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.