JUnit multiple test in single class

Question

public class AccountModel extends AbstractModel {
    public static BigDecimal balance = new BigDecimal(0.00);
    public static BigDecimal balanceEuro = new BigDecimal(0.00);
    public static BigDecimal balanceYen = new BigDecimal(0.00);

    // deposit method
    public static BigDecimal deposit(double amount) {
        balance = balance.add(new BigDecimal(amount));
        return balance;
    }

    // withdraw method
    public static void withdraw(BigDecimal withdraw) {
        balance = balance.subtract(withdraw);

    }

    public static BigDecimal euro() {
        balanceEuro = balance.multiply(new BigDecimal(0.79));
        return balanceEuro;
    }

    public static BigDecimal yen() {
        balanceYen = balance.multiply(new BigDecimal(94.1));
        return balanceYen;
    }

    public static BigDecimal balance() {
        return balance;

    }


    @Override
    public void notifyChange(ModelEvent me) {
        // TODO Auto-generated method stub

    }
}

public class AccountModelTest {
    AccountModel accTest = new AccountModel();

    // test deposit method
    @Test
    public void testDeposit() {
        // BigDecimal balance = new BigDecimal(0.00);

        System.out.println("Deposit");
        accTest.deposit(30.00);
        accTest.deposit(100.00);
        // excepted balance
        BigDecimal expectedBalance = new BigDecimal(130.00);
        System.out.println("Expected: " + expectedBalance.setScale(2, BigDecimal.ROUND_HALF_DOWN) + " Actual: "
            + accTest.balance().setScale(2, BigDecimal.ROUND_HALF_DOWN));

        BigDecimal balance = new BigDecimal(0.00);
        assertEquals(expectedBalance.setScale(2, BigDecimal.ROUND_HALF_DOWN),
            accTest.balance().setScale(2, BigDecimal.ROUND_HALF_DOWN));
        System.out.println("");
    }

    // test withdraw method
    @Test
    public void testWithdraw() {
        System.out.println("Withdraw");
        AccountModel accTest1 = new AccountModel();
        accTest1.deposit(40.00);
        accTest1.withdraw(new BigDecimal(30.00));

        BigDecimal expectedBalance1 = new BigDecimal(10.00);

        System.out.println("Expected: " + expectedBalance1.setScale(2, BigDecimal.ROUND_HALF_DOWN) + " Actual: "
            + accTest1.balance().setScale(2, BigDecimal.ROUND_HALF_DOWN));

        assertEquals(expectedBalance1.setScale(2, BigDecimal.ROUND_HALF_DOWN),
            accTest1.balance().setScale(2, BigDecimal.ROUND_HALF_DOWN));

    }
}

So I'm doing 2 separate test cases for the withdraw method and deposit method. When I implement 1 test case The deposit method works fine, and also the withdraw method works fine. But when I run the test like shown above, the test always fails. The output is

Withdraw Expected: 10.00 Actual: 10.00

Deposit Expected: 130.00 Actual: 140.00 (should be 130.00)

Answer 1

Since you are using static fields inside AccountModel class their value is shared between all AccountModel instances. Make them private non-static:

private BigDecimal balance = BigDecimal.ZERO;
private BigDecimal balanceEuro = BigDecimal.ZERO;
private BigDecimal balanceYen = BigDecimal.ZERO;

JUnit is creating new instance of AccountModelTest class for every @Test with new AccountModel so starting balance will be zero for every test.

Answer 2

every test show run with clean context, you are sharing AccountModel accTest with multiple tests. you should set up and clean the context each time a test run.

public class AccountModelTest {
AccountModel accTest;

@Before // this code will run before any test start, it will create new model for each test
void setUp() {


    AccountModel accTest = new AccountModel();
}