Java 8 Convert Map to List using Stream Example

This article contains Java 8 Convert Map to List using Stream Example like Map to Value List, Map to key List, Map filter based on key, Map filter based on value using java 8 filter.

Example 1: Map Value to List using stream

HashMap<Integer, Employee> employeeHashMap = new HashMap();
       employeeHashMap.put(1, new Employee(1, "Jone"));
       employeeHashMap.put(2, new Employee(2, "Shone"));
       java.util.List<Employee> employeeList = employeeHashMap.entrySet()
                                                              .stream()
                                                              .map(java.util.Map.Entry::getValue)      // map too value
                                                              .collect(Collectors
                                                                      .toList());           // collect value list
       employeeList.forEach(employee -> {
           System.out.println(employee.getName());
       });

Output:

Jone
Shone

Example 2: Map key to List using stream

HashMap<Integer, Employee> employeeHashMap = new HashMap();
        employeeHashMap.put(1, new Employee(1, "Jone"));
        employeeHashMap.put(2, new Employee(2, "Shone"));
        java.util.List<Integer> emplyeeMapKey = employeeHashMap.entrySet()
                                                              .stream()
                                                              .mapToInt(java.util.Map.Entry::getKey)                               // using map get only key
                                                              .collect(ArrayList::new, ArrayList::add, ArrayList::addAll);        // convert Integer Stream to List 

        emplyeeMapKey.forEach(employeeId -> {
            System.out.println(employeeId);
        });

Output:

1
2

Example 3: Map Key to List with filter

HashMap<Integer, Employee> employeeHashMap = new HashMap();
       employeeHashMap.put(1, new Employee(1, "Jone"));
       employeeHashMap.put(2, new Employee(2, "Shone"));
       employeeHashMap.put(3, new Employee(3, "Chapli"));
       employeeHashMap.put(4, new Employee(4, "Bost"));
       employeeHashMap.put(5, new Employee(5, "Novo"));
       java.util.Map<Integer, Employee> filterResultMap = employeeHashMap.entrySet()
                                                                         .stream()
                                                                         .filter((entry -> {
                                                                             return entry.getKey() > 3;                // it will filter based on key
                                                                         }))
                                                                         .collect(Collectors                           // create map with filtered data
                                                                                 .toMap(k -> k.getKey(), v -> v
                                                                                         .getValue()));

       filterResultMap.forEach((k, v) -> {
           System.out.println(k + " " + v.getName());
       });

Output:

4 Bost
5 Novo

Example 4 : Map values to List using filter

HashMap<Integer, Employee> employeeHashMap = new HashMap();
       employeeHashMap.put(1, new Employee(1, "Jone"));
       employeeHashMap.put(2, new Employee(2, "Shone"));
       employeeHashMap.put(3, new Employee(3, "Chapli"));
       employeeHashMap.put(4, new Employee(4, "Bost"));
       employeeHashMap.put(5, new Employee(5, "Novo"));
       java.util.Map<Integer, Employee> filterResultMap = employeeHashMap.entrySet()
                                                                         .stream()
                                                                         .filter((entry -> {
                                                                             return entry.getValue().getName().endsWith("e");      // it will filter based on map value
                                                                         }))   
                                                                         .collect(Collectors
                                                                                 .toMap(k -> k.getKey(), v -> v                     // create map with filtered data
                                                                                         .getValue()));

       filterResultMap.forEach((k, v) -> {
           System.out.println(k + " " + v.getName());
       });

Output:

1 Jone
2 Shone

References:

https://stackoverflow.com/questions/29625529/hashmap-with-streams-in-java-8-streams-to-collect-value-of-map

https://stackoverflow.com/questions/28263518/how-to-revert-a-list-from-a-map-when-filtering-using-streams?rq=1