Typical JUnit example

Account.java

public class Account {

    public static class InsufficientFundsException extends Exception {
    }

    private int balance;

    public synchronized void transferTo(final int amount, final Account destination) throws InsufficientFundsException {
        synchronized (destination) {
            withdraw(amount);
            destination.deposit(amount);
        }
    }

    public synchronized void withdraw(int amount) throws InsufficientFundsException {
        if (sufficientFunds(amount)) {
            balance -= amount;
        } else {
            throw new InsufficientFundsException();
        }
    }

    public synchronized void deposit(int amount) {
        balance += amount;
    }

    public synchronized int balance() {
        return balance;
    }

    private boolean sufficientFunds(int amount) {
        return balance >= amount;
    }

}

AccountTest.java

import org.junit.Test;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertTrue;

public class AccountTest {

    @Test
    public void testTransactions() throws Account.InsufficientFundsException {
        Account account = new Account();
        account.deposit(100);
        assertEquals(100, account.balance());
        account.withdraw(60);
        assertEquals(40, account.balance());
    }

    @Test(expected = Account.InsufficientFundsException.class)
    public void testFailedTransaction() throws Account.InsufficientFundsException {
        Account account = new Account();
        account.deposit(100);
        assertEquals(100, account.balance());
        account.withdraw(101);
    }

    @Test
    public void testTransfer() throws Account.InsufficientFundsException {
        Account account1 = new Account();
        Account account2 = new Account();
        account1.deposit(100);
        account1.transferTo(100, account2);
        assertEquals(100, account2.balance());
        assertEquals(0, account1.balance());
    }

    @Test
    public void testFailedTransfer() throws Account.InsufficientFundsException {
        Account account1 = new Account();
        Account account2 = new Account();
        account1.deposit(100);
        boolean caught = false;
        try {
            account1.transferTo(101, account2);
        } catch (final Account.InsufficientFundsException ife) {
            caught = true;
        }

        assertTrue(caught);
        assertEquals(0, account2.balance());
        assertEquals(100, account1.balance());
    }

}