What are "watches" in IntelliJ and how to use them?

Question

When I debug an app, in the debug tool window there is a Watches window. I have read this manual over and over, but cannot find any practical usage of Watches.

Somehow, I think this is a cool and useful tool and I lack from not using it.

Can someone explain when should I use it and give a few samples? Ideally, the description will be bound to a concrete (imaginary) situation so that I better apply it in my work.

Answer 1

This section allows you to define expressions which you'd like to see how they evolve/change with every step of your debug process, without manually inspecting all the available objects and their properties. Let's take the following simple sample which intentionally throws a NullPointerException (NPE):

public class WatchSample {

    static class Student {
        public static final int CREDITS_REQUIRED_FOR_GRADUATION = 10;
        private String name;
        private Integer credits;

        public Student(String name, Integer credits) {
            this.name = name;
            this.credits = credits;
        }

        String getName() {
            return name;
        }

        public boolean hasGraduated() {
            return credits >= CREDITS_REQUIRED_FOR_GRADUATION;
        }

        public Integer getCredits() {
            return credits;
        }
    }

    public static void main(String[] args) throws Exception {
        List<Student> students = simulateReadingFromDB();

        for (Student student : students) {
            if (student.hasGraduated()) {
                System.out.println("Student [" + student.getName() + "] has graduated with [" + student.getCredits() + "] credits");
            }
        }
    }

    private static List<Student> simulateReadingFromDB() {
        List<Student> students = new ArrayList<>(3);
        students.add(new Student("S1", 15));
        students.add(new Student("S2", null)); // <- simulate some mistake
        students.add(new Student("S3", 10));
        return students;
    }
}

At some point in time you may wonder how come you get a NPE and what needs fixing. So just set a breakpoint, add a few watches and carefully step through the lines. Eventually you'll end up with the troublemaker in sight:

Of course this is a basic example and should be taken as such. Within a regular app you'll probably have more complex scenarios and expressions you'd like to inspect, and this will make more sense, for example: if (((position > 0 && position < MAX) || (position < 0 && position > MIN) && (players(currentPlayer).isNotDead() && move.isAllowed()) && time.notUp())..... In this case you can evaluate the sub-expressions to see which one returns false

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**Note**: You could also make the breakpoint conditional so that the program will pause only when that specific event occurs: