Java Generics : Is any meta information about the generic type preserved at runtime as well?

Question

Background

My understanding of Java generics is it being completely a compile time feature (mainly focusing on type safety checks)

Answer 1

Of course the information that a class is generic is supported.

In other words: when you decompile ArrayList.class you will find hints about the fact that this class allows for one generic type parameter. In other words: class files contain meta information. And using reflection it is possible to inspect this meta information at runtime.

But when you have another class that uses some List object - then you do not find information about that "list uses an Integer" in the compiled class - unless you use some specific patterns, as outlined here for example.

So the answer is basically: for almost all use cases of practical relevance, "generics" are compile time only.

Example:

public class GenericsExample {
  private T member;   
  public T foo(T bar) {
     return member;
  }
}

Now run: javap -p -c GenericsExample

Compiled from "GenericsExample.java"
public class GenericsExample {
  private T member;

  public GenericsExample();
    Code:
       0: aload_0       
       1: invokespecial #1                  // Method java/lang/Object."":()V
       4: return        

  public T foo(T);
    Code:
       0: aload_0       
       1: getfield      #2                  // Field member:Ljava/lang/Object;
       4: areturn       
}

As you can see the decompiler understands that the class uses that generic type T. For more details see here or there.