How to initialize the Map type variable to solve NullPointerException error?

Question

I am not sure which variable is null when the for loop is executed. Is it the entry that is uninitialized?

MainActivity.java

    static class Gist {
        Map<String, GistFile> files;
    }

    static class GistFile {
        String content;
    }

    Gist gist = gistJsonAdapter.fromJson(responseBody.source());

    for (Map.Entry<String, GistFile> entry : gist.files.entrySet()) {
              System.out.println(entry.getKey());
              System.out.println(entry.getValue().content);
    }

Error Message

java.lang.NullPointerException: Attempt to invoke interface method 'java.util.Set java.util.Map.entrySet()' on a null object reference
        at com.example.soccerapi.MainActivity$1.onResponse(MainActivity.java:76)
        at okhttp3.internal.connection.RealCall$AsyncCall.run(RealCall.kt:519)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
        at java.lang.Thread.run(Thread.java:919)

NullPointerException cause by reference to a field or call a method from a null object, so just find out which is null should solve your problem. An easy way to find which is null, you may print variable from top to bottom, in your case, print `gistJsonAdapter` checkout if it was null, then print `responseBody `, then `responseBody.source()` and `gist ` and so on — Wangbo, Oct 10 '20 at 06:39
Could you also show a `JSON` payload you want to deserialise? It must contain `JSON Object` with key `files`. If it does not, `NPE` is thrown later when you want to access it. — Michał Ziober, Oct 10 '20 at 09:56

score 0 · Accepted Answer · answered Oct 10 '20 at 06:36

0

gist.files is null. Since it comes from the responseBody either make sure that it is set in the response or check if it is null before using it.

answered Oct 10 '20 at 06:36

Henry

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How do I initialize the gist.files variable? – oc2020 Oct 10 '20 at 15:55
`gistJsonAdapter.fromJson(...)` should do that. – Henry Oct 10 '20 at 16:02

How to initialize the Map type variable to solve NullPointerException error?

1 Answers1