Collect list of Integer (List) to map with Java 8 Stream API












6















I tried to convert a simple List<Integer> to a Map using Java 8 stream API and got the following compile time error:



The method toMap(Function<? super T,? extends K>, Function<? super T,? 

extends U>) in the type Collectors is not applicable for the arguments 

(Function<Object,Object>, boolean)


My code:



ArrayList<Integer> m_list = new ArrayList<Integer>();



m_list.add(1);

m_list.add(2);

m_list.add(3);

m_list.add(4);



Map<Integer, Boolean> m_map = m_list.stream().collect(

                Collectors.toMap(Function.identity(), true));


I also tried the second method below but got the same error.



Map<Integer, Boolean> m_map = m_list.stream().collect(

                Collectors.toMap(Integer::intValue, true));


What is the correct way to do this using Java 8 stream API?






arraylist collections java-8 hashmap java-stream







share|improve this question





























    6















    I tried to convert a simple List<Integer> to a Map using Java 8 stream API and got the following compile time error:



    The method toMap(Function<? super T,? extends K>, Function<? super T,? 
    
extends U>) in the type Collectors is not applicable for the arguments 
    
(Function<Object,Object>, boolean)


    My code:



    ArrayList<Integer> m_list = new ArrayList<Integer>();
    

    
m_list.add(1);
    
m_list.add(2);
    
m_list.add(3);
    
m_list.add(4);
    

    
Map<Integer, Boolean> m_map = m_list.stream().collect(
    
                Collectors.toMap(Function.identity(), true));


    I also tried the second method below but got the same error.



    Map<Integer, Boolean> m_map = m_list.stream().collect(
    
                Collectors.toMap(Integer::intValue, true));


    What is the correct way to do this using Java 8 stream API?






    arraylist collections java-8 hashmap java-stream







    share|improve this question



























      6












      6








      6


      1






      I tried to convert a simple List<Integer> to a Map using Java 8 stream API and got the following compile time error:



      The method toMap(Function<? super T,? extends K>, Function<? super T,? 
      
extends U>) in the type Collectors is not applicable for the arguments 
      
(Function<Object,Object>, boolean)


      My code:



      ArrayList<Integer> m_list = new ArrayList<Integer>();
      

      
m_list.add(1);
      
m_list.add(2);
      
m_list.add(3);
      
m_list.add(4);
      

      
Map<Integer, Boolean> m_map = m_list.stream().collect(
      
                Collectors.toMap(Function.identity(), true));


      I also tried the second method below but got the same error.



      Map<Integer, Boolean> m_map = m_list.stream().collect(
      
                Collectors.toMap(Integer::intValue, true));


      What is the correct way to do this using Java 8 stream API?






      arraylist collections java-8 hashmap java-stream







      share|improve this question
















      I tried to convert a simple List<Integer> to a Map using Java 8 stream API and got the following compile time error:



      The method toMap(Function<? super T,? extends K>, Function<? super T,? 
      
extends U>) in the type Collectors is not applicable for the arguments 
      
(Function<Object,Object>, boolean)


      My code:



      ArrayList<Integer> m_list = new ArrayList<Integer>();
      

      
m_list.add(1);
      
m_list.add(2);
      
m_list.add(3);
      
m_list.add(4);
      

      
Map<Integer, Boolean> m_map = m_list.stream().collect(
      
                Collectors.toMap(Function.identity(), true));


      I also tried the second method below but got the same error.



      Map<Integer, Boolean> m_map = m_list.stream().collect(
      
                Collectors.toMap(Integer::intValue, true));


      What is the correct way to do this using Java 8 stream API?






      arraylist collections java-8 hashmap java-stream




      arraylist collections java-8 hashmap java-stream






      share|improve this question















      share|improve this question













      share|improve this question




      share|improve this question








      edited Nov 16 '18 at 7:25









      Eran

      291k37479563




      291k37479563










      asked Nov 16 '18 at 7:13









      UnknownUnknown

      4232819




      4232819
























          1 Answer
          1






          active

          oldest

          votes


















          5














          You are passing a boolean for the value mapper. You should pass a Function<Integer,Boolean>.



          It should be:



          Map<Integer, Boolean> m_map = m_list.stream().collect(
          
            Collectors.toMap(Function.identity(), e -> true));





          share|improve this answer
























            Your Answer






            StackExchange.ifUsing("editor", function () {
            StackExchange.using("externalEditor", function () {
            StackExchange.using("snippets", function () {
            StackExchange.snippets.init();
            });
            });
            }, "code-snippets");

            StackExchange.ready(function() {
            var channelOptions = {
            tags: "".split(" "),
            id: "1"
            };
            initTagRenderer("".split(" "), "".split(" "), channelOptions);

            StackExchange.using("externalEditor", function() {
            // Have to fire editor after snippets, if snippets enabled
            if (StackExchange.settings.snippets.snippetsEnabled) {
            StackExchange.using("snippets", function() {
            createEditor();
            });
            }
            else {
            createEditor();
            }
            });

            function createEditor() {
            StackExchange.prepareEditor({
            heartbeatType: 'answer',
            autoActivateHeartbeat: false,
            convertImagesToLinks: true,
            noModals: true,
            showLowRepImageUploadWarning: true,
            reputationToPostImages: 10,
            bindNavPrevention: true,
            postfix: "",
            imageUploader: {
            brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
            contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
            allowUrls: true
            },
            onDemand: true,
            discardSelector: ".discard-answer"
            ,immediatelyShowMarkdownHelp:true
            });


