Getting a NullPointerException at build time

Question

Here is the class that causes the NullPointerException:

public class PersistentList<T extends RealmObject> extends RealmObject {
    private String mTitle;
    private RealmList<T> mValue;

    public PersistentList(){
    }

    public PersistentList(String title, RealmList<T> value) {
        mTitle = title;
        mValue = value;
    }

    public RealmList<T> getValue() {
        return mValue;
    }

    public void setValue(RealmList<T> value) {
        mValue = value;
    }

    public void setValue(int[] itemClassesDisplayOrder) {
        mValue = new RealmList<>();
        for (int i=0;i<itemClassesDisplayOrder.length;i++){
            mValue.add((T) new RealmInt(itemClassesDisplayOrder[i]));
        }
    }
}

The class compiled perfectly fine until I added setValue(int[]). There seems to be no error in the logcat regarding the class. Any idea why I'm getting the NullPointerException?

Here is the output from the Gradle Console:

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:compileDebugJavaWithJavac'.
> java.lang.NullPointerException

* Try:
Run with --debug option to get more log output.

* Exception is:
org.gradle.api.tasks.TaskExecutionException: Execution failed for task ':app:compileDebugJavaWithJavac'.
    at org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter.executeActions(ExecuteActionsTaskExecuter.java:69)
    at org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter.execute(ExecuteActionsTaskExecuter.java:46)
    at org.gradle.api.internal.tasks.execution.PostExecutionAnalysisTaskExecuter.execute(PostExecutionAnalysisTaskExecuter.java:35)
    at org.gradle.api.internal.tasks.execution.SkipUpToDateTaskExecuter.execute(SkipUpToDateTaskExecuter.java:64)
    at org.gradle.api.internal.tasks.execution.ValidatingTaskExecuter.execute(ValidatingTaskExecuter.java:58)
    at org.gradle.api.internal.tasks.execution.SkipEmptySourceFilesTaskExecuter.execute(SkipEmptySourceFilesTaskExecuter.java:52)
    at org.gradle.api.internal.tasks.execution.SkipTaskWithNoActionsExecuter.execute(SkipTaskWithNoActionsExecuter.java:52)
    at org.gradle.api.internal.tasks.execution.SkipOnlyIfTaskExecuter.execute(SkipOnlyIfTaskExecuter.java:53)
    at org.gradle.api.internal.tasks.execution.ExecuteAtMostOnceTaskExecuter.execute(ExecuteAtMostOnceTaskExecuter.java:43)
    at org.gradle.execution.taskgraph.DefaultTaskGraphExecuter$EventFiringTaskWorker.execute(DefaultTaskGraphExecuter.java:203)
    at org.gradle.execution.taskgraph.DefaultTaskGraphExecuter$EventFiringTaskWorker.execute(DefaultTaskGraphExecuter.java:185)
    at org.gradle.execution.taskgraph.AbstractTaskPlanExecutor$TaskExecutorWorker.processTask(AbstractTaskPlanExecutor.java:66)
    at org.gradle.execution.taskgraph.AbstractTaskPlanExecutor$TaskExecutorWorker.run(AbstractTaskPlanExecutor.java:50)
    at org.gradle.execution.taskgraph.DefaultTaskPlanExecutor.process(DefaultTaskPlanExecutor.java:25)
    at org.gradle.execution.taskgraph.DefaultTaskGraphExecuter.execute(DefaultTaskGraphExecuter.java:110)
    at org.gradle.execution.SelectedTaskExecutionAction.execute(SelectedTaskExecutionAction.java:37)
    at org.gradle.execution.DefaultBuildExecuter.execute(DefaultBuildExecuter.java:37)
    at org.gradle.execution.DefaultBuildExecuter.access$000(DefaultBuildExecuter.java:23)
    at org.gradle.execution.DefaultBuildExecuter$1.proceed(DefaultBuildExecuter.java:43)
    at org.gradle.execution.DryRunBuildExecutionAction.execute(DryRunBuildExecutionAction.java:32)
    at org.gradle.execution.DefaultBuildExecuter.execute(DefaultBuildExecuter.java:37)
    at org.gradle.execution.DefaultBuildExecuter.execute(DefaultBuildExecuter.java:30)
    at org.gradle.initialization.DefaultGradleLauncher$4.run(DefaultGradleLauncher.java:154)
    at org.gradle.internal.Factories$1.create(Factories.java:22)
    at org.gradle.internal.progress.DefaultBuildOperationExecutor.run(DefaultBuildOperationExecutor.java:90)
    at org.gradle.internal.progress.DefaultBuildOperationExecutor.run(DefaultBuildOperationExecutor.java:52)
