how to write the perfect code for throwable exception a negative number?Using assertThatThrownBy

Question

I have a unit test and I want to throw an exception on it (because it will always throw in that unit test).

I am trying using try and catch but I don't know what should I do after.

In the unit test (from the first class).

@Test
void subtract_money_from_smaller_money_should_fail() {
    Money oneDinar = new Money(BigDecimal.valueOf(1));
    Money halfDinar = new Money(BigDecimal.valueOf(0.5));

    assertThatThrownBy(new ThrowableAssert.ThrowingCallable() {
        @Override
        public void call() throws Throwable {
            halfDinar.subtract(oneDinar);
        }
    }).isInstanceOf(IllegalArgumentException.class);
}

The second class where we will throw that exception.

public Money subtract(Money SubtractMoney) {
    System.out.println("TheValue of current money "+current_Money.toString());
    Money current = new Money(current_Money);
    System.out.println("TheValue of current  "+ current.current_Money.toString());
    BigDecimal subtractedNumber= BigDecimal.valueOf(0);

    try {
        subtractedNumber = current_Money.subtract(new 
        BigDecimal(String.valueOf(SubtractMoney.current_Money)));

        if (subtractedNumber.intValue() < 0) {
            // throw new IllegalArgumentException("Error the Subtracted is in minus...");
            // throw new Throwable();
        }

        //  System.out.println("TheValue of subtractedNumber" + subtractedNumber.toString());
    }

I made subtraction operation inside the try statement, and checked if that subtracted number (the new number) will be in minus (-1 or less) to throw the exception.

I tried this

throw new Throwable(); 

but didn't work (maybe I didn't use it properly).

I tried this

throw new IllegalArgumentException("Error the Subtracted is in minus...");

inside the if statement in try statement. (didn't work and I don't know if I used in proper way).

When the method in the second class is called, should throw an Exception if the money is in minus (-1 or less).

For example I want to buy an apple and it costs me 2 dollars but I gave him 1 dollar (1-2 = -1 (which activate the exception)), the seller will refuse to complete the payment (Purchase) and he will return to me my dollar and he will get the apple.

Answer 1

Your problem is on the if condition:

if (subtractedNumber.intValue() < 0){

this condition will convert the BigDecimal to int, truncating the value in case of a decimal. In your case, -0.5 will be truncated to 0, and that's why your test is not passing.

Change this condition to

if (subtractedNumber.doubleValue() < 0)

uncomment the Illegal Arg Exception and your test will pass.

Edit: Added more detail

This is the code I ended with:

public class Money {

    BigDecimal current_Money;

    public Money(BigDecimal money) {
        this.current_Money = money;
    }

    public Money subtract(Money subtractedMoney) {

        // TODO: Validation if needed

        BigDecimal subtractedNumber = current_Money.subtract(subtractedMoney.current_Money);

        if (subtractedNumber.doubleValue() < 0) {
            throw new IllegalArgumentException("Error the Subtracted is in minus...");
        }

        return new Money(subtractedNumber);
    }
}

Test class:

import org.assertj.core.api.Assertions;
import org.assertj.core.api.ThrowableAssert;
import org.junit.Test;

import java.math.BigDecimal;

public class MoneyTest {

    @Test
    public void subtract_money_from_smaller_money_should_fail() {
        Money oneDinar = new Money(BigDecimal.valueOf(1));
        Money halfDinar = new Money(BigDecimal.valueOf(0.5));

        Assertions.assertThatThrownBy(new ThrowableAssert.ThrowingCallable() {
            @Override
            public void call() throws Throwable {
                halfDinar.subtract(oneDinar);
            }
        }).isInstanceOf(IllegalArgumentException.class);
    }
}

Personally, I would verify the exception this way:

import org.junit.Assert;
import org.junit.Test;
import org.junit.jupiter.api.Assertions;

public class MoneyTest {

    @Test
    public void subtract_money_from_smaller_money_should_fail() {
        Money oneDinar = new Money(BigDecimal.valueOf(1));
        Money halfDinar = new Money(BigDecimal.valueOf(0.5));

        String expectedMessage = "Error the Subtracted is in minus...";

        Throwable exception = Assertions.assertThrows(IllegalArgumentException.class, ()
                -> {
            halfDinar.subtract(oneDinar);
        });

        Assert.assertEquals(expectedMessage, exception.getMessage());
    }
}

Both are working as expected.