Concatenate set of objects, group by value in java

user_15113491 0
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I'm making a Set of objects that consists of name and value. My requirement is to create a new Set by grouping the Set based on objects that have the same value and concate the property name into a single string separated by ",".

Example:

   Name: Test,     value: Random value
   Name: Test 2,   value: Some random value
   Name: Test 3,   value: Random value

I want the result to be a Set like this:

Name: Test, Test 3,  value: Random value
Name: Test 2 ,   value: Some random value

The class for understanding:

public class Test {
    private String name;
    private String value;
    }

How to create a new Set by grouping the Set based on value and concatenate the name into a string using Java Stream?

java-stream object java
3 Answers

Using toMap with merging (additionally stream elements are before sorted by Name):

Set<Test> result = s.stream()
                    .sorted((t1,t2)-> t1.getName().compareTo(t2.getName())) 
                    .collect(Collectors.toMap(Test::getValue, 
                                              Function.identity(),
                                              (t1,t2)-> new Test(t1.getName()+","+t2.getName(),
                                                                 t1.getValue())))
                                       .values()
                                       .stream()
                                       .collect(Collectors.toSet());
user_134151640

You can do something like this:

var result = setOfTests.stream()
    .collect(
        Collectors.collectingAndThen(
            Collectors.groupingBy(Test::getValue,
                // for each group, map each element to their name and join
                Collectors.mapping(Test::getName, Collectors.joining(", "))
            ), // at this point we have a Map<String, String> mapping from value to joined name
            // now we turn each entry in the map back to a Test object
            // "getValue" below refers to Map.Entry.getValue, not Test.getValue
            map -> map.entrySet().stream().map(x -> new Test(x.getValue(), x.getKey()))
                .collect(
                    Collectors.toList() // or toSet or whatever you like
                )
        )
    );
// result is a List<Test>

Note that the order of the joined names is undefined if your set is unordered, so it could be either "Test, Test 3," or "Test 3, Test,".

user_51335850

You could use

String testValues = tests.stream()
                .map(Test:: getValue)
                .collect(Collectors.joining(","));

You can use the below sample and change the function in map to get either the values or the keys

public static void main(String[] args) {
        Set<Test> tests = new HashSet<>();
        tests.add(new Test("First", "1st"));
        tests.add(new Test("Second", "2nd"));
        tests.add(new Test("Third", "3rd"));
        tests.add(new Test("Fourth", "4th"));
        System.out.println(tests);
        String testValues = tests.stream()
                .map(Test:: getValue)
                .collect(Collectors.joining(","));
        System.out.println(testValues);

    }

You can modify your test class as below

    public class Test {
        public Test(String name, String value) {
            this.name = name;
            this.value = value;
        }

        private String name;
        private String value;

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public String getValue() {
            return value;
        }

        public void setValue(String value) {
            this.value = value;
        }

        @Override
        public String toString() {
            final StringBuilder sb = new StringBuilder("Test{");
            sb.append("name='").append(name).append('\'');
            sb.append(", value='").append(value).append('\'');
            sb.append('}');
            return sb.toString();
        }
    }

Important : This doesn't guarantee the the same order of insertion

user_35668790
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