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

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