Why Unit Tests Will Save You a Lot of Time

When you as a developer hear that you should write unit tests, it sounds like more effort at first. The code you have written works, as you have already seen by trying it.

However, the world around it is constantly changing. And how can you be sure that the code still works despite changes from the outside?

Date Format?

Let's assume that in our program there is a function to parse strings as dates.

 * @param input a date with format "dd.MM.yy HH:mm"
public static Date parseDate(String input) throws ParseException {
    return DateFormat.getInstance().parse(input);

We use a class from the JDK here and even describe in the Javadoc which format the input parameter should have. Once we have tried this, what else can go wrong? Surely this will work forever.

Even after updating from Java 8 to Java 11, everyone would assume that the function still works as described, right? After all, we are only using a class from the JDK.

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Maven Doesn't Execute All JUnit Tests by Default

We as developers write many unit tests. The CI pipeline will run all tests with every build. We trust that, if the status is green, everything should be okay.

But what if the unit tests should have failed? I recently learned that Maven does not execute all tests by default.

Consider this simple class:

public class MyTests {

    public void firstTest() {
        assertEquals(2, 1 + 1);

    public static class NestedTests {

        public void secondTest() {
            assertEquals(3, 1 + 1);

We see that firstTest should succeed and secondTest should fail.

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