Tutorial: Set value
From this tutorial, you will learn how to use one of the most basic, but very useful debugger features: Set Value.
While debugging, you get the information on the values of your variables and examine them in order to understand why a program behaves in a particular way. Other times, you would want to reproduce some bug, which depends on some variable. To do that, you would need this variable to hold a particular value.
Reproducing some conditions without modifying the program at runtime may be tedious and time-consuming, so most of the time you would benefit from setting the variable value right from the debugger.
Let's look at the following simple program:
In this code we have an instance variable (
fish) that is printed out twice. The getter of the
fish variable employs lazy initialization, which means the object is only initialized when it is first needed.
Now what if
fish has already been initialized (suppose we are at
line n1 ), and we want to look into the
initFish method? This method will only be executed if
(fish == null) evaluates to
In this simple case we could just put a breakpoint in the method and restart the session. In more complex cases, however, you may find it very inconvenient to restart the session and perform all the steps that lead to a certain state. Now let's learn the smarter way.
So, we are at
line n1, and we need to step through the
ArrayList initialization in the
Put a breakpoint in the
On the Variables tab, expand
fish, and select Set value.
null. Press Enter.
Press F9 to resume the debugger session.
Now the condition evaluates to
true, and we can step through the
initFish method and see how it performs the initialization.
The example in this tutorial illustrated how you can modify the variables at runtime to change the flow of your program. Although the example is very simple, you can apply the same principle in more complex projects, where this feature will save you a lot of time.