Testing in isolation of Mock Objects: write focused tests that test only a single method, without side effects resulting from other objects relative.

A simple example: Account Manager Problem(based on a Database environment)

Application analysis:
class Account--two attributes: accountId and balance. Three methods: debit(-), credit(+), getBalance--quite simple and not related to other object or enrionment.
class AccountManager--Manage Account, default connect DB to get data from it. Two methods: findAccountForUser and updateAccount. Closely related to object Account and enrionment DataBase.--must be mocked to fit for unit-test.
class AccountService--one attributes: accountManager which you can confige by yourself. Two methods: setAccountManager and transfer.--This is the class we want to test. The transfer method will be testd with class MockAccountManage which implements AccountManager to let the unit-test run easily.

In the real way, solid class which implements AccountManager may connect to a database to gain the data requested. With the MockAccoundManager which tester created to replaced the default behavier of AccounManager, under your control, we can test AccountService singly.

src: http://blog.chinaunix.net/resserver.php?blogId=8755&resource=junit71.txt

public class TestAccountService extends TestCase {
 public void testTransferOK() {
  MockAccountManager mockAccountManager = new MockAccountManager();
  Account senderAccount = new Account("1", 200);
  Account beneficiaryAccount = new Account("2", 100);
  
  mockAccountManager.addAccount("1", senderAccount);
  mockAccountManager.addAccount("2", beneficiaryAccount);
  
  AccountService accountService = new AccountService();
  
  accountService.setAccountManager(mockAccountManager);
  
  accountService.transfer("1", "2", 50);
  
  assertEquals(150, senderAccount.getBalance());
  assertEquals(150, beneficiaryAccount.getBalance());
 }
}