What is the concept of erasure in generics in Java?

Question

What is the concept of erasure in generics in Java?

Answer 1

Just as a side-note, it is an interesting exercise to actually see what the compiler is doing when it performs erasure -- makes the whole concept a little easier to grasp. There is a special flag you can pass the compiler to output java files that have had the generics erased and casts inserted. An example:

javac -XD-printflat -d output_dir SomeFile.java

The -printflat is the flag that gets handed off to the compiler that generates the files. (The -XD part is what tells javac to hand it to the executable jar that actually does the compiling rather than just javac, but I digress...) The -d output_dir is necessary because the compiler needs some place to put the new .java files.

This, of course, does more than just erasure; all of the automatic stuff the compiler does gets done here. For example, default constructors are also inserted, the new foreach-style for loops are expanded to regular for loops, etc. It is nice to see the little things that are happening automagically.

Answer 2

Erasure, literally means that the type information which is present in the source code is erased from the compiled bytecode. Let us understand this with some code.

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);
        }
    }
}

If you compile this code and then decompile it with a Java decompiler, you will get something like this. Notice that the decompiled code contains no trace of the type information present in the original source code.

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();

    }
}