What is T in List T list

Question

Hi guys,
I am wondering what is the T stand for in the List<T> list?
and what's it useful for the following interface and implementation to use <T, PK>?
public interface TestInterface <T, PK extends Serializable>{
     public void sayHello();
public class TestInterfaceImpl<T, PK extends Serializable> implements TestInterface<T, PK>{
     @Override
     public void sayHello() {
          System.out.println("Buenas noches!");
}

Answer 1

That's the type parameter. It tells instances of that class what other kinds of classes can be used with it, so to speak.
It's so you can write a library without knowing what other classes will be used with it.
I'm not explaining this well. Google for tutorials on "java generics"