For general boolean tests, one can assert that some condition is true
public static void assertTrue(boolean condition)
public static void assertTrue(String message, boolean condition)
Test absolute value:
public void testNegative2GreaterThan1() {
Complex z1 = new Complex(-2, 0);
Complex z2 = new Complex(1, 0);
double a1 = z1.abs();
double a2 = z2.abs();
assertTrue(a1 > a2);
}