使用 Java 8 模式匹配将 Map 转换为 List

Question

I have a requirement to store those employees key matching Address1 to a new List<EmpData> , code is given below, I am able to do it by normal iterations.

Can I do it with java8 pattern matching using collect and filter etc.?

public static void main(String[] args) {
    List<EmpData> lst1 = new ArrayList<EmpData>();
    List<EmpData> lst2 = new ArrayList<EmpData>();
    EmpData obj1=new EmpData("100", "Name1", "25/05/1979");
    EmpData obj2=new EmpData("101", "Name2", "25/05/1975");
    EmpData obj3=new EmpData("103", "Name3", "25/05/1976");
    lst1.add(obj1);
    lst1.add(obj2);
    lst2.add(obj3);

    HashMap<String, List<EmpData>> map = new HashMap<>();

    map.put("Address1Emp1", lst1);
    map.put("Address2Emp2", lst2);

    List<EmpData> listEmp = new ArrayList<>();
    //Need to store those employees key matching Address1 to a List<EmpData>

}

Answer 1

I'm guessing you meant streams from java-8 and the requirement, that the key must match "Address1Emp.*" regex.

You can use the following code:

map.entrySet().stream() // create a stream of entries
    .filter(e -> e.getKey().matches("Address1Emp.*")) // leave only those entries, whose keys start with "Address1"
    .map(Map.Entry::getValue) // get values only
    .collect(Collectors.toList()); 

Edit after you have added the inner lists:

map.entrySet().stream() // create a stream of entries
    .filter(e -> e.getKey().matches("Address1Emp.*")) // leave only those entries, whose keys start with "Address1"
    .flatMap(e -> e.getValue().stream()) // get values only
    .collect(Collectors.toList()); 

Answer 2

Depending on which pattern you need you can use startsWith

List<EmpData> listEmp = map.entrySet().stream()
    .filter(e -> e.getKey().startsWith("Address1"))
    .map(Entry::getValue)
    .collect(Collectors.toList());

or with contains :

List<EmpData> listEmp = map.entrySet().stream()
    .filter(e -> e.getKey().contains("Address1"))
    .map(Entry::getValue)
    .collect(Collectors.toList());