Question

我正在编写一个小程序来生成一些随机数，并想获得有关它们的统计信息。我有一个List<AtomicInteger>我想用它来输出一些信息。

我尝试使用以下代码将列表缩减为其总数（例如，所有AtomicIntegers的值的总和）：

// not my real initialization code, just for illustration purposes
List<AtomicInteger> values = new ArrayList<>();
for(int i = 0; i < 10; i++) {
    values.add(new AtomicInteger());
}

// this is the problematic reduce
int total = values
    .parallelStream()
    .reduce(0, 
        (currentValue, currentAtomic) 
            -> currentValue + currentAtomic.get(),
        (combiner1, combiner2) -> combiner1 + combiner2);

我的IDE（IntelliJ）在这一行上没有任何问题，但是使用我当前的javac版本（1.8.0_31），这一行导致编译器出现NullPointerException：

An exception has occurred in the compiler (1.8.0_31). 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.lang.NullPointerException
    at com.sun.tools.javac.code.Types.isConvertible(Types.java:290)
    at com.sun.tools.javac.comp.Check.assertConvertible(Check.java:922)
    at com.sun.tools.javac.comp.Check.checkMethod(Check.java:876)
    at com.sun.tools.javac.comp.Attr.checkMethod(Attr.java:3838)
    at com.sun.tools.javac.comp.Attr.checkIdInternal(Attr.java:3615)
    at com.sun.tools.javac.comp.Attr.checkMethodIdInternal(Attr.java:3522)
    at com.sun.tools.javac.comp.Attr.checkMethodId(Attr.java:3501)
    at com.sun.tools.javac.comp.Attr.checkId(Attr.java:3488)
    at com.sun.tools.javac.comp.Attr.visitSelect(Attr.java:3370)
    at com.sun.tools.javac.tree.JCTree$JCFieldAccess.accept(JCTree.java:1897)
    at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:607)
    at com.sun.tools.javac.comp.Attr.visitApply(Attr.java:1843)
    at com.sun.tools.javac.tree.JCTree$JCMethodInvocation.accept(JCTree.java:1465)
    at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:607)
    at com.sun.tools.javac.comp.Attr.attribExpr(Attr.java:649)
    at com.sun.tools.javac.comp.Attr.visitVarDef(Attr.java:1093)
    at com.sun.tools.javac.tree.JCTree$JCVariableDecl.accept(JCTree.java:852)
    at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:607)
    at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:676)
    at com.sun.tools.javac.comp.Attr.attribStats(Attr.java:692)
    at com.sun.tools.javac.comp.Attr.visitBlock(Attr.java:1142)
    at com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:909)
    at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:607)
    at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:676)
    at com.sun.tools.javac.comp.Attr.visitMethodDef(Attr.java:1035)
    at com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:778)
    at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:607)
    at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:676)
    at com.sun.tools.javac.comp.Attr.attribClassBody(Attr.java:4342)
    at com.sun.tools.javac.comp.Attr.attribClass(Attr.java:4252)
    at com.sun.tools.javac.comp.Attr.attribClass(Attr.java:4181)
    at com.sun.tools.javac.comp.Attr.attrib(Attr.java:4156)
    at com.sun.tools.javac.main.JavaCompiler.attribute(JavaCompiler.java:1248)
    at com.sun.tools.javac.main.JavaCompiler.compile2(JavaCompiler.java:901)
    at com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:860)
    at com.sun.tools.javac.main.Main.compile(Main.java:523)
    at com.sun.tools.javac.main.Main.compile(Main.java:381)
    at com.sun.tools.javac.main.Main.compile(Main.java:370)
    at com.sun.tools.javac.main.Main.compile(Main.java:361)
    at com.sun.tools.javac.Main.compile(Main.java:56)
    at com.sun.tools.javac.Main.main(Main.java:42)

我知道可能有其他方法可以实现相同的reduce，但我真的希望保留流的并行特性（因为我的原始代码有很多值）。任何人都可以建议我另一个并行减少选项，不会崩溃我的编译器，并可以完成相同的结果吗？

我还想知道这是否只是我遇到的问题，或者其他人是否可以重现此编译器错误。

Answer 1

或者，您可以使用reduce和mapToInt来代替使用sum：

int total = values.stream().mapToInt(AtomicInteger::get).sum();

Answer 2

不会使编译器崩溃的版本：

// not my real initialization code, just for illustration purposes
        List<AtomicInteger> values = new ArrayList<>();
        for(int i = 0; i < 10; i++) {
            values.add(new AtomicInteger());
        }

// this is the problematic reduce
        BiFunction<Integer, AtomicInteger, Integer> accumulator = 
                (currentValue, currentAtomic) -> currentValue + currentAtomic.get();
        BinaryOperator<Integer> combiner = (combiner1, combiner2) -> combiner1 + combiner2;
        int total = values
                .parallelStream()
                .reduce(0, accumulator, combiner);

Java 8编译器bug与Stream和reduce

2 个答案: