Collectors. Tomap application

Collectors.toMap Introduce

There are many in real business scenarios aggregate turn map The operation of , for example

@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 
 */
public class ListToMap {

    public static void main(String[] args) {
        House house = new House(1,1,"aa"," Beijing haidian ");
        House house1 = new House(2,2,"bb"," Wuhan, Hubei ");
        House house2 = new House(3,3,"cc"," 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);
        //{aa= Wuhan, Hubei , bb= Hangzhou, Zhejiang , cc= 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=aa, 2=bb, 3=cc}

        /**
         *  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='aa', address=' Beijing haidian '}, 
         * 2=House{id=2, ownerid=2, housename='bb', address=' Wuhan, Hubei '}, 
         * 3=House{id=3, ownerid=3, housename='cc', 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='aa', address=' Beijing haidian '}
        
        // The effect here is the same as houseMap
        System.out.println(houseMap1);
       /**
         * {1=House{id=1, ownerid=1, housename='aa', address=' Beijing haidian '}, 
         * 2=House{id=2, ownerid=2, housename='bb', address=' Wuhan, Hubei '}, 
         * 3=House{id=3, ownerid=3, housename='cc', address=' Hangzhou, Zhejiang '}}
         */

    }

}

You can practice common operations

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

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

We put the test data House house = new House(1,1,“aa”,“ Beijing haidian ”);
House house1 = new House(2,2,“bb”,“ Wuhan, Hubei ”);
House house2 = new House(3,3,“cc”,“ Hangzhou, Zhejiang ”);
house2 It is amended as follows ： House house2 = new House(3,1,“cc”,“ Hangzhou, Zhejiang ”);
such Ownerid Will repeat
When we execute the following code ：houses.stream().collect(Collectors.toMap(House::getOwnerid, House::getHousename);
Will make mistakes ,java.lang.IllegalStateException: Duplicate key

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 aa
	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));

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

List Count the number of duplicates

final String path = "F:/AppData/API-url.txt";
final List<String> list = BlockIOUtil.readLines("file:" + path);
Map<String, Integer> map = 
		list.stream().collect(Collectors.toMap(it -> it, it -> 1, (val1, val2) -> val1 + 1));
for (Map.Entry<String, Integer> entry : map.entrySet()) {
	if (entry.getValue() != null && entry.getValue() > 1) {
		System.out.println(entry.getKey() + "-->" + entry.getValue());
	}
}