JAVA-Stream-List转换Map

「爱情、让人受尽委屈。」 2022-05-17 01:23 265阅读 0赞

使用Java8的Stream Api，将List转换成Map非常方便。

/**
     * @author wzx
     * @time 2018/8/4
     */
    public class ListToMapTest { 
    
        public static void main(String[] args) {
            List<Person> persons = new ArrayList<Person>();
            persons.add(new Person("wzx1", 10));
            persons.add(new Person("wzx2", 11));
            persons.add(new Person("wzx3", 12));
            persons.add(new Person("wzx4", 13));
            persons.add(new Person("wzx5", 14));
            //Collectors.toMap 参数一:key,参数二:value,参数三:mergeFunction,处理相同key函数
            Map<String, Integer> map1 = persons.stream().collect(Collectors.toMap(Person::getName, Person::getAge));
            map1.forEach((k, v) -> System.out.println(k + ":" + v)); //如果key重复报错 persons.add(new Person("wzx5", 16)); // Map<String, Integer> map2 = persons.stream().collect(Collectors.toMap(Person::getName, Person::getAge)); System.out.println("=================="); //处理key相同情况 Map<String, Integer> map3 = persons.stream().collect(Collectors.toMap(Person::getName, Person::getAge, (oldVal, newVal) -> oldVal)); map3.forEach((k, v) -> System.out.println(k + ":" + v)); System.out.println("=================="); ///排序并转换map Map<String, Integer> map4 = persons.stream().sorted(Comparator.comparingInt(Person::getAge).reversed()) .collect(Collectors.toMap(Person::getName, Person::getAge, (oldVal, newVal) -> newVal, LinkedHashMap::new)); map4.forEach((k, v) -> System.out.println(k + ":" + v)); } static class Person { private String name; private int age; public Person(String name, int age) { this.name = name; this.age = age; } public String getName() { return name; } public int getAge() { return age; } } }

输出结果：

wzx5:14
    wzx4:13
    wzx3:12
    wzx2:11
    wzx1:10 ==================
    wzx5:14
    wzx4:13
    wzx3:12
    wzx2:11
    wzx1:10 ==================
    wzx5:14
    wzx4:13
    wzx3:12
    wzx2:11
    wzx1:10

key重复错误：

Exception in thread "main" java.lang.IllegalStateException: Duplicate key 14
        at java.util.stream.Collectors.lambda$throwingMerger$113(Collectors.java:133)
        at java.util.stream.Collectors$$Lambda$3/666641942.apply(Unknown Source)
    	at java.util.HashMap.merge(HashMap.java:1245)
    	at java.util.stream.Collectors.lambda$toMap$171(Collectors.java:1320)
    	at java.util.stream.Collectors$$Lambda$5/186370029.accept(Unknown Source)
        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:512)
        at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:502)
        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 cn.wzx.ListToMapTest.main(ListToMapTest.java:26)