    List<String> list = new ArrayList<String>();
    list.add("Hello");
    Iterator<String> iter = list.iterator();
    while(iter.hasNext()) {
      String s = iter.next();
      System.out.println(s);
    }
  }
}




I am going to take the bytecode generated by the above class and decompile it using JAD.

Here is the decompiled Java class:









// Decompiled by Jad v1.5.8g. Copyright 2001 Pavel Kouznetsov.
// Jad home page: http://www.kpdus.com/jad.html
// Decompiler options: packimports(3) 
// Source File Name:   GenericsErasure.java

package net.adaptivelearningonline.examples.generics;

import java.io.PrintStream;
import java.util.*;

public class GenericsErasure
{

    public GenericsErasure()
    {
    }

    public static void main(String args[])
    {
        List list = new ArrayList();
        list.add("Hello");
        String s;
        for(Iterator iter = list.iterator(); iter.hasNext(); System.out.println(s))
            s = (String)iter.next();

    }
}





As you can see, the decompiled code has absolutely no trace of any generics information. This is because of erasure. When the Java compiler compiles client code which uses a library that has been generified, the client code is compiled in such a way that all generics information is erased from the bytecode.

This also explains the fact that generics information in client code is meant purely for the compiler to do type checking. Once the compiler has done it's job, this information is discarded.

The reason why Java uses type erasure is to maintain backwards compatibility. Let's assume you wrote some code a few years back which used Java Lists, before generics was introduced in the language. Obviously your code had no mention of generics. Now, when engineers at Sun decided to introduce generics, they did not want to break code which people had already written. One possible way was to ensure that client code which uses generics in the classes they invoke, never carries any information about generics in the compiled code. So the above class when compiled should not carry any information about generics. Because compiled client code never carries information about generics anyways, library implementers are free to add generics to their code without the worry of breaking anyone's old code.

Bruce Eckel has written a rather lenghty but good article on generics and erasure.

Comments

jeet said…
Thanks for this article.It is helpful.
Anonymous said…
Awesome article.... keep it up mate
Dhiral V Pandya said…
great example... keep it up ...

Popular posts from this blog

Fuctional Programming Principles in Scala - Getting Started

Sometime back I registered for the Functional Programming Principles in Scala , on Coursera. I have been meaning to learn Scala from a while, but have been putting it on the back burner because of other commitments. But  when I saw this course being offered by Martin Odersky, on Coursera , I just had to enroll in it. This course is a 7 week course. I will blog my learning experience and notes here for the next seven weeks (well actually six, since the course started on Sept 18th). The first step was to install the required tools: JDK - Since this is my work machine, I already have a couple of JDK's installed SBT - SBT is the Scala Build Tool. Even though I have not looked into it in detail, it seems like a replacement for Maven. I am sure we will use it for several things, however upto now I only know about two uses for it - to submit assignments (which must be a feature added by the course team), and to start the Scala console. Installed sbt from here , and added the path

Five Reasons Why Your Product Needs an Awesome User Guide

Photo Credit: Peter Merholz ( Creative Commons 2.0 SA License ) A user guide is essentially a book-length document containing instructions for installing, using or troubleshooting a hardware or software product. A user guide can be very brief - for example, only 10 or 20 pages or it can be a full-length book of 200 pages or more. -- prismnet.com As engineers, we give a lot of importance to product design, architecture, code quality, and UX. However, when it comes to the user manual, we often only manage to pay lip service. This is not good. A usable manual is as important as usable software because it is the first line of help for the user and the first line of customer service for the organization. Any organization that prides itself on great customer service must have an awesome user manual for the product. In the spirit of listicles - here are at least five reasons why you should have an awesome user manual! Enhance User Satisfaction In my fourteen years as a

Inheritance vs. composition depending on how much is same and how much differs

I am reading the excellent Django book right now. In the 4th chapter on Django templates , there is an example of includes and inheritance in Django templates. Without going into details about Django templates, the include is very similar to composition where we can include the text of another template for evaluation. Inheritance in Django templates works in a way similar to object inheritance. Django templates can specify certain blocks which can be redefined in subtemplates. The subtemplates use the rest of the parent template as is. Now we have all learned that inheritance is used when we have a is-a relationship between classes, and composition is used when we have a contains-a relationship. This is absolutely right, but while reading about Django templates, I just realized another pattern in these relationships. This is really simple and perhaps many of you may have already have had this insight... We use inheritance when we want to allow reuse of the bulk of one object in other