Build test cases/contracts for a java program

The class Word is essentially a specialised String whose purpose is to contain character strings that consist of an unbroken sequence of letters. Building on the class Word, the class WordList offers a number of methods for manipulating a list of words. These classes can be used for various purposes such as, for example, building auto-completion routines for text boxes in user interfaces. Task requirement: Create preconditions and postcondition tests (ideally weakest and strongest respectively) for the given program using the requirements set (Can be read on the document attached) and add invariants (ideally the strongest) to the given classes. Each method written must be tested by a series of JUnit tests.