Type Erasure of Generics

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. Type erasure enables Java applications that use generics to maintain binary compatibility with Java libraries and applications that were created before generics.
For instance, Box<String> is translated to type Box, which is called the raw type — a raw type is a generic class or interface name without any type arguments. This means that you can't find out what type of Object a generic class is using at runtime. The following operations are not possible:
public class MyClass<E> {
    public static void myMethod(Object item) {
        // Compiler error
        if (item instanceof E) {
            ...
        }
        // Compiler error
        E item2 = new E();
        // Compiler error
        E[] iArray = new E[10];
        // Unchecked cast warning
        E obj = (E)new Object();
    }
}
The operations shown in bold are meaningless at runtime because the compiler removes all information about the actual type argument (represented by the type parameter E) at compile time.

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Posted in: Generics