Java 8 stream findFirst抛出NPE，无法从流中筛选出非Null对象

Question

我试图访问内部对象，而不会导致空指针异常。我希望避免手动编写if条件null检查，而是使用利用java 8特性。

我从findFirst方法中获得Nullpointer异常。我知道如果所选元素为null，findFirst会返回null指针异常，所以我在调用findFirst之前尝试使用过滤器（objects：：nonNull）过滤出null对象，但它仍然会抛出null指针。

看起来我的过滤不起作用，但我不知道为什么。

Please find below the code

package main.java;

import java.util.*;
import java.util.stream.Stream;


public class OptionalTest {

    public static void main(String args[]) {
        Inner in = new Inner("Has Value");
        Nested nest = new Nested(in);
        List<Nested> list = new ArrayList<Nested>();
        //list.add(nest);  //commented out to test failure scenario
        list.add(null); //added null
        list.add(null); //added null
        Outer outer = new Outer(list);
        OuterMost outermost = new OuterMost(outer);

        Optional<String> innerValue = Optional.ofNullable(outermost) // outermost var can be null, hence using Optional.ofNullable
                .map(OuterMost::getOuter)
                .map(Outer::getNested)
                .map(Collection::stream)
                .filter(Objects::nonNull)  //findFirst throws null pointer if selected element is null, hence filtering out null
                .flatMap(Stream::findFirst)
                .map(Nested::getInner)
                .map(Inner::getFoo);


        System.out.println(innerValue.orElse("No Value"));
    }
}

//Classes OuterMost>Outer>Nested(List)>Inner

class OuterMost {
    public OuterMost(Outer out) {
        outer = out;
    }

    Outer outer;

    Outer getOuter() {
        return outer;
    }
}

class Outer {
    public Outer(List<Nested> nest) {
        nested = nest;
    }

    List<Nested> nested;

    List<Nested> getNested() {
        return nested;
    }
}

class Nested {

    public Nested(Inner in) {
        inner = in;
    }

    Inner inner;

    Inner getInner() {
        return inner;
    }
}

class Inner {
    public Inner(String val) {
        foo = val;
    }

    String foo;

    String getFoo() {
        return foo;
    }
}

Answer 1

您正在调用可选的

使用

Optional<String> innerValue = Optional.ofNullable(outermost) // outermost var can be null, hence using Optional.ofNullable
            .map(OuterMost::getOuter)
            .map(Outer::getNested)
            .map(nestedList -> nestedList.stream().filter(Objects::nonNull))
            .flatMap(Stream::findFirst)
            .map(Nested::getInner)
            .map(Inner::getFoo);