假設有這么一個類:

import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;public class Student {private int id;private String name;public Student(int id, String name) {this.id = id;this.name = name;}public Student() {}public int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}@Overridepublic String toString() {return "Student{" +"id=" + id +", name='" + name + '\'' +'}';}

使用stream流來將list轉map

  public void test(){List<Student> list = Arrays.asList(new Student(1, "張三" ),new Student(2, "李四"),new Student(3, "趙武" ),new Student(4, "王六" ));/***  key: id*value : name*/Map<Integer, String> collect = list.stream().collect(Collectors.toMap(Student::getId, Student::getName));System.out.println(collect);/*** key :id* value: student*/Map<Integer, Student> collect1 = list.stream().collect(Collectors.toMap(Student::getId, v -> v));System.out.println(collect1);/*** key :id* value: student*  但是這種情況是有重復的key 的情況下*/List<Student> list2 = Arrays.asList(new Student(1, "張三" ),new Student(2, "李四"),new Student(3, "趙武" ),new Student(1, "王六" ));
//        //這種會報錯
//        Map<Integer, String> collect2 = list2.stream()
//                .collect(Collectors.toMap(Student::getId, Student::getName));
//        System.out.println(collect2);//稍微處理一下重復的key的問題HashMap<Integer, String> collect3 = list2.stream().collect(Collectors.toMap(Student::getId, Student::getName, (n1, n2) -> n2, HashMap::new));System.out.println(collect3);}

運行結果如下:

{1=張三, 2=李四, 3=趙武, 4=王六}
{1=Student{id=1, name='張三'}, 2=Student{id=2, name='李四'}, 3=Student{id=3, name='趙武'}, 4=Student{id=4, name='王六'}}
{1=王六, 2=李四, 3=趙武}

源碼:

    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);}

這種沒有對key重復的時候做處理,在實際開發中肯定會考慮導key是否重復的場景;

下面這種方式可以將處理后的數據返回到一個新的map(指定的容器)之中

    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);}

并行流: