Операция добавления в JUnit тестах

int operation = 0; // 1 - операция +
 
@Override
public void pressButtonPlus() {
operation = 1;}
 
@Override
public void pressButton(int i) {
if (operation == 0) {
firstOperand = i;
} else {
secondOperand = i;}
displayNumber = i;}
 
@Override
public void pressButtonResult() {
if (operation == 1) {
displayNumber = firstOperand + secondOperand;}}
 
Вот сам тестовый случай:
@Test
public void multiplePlusOperationTest() {
CalculatorInterface calculator = new Calculator(); 
 
calculator.pressButtonOn(); 
calculator.pressButton(1);
Assert.assertEquals(1, calculator.getDisplayNumber()); 
calculator.pressButtonPlus();
Assert.assertEquals(1, calculator.getDisplayNumber()); 
calculator.pressButton(6);
Assert.assertEquals(6, calculator.getDisplayNumber()); 
calculator.pressButtonPlus();
Assert.assertEquals(7, calculator.getDisplayNumber());
calculator.pressButton(8);
Assert.assertEquals(8, calculator.getDisplayNumber());
calculator.pressButtonResult();
Assert.assertEquals(15, calculator.getDisplayNumber());
}