采坑系列 Collectors.toMap

女爷i 2023-02-21 09:06 194阅读 0赞

java8出的流操作，由于可以快速转化为map并且便于代码阅读，所以推荐使用java8语法转化List为map。

这里给出例子：

public class GroupByTest {
    
    
        public class Student{
            private String name;
            private Integer 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;
            }
    
            public Student name(String name){
                this.name = name;
                return this;
            }
    
            public Student age(Integer age){
                this.age = age;
                return this;
            }
    
    
    
    
        }
    
    
        @Test
        public void groupBy(){
    
            List<Student> studentList = new ArrayList<>();
            studentList.add(new Student().age(20).name("张三"));
            studentList.add(new Student().age(20).name("李四"));
            studentList.add(new Student().age(21).name("王五"));
    
            Map<Integer, Student> collect = studentList.stream().collect(Collectors.toMap(Student::getAge, v -> v));
    
        }
    }

结果上诉代码报错

java.lang.IllegalStateException: Duplicate key com.example.demo.GroupByTest$Student@22680f52
    
    	at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
    	at java.util.HashMap.merge(HashMap.java:1254)

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Rvbmd5aW5nMTc1MQ_size_16_color_FFFFFF_t_70][]

分析原因：

Collectors源码

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

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);
        }

merge方法源码

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Rvbmd5aW5nMTc1MQ_size_16_color_FFFFFF_t_70 1][]

上面的源码可以看到。

Collectors.toMap 传入了一个  参数

throwingMerger()

我们看这个方法中代码，发现直接扔了一个异常。

private static <T> BinaryOperator<T> throwingMerger() {
            return (u,v) -> { throw new IllegalStateException(String.format("Duplicate key %s", u)); };
        }

remappingFunction 一直传到了merge 方法中 。

*  判断key对应的value是否有值，如果有值，执行 入参 remappingFunction 的操作。但是我们传入的是 抛出异常，所以。这里直接抛出了异常；

我们打断点调试看到

![202006291955285.png][]

![20200629195602246.png][]

解决方法，调用重载方法

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);
        }

自己传入 function

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Rvbmd5aW5nMTc1MQ_size_16_color_FFFFFF_t_70 2][]

可以看到，这里已经对重复值进行了覆盖。

**解决方案：调用重载方法，传入处理的 function**

Collectors.toMap(Student::getAge, v -> v, (u, v) -> v)

**总结：使用api时，如果遇到重载的api，需要搞清楚可能会发生的问题，每个参数的含义。**

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[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Rvbmd5aW5nMTc1MQ_size_16_color_FFFFFF_t_70 1]: https://img-blog.csdnimg.cn/2020062919392534.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Rvbmd5aW5nMTc1MQ==,size_16,color_FFFFFF,t_70
[202006291955285.png]: https://img-blog.csdnimg.cn/202006291955285.png
[20200629195602246.png]: https://img-blog.csdnimg.cn/20200629195602246.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Rvbmd5aW5nMTc1MQ_size_16_color_FFFFFF_t_70 2]: https://img-blog.csdnimg.cn/20200629195217741.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Rvbmd5aW5nMTc1MQ==,size_16,color_FFFFFF,t_70