    at org.gradle.initialization.DefaultGradleLauncher.doBuildStages(DefaultGradleLauncher.java:151)
    at org.gradle.initialization.DefaultGradleLauncher.access$200(DefaultGradleLauncher.java:32)
    at org.gradle.initialization.DefaultGradleLauncher$1.create(DefaultGradleLauncher.java:99)
    at org.gradle.initialization.DefaultGradleLauncher$1.create(DefaultGradleLauncher.java:93)
    at org.gradle.internal.progress.DefaultBuildOperationExecutor.run(DefaultBuildOperationExecutor.java:90)
    at org.gradle.internal.progress.DefaultBuildOperationExecutor.run(DefaultBuildOperationExecutor.java:62)
    at org.gradle.initialization.DefaultGradleLauncher.doBuild(DefaultGradleLauncher.java:93)
    at org.gradle.initialization.DefaultGradleLauncher.run(DefaultGradleLauncher.java:82)
    at org.gradle.launcher.exec.InProcessBuildActionExecuter$DefaultBuildController.run(InProcessBuildActionExecuter.java:94)
    at org.gradle.tooling.internal.provider.runner.BuildModelActionRunner.run(BuildModelActionRunner.java:46)
    at org.gradle.launcher.exec.ChainingBuildActionRunner.run(ChainingBuildActionRunner.java:35)
    at org.gradle.tooling.internal.provider.runner.SubscribableBuildActionRunner.run(SubscribableBuildActionRunner.java:58)
    at org.gradle.launcher.exec.ChainingBuildActionRunner.run(ChainingBuildActionRunner.java:35)
    at org.gradle.launcher.exec.InProcessBuildActionExecuter.execute(InProcessBuildActionExecuter.java:43)
    at org.gradle.launcher.exec.InProcessBuildActionExecuter.execute(InProcessBuildActionExecuter.java:28)
    at org.gradle.launcher.exec.ContinuousBuildActionExecuter.execute(ContinuousBuildActionExecuter.java:78)
    at org.gradle.launcher.exec.ContinuousBuildActionExecuter.execute(ContinuousBuildActionExecuter.java:48)
    at org.gradle.launcher.daemon.server.exec.ExecuteBuild.doBuild(ExecuteBuild.java:52)
    at org.gradle.launcher.daemon.server.exec.BuildCommandOnly.execute(BuildCommandOnly.java:36)
    at org.gradle.launcher.daemon.server.api.DaemonCommandExecution.proceed(DaemonCommandExecution.java:120)
    at org.gradle.launcher.daemon.server.exec.WatchForDisconnection.execute(WatchForDisconnection.java:37)
    at org.gradle.launcher.daemon.server.api.DaemonCommandExecution.proceed(DaemonCommandExecution.java:120)
    at org.gradle.launcher.daemon.server.exec.ResetDeprecationLogger.execute(ResetDeprecationLogger.java:26)
    at org.gradle.launcher.daemon.server.api.DaemonCommandExecution.proceed(DaemonCommandExecution.java:120)
    at org.gradle.launcher.daemon.server.exec.RequestStopIfSingleUsedDaemon.execute(RequestStopIfSingleUsedDaemon.java:34)
    at org.gradle.launcher.daemon.server.api.DaemonCommandExecution.proceed(DaemonCommandExecution.java:120)
    at org.gradle.launcher.daemon.server.exec.ForwardClientInput$2.call(ForwardClientInput.java:74)
    at org.gradle.launcher.daemon.server.exec.ForwardClientInput$2.call(ForwardClientInput.java:72)
    at org.gradle.util.Swapper.swap(Swapper.java:38)
    at org.gradle.launcher.daemon.server.exec.ForwardClientInput.execute(ForwardClientInput.java:72)
    at org.gradle.launcher.daemon.server.api.DaemonCommandExecution.proceed(DaemonCommandExecution.java:120)
    at org.gradle.launcher.daemon.server.health.DaemonHealthTracker.execute(DaemonHealthTracker.java:47)
    at org.gradle.launcher.daemon.server.api.DaemonCommandExecution.proceed(DaemonCommandExecution.java:120)
    at org.gradle.launcher.daemon.server.exec.LogToClient.doBuild(LogToClient.java:66)
    at org.gradle.launcher.daemon.server.exec.BuildCommandOnly.execute(BuildCommandOnly.java:36)
    at org.gradle.launcher.daemon.server.api.DaemonCommandExecution.proceed(DaemonCommandExecution.java:120)
    at org.gradle.launcher.daemon.server.exec.EstablishBuildEnvironment.doBuild(EstablishBuildEnvironment.java:72)
    at org.gradle.launcher.daemon.server.exec.BuildCommandOnly.execute(BuildCommandOnly.java:36)
    at org.gradle.launcher.daemon.server.api.DaemonCommandExecution.proceed(DaemonCommandExecution.java:120)
