Internal compiler error for Lambda Expression in Java8

Question

I am running a small code to test the reduce() functionality of Java8 lambda. I have a list of objects of type Person having fields name and age. I am trying to find the sum of ages of all persons using reduce functionality of Java8. The code is given below:

public class Person {

    String name;
    Integer age;

    public Person(String name, Integer age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}


public class TestReduce {
    public static void main(String[] args){
       Person p1 = new Person("P1", 10);
       Person p2 = new Person("P2", 20);
       Person p3 = new Person("P3", 30);

       List<Person> personList = new ArrayList<Person>();
       personList.add(p1);
       personList.add(p2);
       personList.add(p3);

       int sumAge = personList.stream()
                       .reduce(0, (result, person) -> result += person.age, Integer::sum);
    }
}

When I run the above code, I am getting the following error although my IDE does not show any compilation error.

Using javac 1.8.0_25 to compile java sources
java: An exception has occurred in the compiler (1.8.0_25). Please file a bug at the Java Developer Connection (http://java.sun.com/webapps/bugreport)  after checking the Bug Parade for duplicates. Include your program and the following diagnostic in your report.  Thank you.
java: java.lang.NullPointerException
java:   at com.sun.tools.javac.code.Types.isConvertible(Types.java:290)
java:   at com.sun.tools.javac.comp.Check.assertConvertible(Check.java:922)
java:   at com.sun.tools.javac.comp.Check.checkMethod(Check.java:876)
java:   at com.sun.tools.javac.comp.Attr.checkMethod(Attr.java:3838)
java:   at com.sun.tools.javac.comp.Attr.checkIdInternal(Attr.java:3615)
java:   at com.sun.tools.javac.comp.Attr.checkMethodIdInternal(Attr.java:3522)
java:   at com.sun.tools.javac.comp.Attr.checkMethodId(Attr.java:3501)
java:   at com.sun.tools.javac.comp.Attr.checkId(Attr.java:3488)
java:   at com.sun.tools.javac.comp.Attr.visitSelect(Attr.java:3370)
java:   at com.sun.tools.javac.tree.JCTree$JCFieldAccess.accept(JCTree.java:1897)
java:   at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:607)
java:   at com.sun.tools.javac.comp.Attr.visitApply(Attr.java:1843)
java:   at com.sun.tools.javac.tree.JCTree$JCMethodInvocation.accept(JCTree.java:1465)
java:   at com.sun.tools.javac.comp.Attr.attribExpr(Attr.java:649)
java:   at com.sun.tools.javac.comp.Attr.visitVarDef(Attr.java:1093)
java:   at com.sun.tools.javac.tree.JCTree$JCVariableDecl.accept(JCTree.java:852)
java:   at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:676)
java:   at com.sun.tools.javac.comp.Attr.attribStats(Attr.java:692)
java:   at com.sun.tools.javac.comp.Attr.visitBlock(Attr.java:1142)
java:   at com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:909)
java:   at com.sun.tools.javac.comp.Attr.visitMethodDef(Attr.java:1035)
java:   at com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:778)
java:   at com.sun.tools.javac.comp.Attr.attribClassBody(Attr.java:4342)
java:   at com.sun.tools.javac.comp.Attr.attribClass(Attr.java:4252)
java:   at com.sun.tools.javac.comp.Attr.attribClass(Attr.java:4181)
java:   at com.sun.tools.javac.comp.Attr.attrib(Attr.java:4156)
java:   at com.sun.tools.javac.main.JavaCompiler.attribute(JavaCompiler.java:1248)
java:   at com.sun.tools.javac.main.JavaCompiler.compile2(JavaCompiler.java:901)
java:   at com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:860)
java:   at com.sun.tools.javac.main.Main.compile(Main.java:523)
java:   at com.sun.tools.javac.api.JavacTaskImpl.doCall(JavacTaskImpl.java:129)
java:   at com.sun.tools.javac.api.JavacTaskImpl.call(JavacTaskImpl.java:138)
java:   at org.jetbrains.jps.javac.JavacMain.compile(JavacMain.java:165)
java:   at org.jetbrains.jps.incremental.java.JavaBuilder.compileJava(JavaBuilder.java:407)
java:   at org.jetbrains.jps.incremental.java.JavaBuilder.compile(JavaBuilder.java:304)
java:   at org.jetbrains.jps.incremental.java.JavaBuilder.doBuild(JavaBuilder.java:210)
java:   at org.jetbrains.jps.incremental.java.JavaBuilder.build(JavaBuilder.java:182)
java:   at org.jetbrains.jps.incremental.IncProjectBuilder.runModuleLevelBuilders(IncProjectBuilder.java:1106)
java:   at org.jetbrains.jps.incremental.IncProjectBuilder.runBuildersForChunk(IncProjectBuilder.java:814)
java:   at org.jetbrains.jps.incremental.IncProjectBuilder.buildTargetsChunk(IncProjectBuilder.java:862)
java:   at org.jetbrains.jps.incremental.IncProjectBuilder.buildChunkIfAffected(IncProjectBuilder.java:777)
java:   at org.jetbrains.jps.incremental.IncProjectBuilder.buildChunks(IncProjectBuilder.java:600)
java:   at org.jetbrains.jps.incremental.IncProjectBuilder.runBuild(IncProjectBuilder.java:352)
java:   at org.jetbrains.jps.incremental.IncProjectBuilder.build(IncProjectBuilder.java:184)
java:   at org.jetbrains.jps.cmdline.BuildRunner.runBuild(BuildRunner.java:129)
java:   at org.jetbrains.jps.cmdline.BuildSession.runBuild(BuildSession.java:224)
java:   at org.jetbrains.jps.cmdline.BuildSession.run(BuildSession.java:113)
java:   at org.jetbrains.jps.cmdline.BuildMain$MyMessageHandler$1.run(BuildMain.java:157)
java:   at org.jetbrains.jps.service.impl.SharedThreadPoolImpl$1.run(SharedThreadPoolImpl.java:41)
java:   at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
java:   at java.util.concurrent.FutureTask.run(FutureTask.java:266)
java:   at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
java:   at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
java:   at java.lang.Thread.run(Thread.java:745)
java: Errors occurred while compiling module 'Lambda'
Compilation completed with 1 error and 0 warnings in 2 sec
1 error
0 warnings
Error:java: Compilation failed: internal java compiler error

Answer 1

Although Tagir's answer more directly answers your question (about the compiler error), it's worth pointing out that there's a better way to code your solution that is not only far cleaner and simpler, but would not encounter that error:

int sumAge = personList.stream().mapToInt(Person::age).sum();

Answer 2

Nice. Probably you should raise a JDK bug (but try the code with the latest version first). But the code is not doing what you are trying to do. The tutorial has an example of how to do it properly: https://docs.oracle.com/javase/tutorial/collections/streams/reduction.html

In your code result += ... makes no sense, as it is a local parameter. And, I believe the compiler struggles to infer its type.

BTW, reduce takes only two arguments, and you are providing three.

Answer 3

This is known bug in javac compiler. Either update the compiler to version 1.8.0_40 (at least) or specify the lambda argument types explicitly ((Integer result, Person person) -> ...).