1、JDK1.8流方式

用户类

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import lombok.Data;

@Data
public class User {

    private String id;
    private String name;

    public User(String id, String name) {
        this.id = id;
        this.name = name;
    }
}

1. 收集成key为id，value为name：

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List<User> userList = new ArrayList<>();
User user1 = new User("1", "name1");
User user2 = new User("2", "name2");
User user3 = new User("3", "name3");
userList.add(user1);
userList.add(user2);
userList.add(user3);

Map<String, String> collect = userList.stream().collect(Collectors.toMap(User::getId, User::getName));

for (Map.Entry<String, String> next : collect.entrySet()) {
    System.out.println("key: " + next.getKey() + " , " + "value: " + next.getValue());
}

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2. 收集成实体本身

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List<User> userList = new ArrayList<>();
User user1 = new User("1", "name1");
User user2 = new User("2", "name2");
User user3 = new User("3", "name3");
userList.add(user1);
userList.add(user2);
userList.add(user3);

Map<String, User> collect = userList.stream().collect(Collectors.toMap(User::getId, user -> user));

for (Map.Entry<String, User> next : collect.entrySet()) {
    System.out.println("key: " + next.getKey() + " , " + "value: " + next.getValue());
}

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user -> user是一个返回本身的lambda表达式，其实还可以使用Function接口中的一个默认方法代替，使整个方法更简洁优雅：

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Map<String, User> collect1 = userList.stream().collect(Collectors.toMap(User::getId, Function.identity()));

重复key的情况：
代码如下：

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Map<String, User> collect2 = userList.stream().collect(Collectors.toMap(User::getName, Function.identity());

这个方法可能报错（java.lang.IllegalStateException: Duplicate key），因为name是有可能重复的。
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toMap有个重载方法，可以传入一个合并的函数来解决key冲突问题：

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List<User> userList = new ArrayList<>();
User user1 = new User("1", "name2");
User user2 = new User("2", "name2");
User user3 = new User("3", "name3");
userList.add(user1);
userList.add(user2);
userList.add(user3);

Map<String, User> collect = userList.stream().collect(Collectors.toMap(User::getName, Function.identity(), (key1, key2) -> key2));

for (Map.Entry<String, User> next : collect.entrySet()) {
    System.out.println("key: " + next.getKey() + " , " + "value: " + next.getValue());
}

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这里只是简单的使用后者覆盖前者来解决key重复问题。还有一种分组的方法：

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List<User> userList = new ArrayList<>();
User user1 = new User("1", "name2");
User user2 = new User("2", "name2");
User user3 = new User("3", "name3");
userList.add(user1);
userList.add(user2);
userList.add(user3);

Map<String, List<User>> collect3 = userList.stream().collect(Collectors.groupingBy(User::getName));

for (Map.Entry<String, List<User>> next : collect3.entrySet()) {
    System.out.println("key: " + next.getKey() + " , " + "value: " + next.getValue());
}

图片

3. 指定具体收集的map

toMap还有另一个重载方法，可以指定一个Map的具体实现，来收集数据：

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List<User> userList = new ArrayList<>();
User user1 = new User("1", "name2");
User user2 = new User("2", "name2");
User user3 = new User("3", "name3");
userList.add(user1);
userList.add(user2);
userList.add(user3);

Map<String, User> collect3 = userList.stream().collect(Collectors.toMap(User::getName, Function.identity(), (key1, key2) -> key2, LinkedHashMap::new));

for (Map.Entry<String, User> next : collect3.entrySet()) {
    System.out.println("key: " + next.getKey() + " , " + "value: " + next.getValue());
}

图片