1、现象

import lombok.Data;
import org.apache.commons.lang3.StringUtils;
 
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.Optional;
import java.util.stream.Collectors;
 
public class Main {
    public static void main(String[] args) {
        List<Student> list = new ArrayList<Student>(){{
			add(new Student("a",1));
			add(new Student("b",2));
			add(new Student("c",3));
		}};
        // 当value为空时,报NullPointerException 
        Map<String, String> collect = list.stream().collect(Collectors.toMap(x -> x.getName(), x -> x.getAge(), (a, b) -> a));
 
    }
 
    @Data
    public static class Student {
        private String name;
        private int age;
		
		public Student(name,age){
			this.name = name;
			this.age = age;
		}
    }
}

Exception in thread "main" java.lang.NullPointerException
	at java.util.HashMap.merge(HashMap.java:1225)
	at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
	at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
	at java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:948)
	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.eju.goodhouse.test.T.main(T.java:22)

2、原因

Collectors.toMap()，key可以为null,但 value不能为null， 否则抛空指针异常nullPointerException

 3、解决

// 对空判断
Map<String, String> collect1 = list.stream().collect(
	Collectors.toMap(
		x -> x.getName(),
        x -> Objects.nonNull(x.getPetName()) ? x.getPetName() : "",
        (a, b) -> StringUtils.isNotBlank(a) ? a : b
	);

// 使用Optional对空值进行处理
Map<String, String> collect2 = list.stream().collect(
	Collectors.toMap(
		x -> x.getName(),
        x -> Optional.ofNullable(x.getPetName()).orElse(""),
        (a, b) -> StringUtils.isNotBlank(a) ? a : b
	);