java - 我的 ImmutableList 收集器不工作？

Question

我正在尝试为 Guava 的 ImmutableList 构建一个 Java 8 收集器实现。但是我不断收到奇怪的空指针错误。我的收集器出了什么问题？

java.lang.NullPointerException at java.util.stream.ReferencePipeline.collect(Unknown Source) 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 -> i % 2 == 0) .collect(new ImmutableListCollector()) .forEach(i -> System.out.println(i));

调用这个收集器

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);
 }

}

更新:这是最终的实现，现在似乎可以工作了。

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() 函数是null；我不确定您的期望是什么。