    at org.gradle.launcher.daemon.server.health.HintGCAfterBuild.execute(HintGCAfterBuild.java:41)
    at org.gradle.launcher.daemon.server.api.DaemonCommandExecution.proceed(DaemonCommandExecution.java:120)
    at org.gradle.launcher.daemon.server.exec.StartBuildOrRespondWithBusy$1.run(StartBuildOrRespondWithBusy.java:50)
    at org.gradle.launcher.daemon.server.DaemonStateCoordinator$1.run(DaemonStateCoordinator.java:246)
    at org.gradle.internal.concurrent.ExecutorPolicy$CatchAndRecordFailures.onExecute(ExecutorPolicy.java:54)
    at org.gradle.internal.concurrent.StoppableExecutorImpl$1.run(StoppableExecutorImpl.java:40)
Caused by: java.lang.RuntimeException: java.lang.NullPointerException
    at com.sun.tools.javac.main.Main.compile(Main.java:553)
    at com.sun.tools.javac.api.JavacTaskImpl.doCall(JavacTaskImpl.java:129)
    at com.sun.tools.javac.api.JavacTaskImpl.call(JavacTaskImpl.java:138)
    at org.gradle.api.internal.tasks.compile.JdkJavaCompiler.execute(JdkJavaCompiler.java:45)
    at org.gradle.api.internal.tasks.compile.JdkJavaCompiler.execute(JdkJavaCompiler.java:33)
    at org.gradle.api.internal.tasks.compile.NormalizingJavaCompiler.delegateAndHandleErrors(NormalizingJavaCompiler.java:101)
    at org.gradle.api.internal.tasks.compile.NormalizingJavaCompiler.execute(NormalizingJavaCompiler.java:50)
    at org.gradle.api.internal.tasks.compile.NormalizingJavaCompiler.execute(NormalizingJavaCompiler.java:36)
    at org.gradle.api.internal.tasks.compile.CleaningJavaCompilerSupport.execute(CleaningJavaCompilerSupport.java:34)
    at org.gradle.api.internal.tasks.compile.CleaningJavaCompilerSupport.execute(CleaningJavaCompilerSupport.java:25)
    at org.gradle.api.tasks.compile.JavaCompile.performCompilation(JavaCompile.java:157)
    at org.gradle.api.tasks.compile.JavaCompile.compile(JavaCompile.java:139)
    at org.gradle.api.tasks.compile.JavaCompile.compile(JavaCompile.java:93)
    at com.android.build.gradle.tasks.factory.AndroidJavaCompile.compile(AndroidJavaCompile.java:49)
    at org.gradle.internal.reflect.JavaMethod.invoke(JavaMethod.java:75)
    at org.gradle.api.internal.project.taskfactory.AnnotationProcessingTaskFactory$IncrementalTaskAction.doExecute(AnnotationProcessingTaskFactory.java:244)
    at org.gradle.api.internal.project.taskfactory.AnnotationProcessingTaskFactory$StandardTaskAction.execute(AnnotationProcessingTaskFactory.java:220)
    at org.gradle.api.internal.project.taskfactory.AnnotationProcessingTaskFactory$IncrementalTaskAction.execute(AnnotationProcessingTaskFactory.java:231)
    at org.gradle.api.internal.project.taskfactory.AnnotationProcessingTaskFactory$StandardTaskAction.execute(AnnotationProcessingTaskFactory.java:209)
    at org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter.executeAction(ExecuteActionsTaskExecuter.java:80)
    at org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter.executeActions(ExecuteActionsTaskExecuter.java:61)
    ... 70 more
Caused by: java.lang.NullPointerException
    at io.realm.processor.ClassMetaData.checkListTypes(ClassMetaData.java:175)
    at io.realm.processor.ClassMetaData.generate(ClassMetaData.java:118)
    at io.realm.processor.RealmProcessor.process(RealmProcessor.java:146)
    at com.sun.tools.javac.processing.JavacProcessingEnvironment.callProcessor(JavacProcessingEnvironment.java:794)
    at com.sun.tools.javac.processing.JavacProcessingEnvironment.discoverAndRunProcs(JavacProcessingEnvironment.java:705)
    at com.sun.tools.javac.processing.JavacProcessingEnvironment.access$1800(JavacProcessingEnvironment.java:91)
    at com.sun.tools.javac.processing.JavacProcessingEnvironment$Round.run(JavacProcessingEnvironment.java:1035)
    at com.sun.tools.javac.processing.JavacProcessingEnvironment.doProcessing(JavacProcessingEnvironment.java:1176)
    at com.sun.tools.javac.main.JavaCompiler.processAnnotations(JavaCompiler.java:1170)
    at com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:856)
    at com.sun.tools.javac.main.Main.compile(Main.java:523)
    ... 90 more


