How to modify a List value while debugging in IntelliJ

Question

I need to change a variable during debugging an application. Until now it was just basic variables which could directly be set. Now I need to clear an array so that isEmpty() returns true;

ArrayList<String> someList = new ArrayList<String>;
someList.add("1");
...
if(someList.isEmpty()){ //break point
//need to enter here
}

In the intellij debugger I see:

someList={ArrayList@4271} size=1

I used the 'setValue' method of the debugger and tried: new ArrayList<String>() or someList = new ArrayList<String>()

which results in

someList={ArrayList@4339} size=0

However if I continue I get a NullPointerException when the isEmpty() is called. So my question: How can I inject an empty ArrayList without getting a NPE?

The text of the NPe is: java.lang.NullPointerException: Attempt to invoke interface method 'boolean java.util.List.isEmpty()' on a null object reference

score 27 · Accepted Answer · edited Apr 20 '16 at 13:43

27

Did you try to use the "Evaluate expression" during debug ("Alt + F8" on Windows) ?

In this window you can write :

 someList.clear();

or

someList = new ArrayList<String>();

And it should do the trick.

edited Apr 20 '16 at 13:43

Lonzak

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answered Apr 20 '16 at 13:34

Guillaume M

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Oh Man, I was searching for this everywhere, finally found it thanks a lot for answering :) – Sudhanshu Gaur Dec 03 '18 at 20:27
3

Doesn't work with Kotlin: `Assignments are not expressions, and only expressions are allowed in this context` – Nicolai Weitkemper Jul 10 '19 at 17:36

score 7 · Answer 2 · answered Apr 20 '16 at 13:36

7

Stop the breakpoint at if(someList.isEmpty()), press ALT + F8 (evaluate expression), type someList.clear(), press Evaluate and just proceed on debugging. Now it will definitly enter the if condition.

answered Apr 20 '16 at 13:36

dambros

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thanks, yeah that did the trick - your answer is correct. Guillaume was a bit faster so I accepted his answer - can't accept both ;-) – Lonzak Apr 20 '16 at 13:47
No problem. Glad to be able to help. – dambros Apr 20 '16 at 13:49

score 0 · Answer 3 · answered Nov 20 '18 at 20:22

0

I have this issue, but I am using Kotlin, which you can do (someList as ArrayList<*>).clear()

answered Nov 20 '18 at 20:22

Xianwei

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How to modify a List value while debugging in IntelliJ

3 Answers3