Skip to main content

Type Erasure in Java

While reading some posts online I came across the following post -
List<integer> list = new ArrayList<integer>();
List&lt;integer&gt; list = new ArrayList&lt;integer&gt;();
list.add(1);
list.add(2);
List list1 = new ArrayList();
list1.add(&quot;Some String&quot;);
list.addAll(list1);


This code compiles and executes as well, this works because of Type Erasure concept in Java. I knew about Type Erasure but while reading more about it on internet I found some posts which were questioning on Type Erasure concept.
Following code makes more confusing about Type Erasure concept, if compiler removes all the information about type parameters and type arguments then how does following code works??

class A<T>{
}
class B extends A<String>{
 public B() {
  System.out.println(((ParameterizedType)getClass().
                      getGenericSuperclass()).getActualTypeArguments()[0]);
 }
}


public static void main(String args[]){
     new B();
}



Output of above code-
java.lang.String
Well some people might get confused by this but if you read carefully, Type Erasure docs says -
When a generic type is instantiated, the compiler translates those types by a technique called type erasure — a process where the compiler removes all information related to type parameters and type arguments within a class or method.
Compiler does removes all the type parameter and type arguments information within the class but it maintains the information about the super class so the type arguments with it, and that is the reason why above code prints the java.lang.String.
Now lets see the first code snippet again-
List<Integer>list = new ArrayList<Integer>();
list.add(1);
list.add(2);
List list1 = new ArrayList();
list1.add("Some String");
list.addAll(list1);
This will not work with above code

String str = list.get(2);

Compiler will give error for above statement due to generics, but following code will work

Object str = list.get(2);

While iterating through list you can check data from the list by
for( int i = 0 ; i < list.size() ; i++ ){
     Object data = list.get(i);
     if(data instanceof Integer){
          System.out.println("Integer:"+data);
     }
     if(data instanceof String){
          System.out.println("String:"+data);
     }
}
Type Erasure concept allows Java code to work with the old Java libraries which were written before generics.

Comments

Popular posts from this blog

Spring AOP : JDK Proxy vs CGLib Proxy

Spring framework is one of the most widely used framework in Enterprise application development. It has so many features such as Dependency injection, Data access integration, MVC, AOP which takes care of most of the boilerplate part of project, and developers can then focus on business logic only.

One of the important feature in Spring is AOP. It is used by almost every enterprise application which is being developed using Spring.

AOP So what is AOP? Definition of AOP -
Aspect oriented programming is a programming paradigm which aims to increase modularity by allowing the separation of cross cutting concerns. Well, my understanding of AOP is - AOP allows us to introduce/join new modules in your project at pre-specified dynamic locations without having to code for it. Traditional examples of AOP is Transaction management, logging etc. You can read more about AOP at here.

This article is for those who know AOP, have used AOP in their projects but do not know how Spring or other DI fra…

Java Tools Plugin for Notepad++

This post and the plugin is outdated. Please use plugin from this link which is updated and allows customization of shortcut key mappings and much more.
Notepad++ is a great free editor. I like notepad++ because its a light weight editor and loads instantly. I use notepad++ for editing many files everyday. One plugin I always wanted was a plugin which lets me compile my java files and execute them from editor itself, but I couldn't find it so I wrote a myself. This plugin can be downloaded from this link.
I am working on this plugin to make it more effective, so that if user have more than one installations of JDK or JRE then user should be able to choose which one to use etc.
This is a simple plugin which has two commands-
Compile - compiles a java file.
Run - executes a java file.
For using this plugin Java must be installed on the system.

Notepad++ Compile and Run Java Programs

Notepad++ is a great file editor. It has many features. The most important feature which I like about Notepad++ is its light weight. It loads up so quickly, that's great. It also provides syntax highlighting for many languages. I use notepad++ to edit general files as well as my simple Java programs.
Although notepad++ provides functionality to run external programs, I prefer NppJavaTools plugin to compile and run Java programs using notepad++.
You can download NppJavaTools plugin from this page - NppJavaTools.

Installation
Installation of plugins in notepad++ is very simple process. All you have to do is copy plugin dll into plugins folder of notepad++ installation directory and restart the notepad++.

Features
This plugin provides following features,


Compile and run your Java files within Notepad++Set custom hotkeys for compiling and running Java CodeLibrary support Compile and RunThis function allows you to run your Java programs to compile and run from Notepad++. Set Custom hotkeysThi…