如何循环访问ArrayList的集合

Question

i had a Hash Map, the key is a string and the value is array list, i want to check if Array List include a value then retrieve the key for the value. so i did this:

Collection<ArrayList<String>> emailList = AddressBook.addressMap.values();
    Iterator<ArrayList<String>> iterator = emailList.iterator();

using a loop i can print the lists but i can't check if it equals to a string. how can i access each string in Array List and use equals

Answer 1

Use a nested loop:

for(List<String> list:emailList)
   for(String s:list) {
      // Logic
   }

This will allow you to iterate over each String in the entire Collection of ArrayList s.

Answer 2

You could try something like this. Since the Value in the hashmap is that of ArrayList, you have to loop through the collection of arraylists and then check the arraylist for your string:

For (ArrayList list : emailList ){

    if(emailList.contains(stringVALUE)){
        // do some stuff
    }
}

OR

 For (ArrayList list : emailList ){

   For(String s: list){
       if(s.equals(theValueYouAreLookingFor)){
          //do some stuff
       }

   }

}

Answer 3

You could do it like this using streams. This returns all keys for each mapped list that contains the desired value.

      Map<String, List<String>> map = new HashMap<>();
      map.put("list1", Arrays.asList("Apples", "Oranges", "Bananas"));
      map.put("list2", Arrays.asList("Grapes", "Kiwi", "Apples"));
      map.put("list3", Arrays.asList("Mangos", "Kiwi", "Berries"));

      String val = "Kiwi";
      List<String> keys = map.entrySet().stream().filter(
            entry -> entry.getValue().contains(val)).map(
                  entry -> entry.getKey()).collect(Collectors.toList());

      System.out.println(keys);

It works as follows:

• Map the entrySet of the map to a stream of entrySets.
• Filter the entry value by checking the value (a list of items) to see if it contains the desired item.
• If it does, map the result to the entry key for that list.
• Collect the keys in a list.

For the search val of Kiwi
This produces the output  [list3, list2]

If you want to stop after you find the first entry you could alter it slightly.

       for (String item : List.of("Apples", "Fish")) {

          String key = map.entrySet().stream().filter(
               entry -> entry.getValue().contains(item)).map(
                     entry -> entry.getKey()).findFirst().orElse(
                           "Not Found");

         System.out.println("Key for item " + item + " = " + key);

      }

This produces the following output.

Key for item Apples = list1
Key for item Fish = Key Not Found

This uses a findFirst to stop after the first one is found. Since it returns an Optional, the orElse method can be used to specify a default value to indicate that no list contained the item.

You can also specify parallel to process these streams using threads. This can greatly increase processing speed. Especially if you have multiple cores.

Answer 4

Please take a look at:

String key = null;
for (Map.Entry<String, ArrayList<String>> emailListEntry : AddressBook.addressMap.entrySet()) {
    for (String email: emailListEntry.getValue()){
        if(email.equals("TEST")){
            key =  emailListEntry.getKey(); 
            break;
        }
    }
    if (key!=null){
        break;
    }
}

Answer 5

Map<String,List> map = new HashMap<>();
    map.put( "list1", Arrays.asList( "Hello1", "Hello2" ) );
    map.put( "list2", Arrays.asList( "Hi", "Hi2" ) );

    for ( String key : map.keySet() ) {

        List list = map.get( key );
        if ( list.contains( "Hi2" ) ) {
            System.out.println( "This key contained the string " + key );
            break;
        }
    }