            }
            });














            draft saved

            draft discarded


















            StackExchange.ready(
            function () {
            StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f53333077%2fcollect-list-of-integer-listinteger-to-map-with-java-8-stream-api%23new-answer', 'question_page');
            }
            );

            Post as a guest















            Required, but never shown

























            1 Answer
            1






            active

            oldest

            votes








            1 Answer
            1






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes









            5














            You are passing a boolean for the value mapper. You should pass a Function<Integer,Boolean>.



            It should be:



            Map<Integer, Boolean> m_map = m_list.stream().collect(
            
            Collectors.toMap(Function.identity(), e -> true));





            share|improve this answer




























              5














              You are passing a boolean for the value mapper. You should pass a Function<Integer,Boolean>.



              It should be:



              Map<Integer, Boolean> m_map = m_list.stream().collect(
              
            Collectors.toMap(Function.identity(), e -> true));





              share|improve this answer


























                5












                5








                5







                You are passing a boolean for the value mapper. You should pass a Function<Integer,Boolean>.



                It should be:



                Map<Integer, Boolean> m_map = m_list.stream().collect(
                
            Collectors.toMap(Function.identity(), e -> true));





                share|improve this answer













                You are passing a boolean for the value mapper. You should pass a Function<Integer,Boolean>.



                It should be:



                Map<Integer, Boolean> m_map = m_list.stream().collect(
                
            Collectors.toMap(Function.identity(), e -> true));






                share|improve this answer












                share|improve this answer



                share|improve this answer










                answered Nov 16 '18 at 7:13









                EranEran

                291k37479563




                291k37479563
































                    draft saved

                    draft discarded




















































                    Thanks for contributing an answer to Stack Overflow!


                    • Please be sure to answer the question. Provide details and share your research!

                    But avoid …



                    • Asking for help, clarification, or responding to other answers.

                    • Making statements based on opinion; back them up with references or personal experience.


                    To learn more, see our tips on writing great answers.




                    draft saved


                    draft discarded














                    StackExchange.ready(
                    function () {
                    StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f53333077%2fcollect-list-of-integer-listinteger-to-map-with-java-8-stream-api%23new-answer', 'question_page');
                    }
                    );

                    Post as a guest















                    Required, but never shown





















































                    Required, but never shown














                    Required, but never shown












                    Required, but never shown







                    Required, but never shown

































                    Required, but never shown














                    Required, but never shown












                    Required, but never shown







                    Required, but never shown







                    -

                    Popular posts from this blog

                    Florida Star v. B. J. F.

                    Image
                    Florida Star v. B. J. F. From Wikipedia, the free encyclopedia Jump to navigation Jump to search United States Supreme Court case Florida Star v. B. J. F. Supreme Court of the United States Argued March 21, 1989 Decided June 21, 1989 Full case name The Florida Star v. B. J. F. Citations 491 U.S. 524 ( more ) 109 S. Ct. 2603; 105 L. Ed. 2d 443; 1989 U.S. LEXIS 3120; 57 U.S.L.W. 4816; 16 Media L. Rep. 1801 Prior history The Florida Star v. B.J.F., 530 So.2d 286 (1988) Supreme Court of Florida; Florida Star v. B.J.F., 499 So.2d 883 (1986) Fla. Dist. Court of Appeals Holding Florida Stat. § 794.03 is unconstitutional to the extent it makes the truthful reporting of information that was a matter of public record unlawful, as it violates the First Amendment. Court membership Chief Justice William Rehnquist Associate Justices William J. Brennan Jr.  · Byron White Thurgood Marshall  · Harry Blac
                    Read more

                    Error while running script in elastic search , gateway timeout

                    Image
                    0 while running script in elastic search, I got 504 Gateway timeout error. { "query": { "bool": { "filter": { "script": { "script": " doc['creted_date'].date.getMonthOfYear() == 12 " } } } }, "aggs": { "test": { "date_histogram": { "field": "creted_date", "interval": "month", "format": "MMM" }, "aggs": { "cost": { "sum": { "field": "cost" } } }
                    Read more

                    Adding quotations to stringified JSON object values

                    Image
                    0 My stringified object data doesn't include quotation marks around object values, which errors when attempting JSON.parse() : '{ "affiliation": CORPORATE, "userId": 75c35d1c-5d12-4485-8fa8-b2f1551a3e6e }' I need the string to be: '{ "affiliation": "CORPORATE", "userId": "75c35d1c-5d12-4485-8fa8-b2f1551a3e6e" }' I'm using this regex to add quotes to the object keys: var newStr = str.replace(/(['"])?([a-z0-9A-Z_]+)(['"])?:/g, '"$2": '); For instance: '{ affiliation: CORPORATE }' to '{ "affiliation": CORPORATE }' There are only string values in my data, so I don't need to check value types. How can I alter my regex expression to add quotations to
                    Read more