Question

我正在尝试为Guava的ImmutableList构建Java 8 Collector实现。但我不断得到奇怪的空指针错误。我的收藏家有什么问题？

java.lang.NullPointerException at java.util.stream.ReferencePipeline.collect（未知来源）at com.swa.rm.common.test.TestLauncher.main（TestLauncher.java:50）

是我尝试运行此代码时收到的错误...

ImmutableList.of（1,5,2,7,8,2,4） .stream（）。filter（i - ＆gt; i％2 == 0） .collect（new ImmutableListCollector（）） .forEach（i - ＆gt; System.out.println（i））;

调用此收集器

package com.swa.rm.common.stream;

import java.util.EnumSet;
import java.util.Set;
import java.util.function.BiConsumer;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.stream.Collector;

import com.google.common.collect.ImmutableList;

public class ImmutableListCollector<T> implements Collector<T, ImmutableList.Builder<T>, ImmutableList<T>> {

 @Override
 public Supplier<ImmutableList.Builder<T>> supplier() {
  return ImmutableList::builder;
 } 

 @Override
 public BiConsumer<ImmutableList.Builder<T>, T> accumulator() {
  return (builder, t) -> builder.add(t);
 }

 @Override
 public BinaryOperator<ImmutableList.Builder<T>>  combiner() {
  return (left, right) -> { 
   left.addAll(right.build());
   return left;
  };
 }

 @Override
 public Function<ImmutableList.Builder<T>, ImmutableList<T>> finisher() {
  return null;
 }

 @Override
 public Set<Characteristics> characteristics() {
  return EnumSet.of(Characteristics.UNORDERED);
 }

}

UPDATE ::这是最终的实现，现在似乎可以正常工作。

import java.util.EnumSet;
import java.util.Set;
import java.util.function.BiConsumer;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.stream.Collector;

import com.google.common.collect.ImmutableList;

public class ImmutableListCollector<T> implements Collector<T, ImmutableList.Builder<T>, ImmutableList<T>> {


         @Override
         public Supplier<ImmutableList.Builder<T>> supplier() {
          return (() -> ImmutableList.builder());
         } 

         @Override
         public BiConsumer<ImmutableList.Builder<T>, T> accumulator() {
          return (builder, t) -> builder.add(t);
         }

         @Override
         public BinaryOperator<ImmutableList.Builder<T>>  combiner() {
          return (left, right) -> { 
           left.addAll(right.build());
           return left;
          };
         }

         @Override
         public Function<ImmutableList.Builder<T>, ImmutableList<T>> finisher() {
             return (b -> builder.build());
         }

         @Override
         public Set<Characteristics> characteristics() {
          return EnumSet.of(Characteristics.UNORDERED);
         }

}

Answer 1

您的finisher()函数为null;我不确定你的期望。

我的ImmutableList收集器不工作？

1 个答案: