Question

我正在运行一个小代码来测试Java8 lambda的Person功能。我有name类型的对象列表，其中包含字段age和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); } }。我试图找到使用Java8的reduce功能的所有人的年龄总和。代码如下：

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

当我运行上面的代码时，我收到以下错误，虽然我的IDE没有显示任何编译错误。

{{1}}

Answer 1

这是javac编译器中的已知错误。将编译器更新到版本1.8.0_40（至少）或明确指定lambda参数类型（(Integer result, Person person) -> ...）。

Answer 2

虽然Tagir's answer更直接地回答了您的问题（关于编译器错误），但值得指出的是，有一种更好的方法来编写您的解决方案，这不仅更清晰，更简单，但不会遇到这个错误：

class Data {
    private $property, $property2;

    public function  __construct($property,$property2) {
        $this->Property = $property;
        $this->Property2 = $property2;
    }
}

$dataArray = array();

$dataArray[] = new Data('Name',Image);
$dataArray[] = new Data('Name',Image);

print "<pre>";
print_r($dataArray);
print "</pre>";

for ($i=0; $i < count($dataArray); $i++) { 
    // do something with the array of objects
}

Answer 3

尼斯。可能你应该提出一个JDK错误（但首先尝试使用最新版本的代码）。但是代码没有做你想要做的事情。本教程有一个如何正确执行此操作的示例：https://docs.oracle.com/javase/tutorial/collections/streams/reduction.html

在您的代码中result += ...毫无意义，因为它是一个本地参数。而且，我相信编译器很难推断它的类型。

BTW，reduce只有两个参数，你提供三个。

Java8中Lambda表达式的内部编译器错误

3 个答案: