Generics exist only till compile time. They do not exist at run time

All your generic code is strictly for the compiler. Through a process called "type erasure," the compiler does all of its verifications on your generic code and then strips the type information out of the class bytecode. At runtime, ALL collection code—both legacy and new Java 5 code you write using generics—looks exactly like the pre-generic version of collections. None of your typing information exists at runtime. In other words, even though you WROTE

List myList = new ArrayList();


By the time the compiler is done with it, the JVM sees what it always saw before Java 5 and generics:

List myList = new ArrayList();

The compiler even inserts the casts for you—the casts you had to do to get things out of a pre-Java 5 collection.

Think of generics as strictly a compile-time protection. The compiler uses generic type information (the in the angle brackets) to make sure that your code doesn't put the wrong things into a collection, and that you do not assign what you get from a collection to the wrong reference type. But NONE of this protection exists at runtime.


This compiler warning isn't very descriptive, but the second note suggests that you recompile with -xlint:unchecked. If you do, you'll get something like this:

javac -Xlint:unchecked TestBadLegacy.java
TestBadLegacy.java:17: warning: [unchecked] unchecked call to
add(E) as a member of the raw type java.util.List
list.add(new String("42"));
^
1 warning

When you compile with the -Xlint:unchecked flag, the compiler shows you exactly which method(s) might be doing something dangerous.