Java 8中Collectors.toMap空指针异常源码分析

当需要将一个List转换为Map时，可以使用 Java 8 中的 Collectors.toMap() 方法，Map是由key-value组成的键值对集合，在使用Collectors.toMap() 方法时，如果值为空，会报空指针异常，下面通过一个实例来验证一下。

首先定义一个 Student.java 类

package com.magic.npe;

public class Student {

    private String name;
    private Integer age;

    public Student(String name, Integer age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }
}

再创建一个 Test.java 类，用来验证将 List<Student> 转换为 Map<String, Integer>。

package com.magic.npe;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Test {

    public static void main(String[] args) {
        List<Student> students = new ArrayList<>();
        students.add(new Student("张三", 18));
        students.add(new Student("李四", 21));
        students.add(new Student("王五", null));

        Map<String, Integer> studentMap = students.stream().collect(Collectors.toMap(Student::getName, Student::getAge));
        System.out.println(studentMap);
    }
}

运行程序，直接报出如下的错误信息

Exception in thread "main" java.lang.NullPointerException
    at java.util.HashMap.merge(HashMap.java:1224)
    at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
    at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
    at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
    at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
    at com.magic.npe.Test.main(Test.java:16)

查看一下 Collectors.toMap() 方法的源码，如下：

public static <T, K, U>
    Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,
                                    Function<? super T, ? extends U> valueMapper,
                                    BinaryOperator<U> mergeFunction) {
    return toMap(keyMapper, valueMapper, mergeFunction, HashMap::new);
}

默认会创建一个 HashMap，继续查看源码：

public static <T, K, U, M extends Map<K, U>>
    Collector<T, ?, M> toMap(Function<? super T, ? extends K> keyMapper,
                             Function<? super T, ? extends U> valueMapper,
                             BinaryOperator<U> mergeFunction,
                             Supplier<M> mapSupplier) {
    BiConsumer<M, T> accumulator
        = (map, element) -> map.merge(keyMapper.apply(element),
                                      valueMapper.apply(element), mergeFunction);
    return new CollectorImpl<>(mapSupplier, accumulator, mapMerger(mergeFunction), CH_ID);
}

通过 Map.merge() 方法来合并，源码如下：

default V merge(K key, V value,
                BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
    Objects.requireNonNull(remappingFunction);
    Objects.requireNonNull(value);
    V oldValue = get(key);
    V newValue = (oldValue == null) ? value :
    remappingFunction.apply(oldValue, value);
    if(newValue == null) {
        remove(key);
    } else {
        put(key, newValue);
    }
    return newValue;
}

可以看到，在 merge() 方法中，要求 value 值不能为空

Objects.requireNonNull(value);

继续查看一下 Objects.requireNonNull() 方法的源码

public static <T> T requireNonNull(T obj) {
    if (obj == null)
        throw new NullPointerException();
    return obj;
}

如果值为空，则会直接报出 NullPointerException 异常。

那么，对于这个空指针异常问题，如何解决呢？一般有两种方式：

（1）替换空值null为一个默认值，比如0

package com.magic.npe;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.stream.Collectors;

public class Test {

  public static void main(String[] args) {
      List<Student> students = new ArrayList<>();
      students.add(new Student("张三", 18));
      students.add(new Student("李四", 21));
      students.add(new Student("王五", null));

      // 如果age为null，则直接赋值默认值0
      Map<String, Integer> studentMap = students.stream().collect(Collectors.toMap(Student::getName, s -> Optional.ofNullable(s.getAge()).orElse(0)));
      System.out.println(studentMap);
  }
}

运行程序，可以看到如下输出

{李四=21, 张三=18, 王五=0}

（2）调用 collect() 的其他实现方法

package com.magic.npe;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Test {

  public static void main(String[] args) {
      List<Student> students = new ArrayList<>();
      students.add(new Student("张三", 18));
      students.add(new Student("李四", 21));
      students.add(new Student("王五", null));

      Map<String, Integer> studentMap = students.stream().collect(HashMap::new, (map, student) -> map.put(student.getName(), student.getAge()), HashMap::putAll);
      System.out.println(studentMap);
  }
}

运行程序，可以看到如下输出

{李四=21, 张三=18, 王五=null}