[list to map] collectors Tomap syntax sharing (case practice)

【 Chen Xi has to work hard 】：hello Hello, I'm Chen Xi , I'm glad you came to read , Nickname is the hope that you can constantly improve , Moving forward to good programmers ！

The blog comes from the project and the summary of the problems encountered in programming , Occasionally there are books to share , I'll keep updating Java front end 、 backstage 、 database 、 Project cases and other related knowledge points summary , Thank you for your reading and attention , I hope my blog can help more people , Share and gain new knowledge , Make progress together !

We quarryers , The heart of a cathedral , I wish you all to go in your love …

---

One 、 First time to know Collectors.toMap

In the real business scenario, there are many collections map The operation of , Let's study together today Collectors.toMap Related use

Create a house The object of

@Data
public class House {
    

    private Integer id;        //id
    private Integer ownerid;   // Home owner No 
    private String housename;  // Family name 
    private String address;    // Home address 
    
}

Analog data

/** * @description: List  turn  Map  operation  * @author:  Chen Xi has to work hard  * @create: 2021-09-11 12:57 */
public class ListToMap {
    

    public static void main(String[] args) {
    
        House house = new House(1,1," Chen Xi "," Beijing haidian ");
        House house1 = new House(2,2," Chen Xi has to work hard "," Wuhan, Hubei ");
        House house2 = new House(3,3," Chen Xiaoxi "," Hangzhou, Zhejiang ");
        ArrayList<House> houses = new ArrayList<>();
        houses.add(house);
        houses.add(house1);
        houses.add(house2);
        // In actual projects, we often use  List  turn  Map  operation  -> It used to be for Operation of the cycle , Now you can learn the following methods Collectors.toMap
        /** *  Let's collect two separate attributes for each object in the collection  */
        Map<String, String> mapHouse = houses.stream().collect(Collectors.toMap(House::getHousename, House::getAddress));
        System.out.println(mapHouse);
        //{ Chen Xi has to work hard = Wuhan, Hubei ,  Chen Xiaoxi = Hangzhou, Zhejiang ,  Chen Xi = Beijing haidian }
        
        /** *  The data types of the attributes before and after should correspond to   Collect business data with unique representation in general time business  */
        Map<Integer, String> map = houses.stream().collect(Collectors.toMap(House::getOwnerid, House::getHousename));
        System.out.println(map);
        //{1= Chen Xi , 2= Chen Xi has to work hard , 3= Chen Xiaoxi }

        /** *  Collect the attributes and the object itself  */
        Map<Integer, House> houseMap = houses.stream().collect(Collectors.toMap(House::getOwnerid, o -> o));
        Map<Integer, House> houseMap1 = houses.stream().collect(Collectors.toMap(House::getOwnerid,  Function.identity()));
        System.out.println(houseMap);
       /** * {1=House{id=1, ownerid=1, housename=' Chen Xi ', address=' Beijing haidian '}, * 2=House{id=2, ownerid=2, housename=' Chen Xi has to work hard ', address=' Wuhan, Hubei '}, * 3=House{id=3, ownerid=3, housename=' Chen Xiaoxi ', address=' Hangzhou, Zhejiang '}} */
         
        // Business scenario ： Generally, it will be based on the specific key value   Take specific objects 
        System.out.println(houseMap.get(1));
        //House{id=1, ownerid=1, housename=' Chen Xi ', address=' Beijing haidian '}
        
        // The effect here is the same as houseMap
        System.out.println(houseMap1);
       /** * {1=House{id=1, ownerid=1, housename=' Chen Xi ', address=' Beijing haidian '}, * 2=House{id=2, ownerid=2, housename=' Chen Xi has to work hard ', address=' Wuhan, Hubei '}, * 3=House{id=3, ownerid=3, housename=' Chen Xiaoxi ', address=' Hangzhou, Zhejiang '}} */

    }

}

Output effect ： ditto

You can practice common operations

houses.stream().collect(Collectors.toMap(House::getOwnerid, House::getHousename));

Learn grammar in detail ： uncover System.out::println The veil of mystery

---

Two 、 thorough Collectors.toMap

Collectors.toMap There are three ways to overload ：

toMap(Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper);
toMap(Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper,
        BinaryOperator<U> mergeFunction);
toMap(Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper,
        BinaryOperator<U> mergeFunction, Supplier<M> mapSupplier);

The meanings of the parameters are ：

keyMapper：Key The mapping function of

valueMapper：Value The mapping function of

mergeFunction： When Key When the conflict , The merge method called

mapSupplier：Map Constructors , When you need to return a specific Map When using

---

The most business scenarios are map The key of is a unique identifier , The value is the object itself ！

If you want to get Map Of value For the object itself , It can be written like this

       /** *  Collect the attributes and the object itself  */
Map<Integer, House> houseMap = houses.stream().collect(Collectors.toMap(House::getOwnerid, o -> o));
Map<Integer, House> houseMap1 = houses.stream().collect(Collectors.toMap(House::getOwnerid,  Function.identity()));

---

common java.lang.IllegalStateException: Duplicate key Problem handling

The online business code appears Duplicate Key It's abnormal , It affects the business logic , Look at the code that throws the exception

Exception in thread "main" java.lang.IllegalStateException: Duplicate key  Chen Xi 
	at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
	at java.util.HashMap.merge(HashMap.java:1254)
	at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
	at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
	at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
	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 com.example.cwgl.ListToMap.main(ListToMap.java:28)

terms of settlement ： When duplicate occurs , Take the front value Value , Or take what you put in the back value value , Then the previous value value

houses.stream().collect(Collectors.toMap(House::getOwnerid, House::getHousename,(v1,v2)->v2));
houses.stream().collect(Collectors.toMap(House::getOwnerid, House::getHousename,(v1,v2)->v1));

The implementation effect is as follows ： Don't complain , The normal value covers

There are many ways to deal with the operation of results , Such as splicing

houses.stream().collect(Collectors.toMap(House::getOwnerid, House::getHousename,(v1,v2)->v1+v2));

Execution effect

{
    1= Chen Xi, Chen Xiao Xi , 2= Chen Xi has to work hard }

The specific practice is to operate the data according to the specific business

---

Record Chen Xi's first 200 blog ！ Clear objectives , perform , reflection , Refinement ！ Become an excellent programmer as soon as possible ！

---

Thank you very much for reading here , If this article helps you , I hope I can leave your praise Focus on ️ Share Leaving a message. thanks！！！

2021 year 9 month 12 Japan 17:56:48 May you go to your love ！