BUILD FAILED

Total time: 3.913 secs
Stopped 0 compiler daemon(s).

There's a chance that `PersistentList` can be instantiated with `mTitle` and `mValue` defaulting to `null`. — Makoto, Oct 10 '16 at 19:06
@Makoto yes but in the method, `mValue` is initialized, so there's never a possibility of a `NullPointerException` within the class — shoe, Oct 10 '16 at 19:08
If I declare `new PersistentList()` anywhere - even by a framework - then `mValue` and `mTitle` can be null. Perhaps you should give us more context into how this class is instantiated? — Makoto, Oct 10 '16 at 19:09
@Makoto yes, but it is not illegal in java to have a null field, so why am I getting this NullPointerException? it seems like a couple of you didn't read the question, because if you did, you'd know this is a build time error, and the class never gets instantiated. — shoe, Oct 10 '16 at 19:12
You seem to be misunderstanding me. There's nothing wrong with having a `null` field. Attempting to dereference a `null` field leads to the NPE. NPEs don't occur at compile time unless there's some process that's actually invoking or executing your code at compile time which trips on this line. You are **strongly encouraged** to provide a stack trace to this so that we can get closer to the bottom of this. — Makoto, Oct 10 '16 at 19:13
That's likely not code you're controlling...Remove the line and ensure that the compilation doesn't occur again. I'm unfamiliar with Realm (but very familiar with Java), so this kind of error would imply some strange processing error while Realm was doing some kind of annotation processing on your classes. — Makoto, Oct 10 '16 at 23:13

score 1 · Answer 1 · answered Oct 11 '16 at 12:11

1

RealmObjects with template parameters are not supported by Realm.

That's why you're getting this exception.

answered Oct 11 '16 at 12:11

EpicPandaForce

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score 0 · Answer 2 · answered Oct 11 '16 at 17:21

Posted an issue on Realm's github, and they made it clear that using generics as I did is not supported. Here is their response:

Using generic type there is not supported. We need to fix the target class of the RealmList as part of the schema.

I think we can improve the error message to make it clear that the generic type is not supported.

Getting a NullPointerException at build time

2 Answers2