Learn from Mistakes: A Quick Guide from 100 Java Mistakes and How to Avoid Them

Learn a little today, avoid a lot of bugs tomorrow

The book 100 Java Mistakes and How to Avoid Them is a great book for developers who want to grow faster by learning from real-world problems. Juniors and mid-level engineers will benefit the most, but even experienced developers will find surprising pitfalls they may have never seen before.

In this short guide, we look at one example from the book, the unexpected behavior of the peek method, and how a small detail can change the output of your code.

A Peek Problem You Might Not Expect

public static void main(String[] args) {
    AtomicLong counter = new AtomicLong();
    long count = Stream.of(1,2,3,4,5,6,7,8,9)
            .peek(p -> counter.incrementAndGet())
            .count();
    System.out.println(count);
    System.out.println(counter.get());
}

What is the output?
Answer: it depends.

Java 8 output

9
9

Java 9 or higher

9
0

From Java 9 onward, count() can detect that the stream source has a known size.
Because of this optimization, map() and peek() may not run at all, and the runtime simply returns the list’s size.

Why This Happens

The documentation clearly states:

peek() exists mainly to support debugging

It is not guaranteed to run. It was never designed for side effects.

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Why This Book Helps You Level Up 📘

The book 100 Java Mistakes and How to Avoid Them contains a collection of mistakes from real scenarios.
Many are common, but some are deep and subtle, the kind you only learn after years of debugging or reading the right material.

Tagir Valeev explains each mistake clearly, shows why it happens, and teaches you how to avoid it.
Even if you fix only one bad habit, the book pays for itself.

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Conclusion

Learning from other developers’ mistakes is one of the fastest ways to grow.
The peek() example is a reminder that even familiar features can behave differently than we expect.

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