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I am well aware that generic types are erased from Java code when it is compiled. What information (attributes?) do 1.5+ JVMs use to implement getGenericType , etc. ?
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The class files actually reside as jar files inside the Java distribution being used. Most files are in rt. jar (runtime).
A Generic class simply means that the items or functions in that class can be generalized with the parameter(example T) to specify that we can add any type as a parameter in place of T like Integer, Character, String, Double or any other user-defined type.
A Generic Version of the Box Class To update the Box class to use generics, you create a generic type declaration by changing the code "public class Box" to "public class Box<T>". This introduces the type variable, T, that can be used anywhere inside the class.
They are stored in Signature attributes; see section 4.8.8 of the updated Java Virtual Machine Specification, as well as section 4.4.4 for the format of the field type signature.
Here's an example using javap -verbose java.util.Map:
public interface java.util.Map
SourceFile: "Map.java"
Signature: length = 0x2
00 1E
[other attributes omitted]
The Signature attribute here specifies (if you read this as big-endian, like all integer quantities in the JVM class file format are) constant pool value #30 (30 = 0x1E). So let's have a look there:
const #30 = Asciz <K:Ljava/lang/Object;V:Ljava/lang/Object;>Ljava/lang/Object;;
Read this in the context of the grammar specified in 4.4.4. So, this uses two type parameters, K extends java.lang.Object and V extends java.lang.Object. The type itself (Map) also extends class java.lang.Object, and no interfaces.
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