Generics - Things to know before your OCJP Exam - Generics

Following Generic Declarations are good

List<Exception> li = new ArrayList<Exception>();

List<? extends Exception> li = new ArrayList<FileNotFoundException>(); 

ArrayList<? extends Exception> li = new ArrayList<FileNotFoundException>();

ArrayList<?> li = new ArrayList<FileNotFoundException>();

ArrayList<?> li = new ArrayList<>();

ArrayList<String> li = new ArrayList<>();

List<? super FileNotFoundException> li = new ArrayList<FileNotFoundException>();

List<? super FileNotFoundException> li = new ArrayList<Exception>();

List<? extends Exception> li = new ArrayList<Exception>();

Following declarations result in compile time error

List<Exception> li = new ArrayList<FileNotFoundException>(); // argument type should be same.

ArrayList<Exception> li = new ArrayList<FileNotFoundException>(); // argument type should be same.

List<Exception> li = new List<Exception>(); // List is not a concrete class but an interface

ArrayList<? extends String> li = new ArrayList<FileNotFoundException>(); // FileNotFoundException doesn't extend String

List<> li = new ArrayList<>(); // incorrect argument

List<? super Exception> li = new ArrayList<FileNotFoundException>(); // FileNotFoundException is not super class of Exception