SoFunction
Updated on 2025-04-13

How to elegantly implement Collection to Map

Converting Collection to Map is a common requirement, especially when processing data requires quick searches or deduplication.

Here are a few common methods, including three ways to use Google, Hutool, and Java Stream APIs.

Google's

/**
  * Use #uniqueIndex(, ) method
  *
  * @author Fu Cong
  * @time 2025-03-13 09:29:51
  */
@Test
public void testGoogleMapsUniqueIndex() {

    User user1 = new User();
    (1);
    ("Zhang San");
    User user2 = new User();
    (2);
    ("Li Si");
    User user3 = new User();
    // Repeat id (2)    (2);
    ("Wang Wu");

    List<User> userList = ();
    (user1);
    (user2);
    //(user3);

    Set<User> userSet = ();
    (user1);
    (user2);
    //(user3);

    // If the key is repeated, an exception will be reported.    Map<Integer, User> userListMap = (userList, user -> ());
    (("userListMapyes：{}", ()));

    Map<Integer, User> userSetMap = (userSet, user -> ());
    (("userSetMapyes：{}", ()));

}
———————————————————————— Start testing a single method ————————————————————————

userListMapyes：{1=User(id=1, name=Zhang San, remark=null), 2=User(id=2, name=Li Si, remark=null)}
userSetMapyes：{1=User(id=1, name=Zhang San, remark=null), 2=User(id=2, name=Li Si, remark=null)}

———————————————————————— End testing a single method ————————————————————————

Note: If the key is repeated, an exception will be reported.

Hutool's

/**
  * Use #toMap(, , .Func1, .Func1)
  *
  * @author Fu Cong
  * @time 2025-03-12 09:29:35
  */
@Test
public void testHutoolCollUtilToMap() {

    User user1 = new User();
    (1);
    ("Zhang San");
    User user2 = new User();
    (2);
    ("Li Si");
    User user3 = new User();
    // Repeat id (2)    (2);
    ("Wang Wu");

    List<User> userList = ();
    (user1);
    (user2);
    (user3);

    Set<User> userSet = ();
    (user1);
    (user3);
    (user2);

    // If the key is repeated, one of them will be taken. I am not sure which one is currently available.    Map<Integer, User> userListMap = (userList, (), user -> (), user -> user);
    (("userListMapyes：{}", ()));

    // If the key is repeated, one of them will be taken. I am not sure which one is currently available.    Map<Integer, User> userSetMap = (userSet, (), user -> (), user -> user);
    (("userSetMapyes：{}", ()));

}
———————————————————————— Start testing a single method ————————————————————————

userListMapyes：{1=User(id=1, name=Zhang San, remark=null), 2=User(id=2, name=Wang Wu, remark=null)}
userSetMapyes：{1=User(id=1, name=Zhang San, remark=null), 2=User(id=2, name=Wang Wu, remark=null)}

———————————————————————— End testing a single method ————————————————————————

Note: If the key is repeated, one of them will be taken. I am not sure which one is currently available.

Java Stream API

/**
  * Use #toMap(, , )
  *
  * @author Fu Cong
  * @time 2025-03-12 09:29:35
  */
@Test
public void testJavaStreamAPICollectorsToMap() {

    User user1 = new User();
    (1);
    ("Zhang San");
    User user2 = new User();
    (2);
    ("Li Si");
    User user3 = new User();
    // Repeat id (2)    (2);
    ("Wang Wu");

    List<User> userList = ();
    (user1);
    (user2);
    (user3);

    Set<User> userSet = ();
    (user1);
    (user2);
    (user3);

    // If the key is repeated, an exception will be reported.    //Map<Integer, User> userListMap1 = ().collect((user -> (), user -> user));
    //(("userListMap1 is: {}", ()));    // (key1, key2) -> key1: If the key is repeated, take the first one.    Map<Integer, User> userListMap2 = ().collect((user -> (), user -> user, (key1, key2) -> key1));
    (("userListMap2yes：{}", ()));

    // If the key is repeated, an exception will be reported.    //Map<Integer, User> userSetMap1 = ().collect((user -> (), user -> user));
    //(("userSetMap1 is: {}", ()));    // (key1, key2) -> key2: If the key is repeated, take the second one.    Map<Integer, User> userSetMap2 = ().collect((user -> (), user -> user, (key1, key2) -> key2));
    (("userSetMap2yes：{}", ()));

}
———————————————————————— Start testing a single method ————————————————————————

userListMap2yes：{1=User(id=1, name=Zhang San, remark=null), 2=User(id=2, name=Li Si, remark=null)}
userSetMap2yes：{1=User(id=1, name=Zhang San, remark=null), 2=User(id=2, name=Wang Wu, remark=null)}

———————————————————————— End testing a single method ————————————————————————

Difference comparison

Is it supported to handle duplicate keys What happens if the key is repeated
Google's no Report an error
Hutool's no Take one of them
Java Stream API yes Values ​​can be obtained according to the merge function

Other Instructions

If it does not existStitch multiple field values ​​into keysIn the case of this, you can also change [user -> ()] to [User::getId], but if it existsStitch multiple field values ​​into keysIn the case of , you can use the form of [->] to splice it.

Map<String, User> userListMap2 = ().collect((user -> ("{}_{}", (), ()), user -> user, (key1, key2) -> key1));

Summarize

The above is personal experience. I hope you can give you a reference and I hope you can support me more.