Map Entry How to use it

Question

I m working on creating a calculator  I put my buttons in a HashMap collection and when I want to add them to my class  which extends JPanel  I don t know how can I get the buttons from my collection  So I found on the internet the 2 last lines of my code  but I don t know their meaning   Here is my code   import java awt Component  import java util HashMap  import java util Iterator  import java util Map  import java util Set   import javax swing JButton  import javax swing JPanel    public class PanneauCalcul extends JPanel        private HashMap lt String  JButton gt  listbouton   new HashMap lt String  JButton gt           public PanneauCalcul             for int i   0  i  lt  10  i                   listbouton put  num    i  new JButton      i                        listbouton put  add   new JButton                 listbouton put  soustract   new JButton  -              listbouton put  multiply   new JButton  x              listbouton put  divise   new JButton                 listbouton put  equal   new JButton                  Set entrys   listbouton entrySet              Iterator iter   entrys iterator              while iter hasNext                  Map Entry me    Map Entry iter next       don t understand              this add  Component  me getValue          don t understand                    EcouteCalcul ecout   new EcouteCalcul this              I don t understand how can we use Map Entry--which is an interface--without redefining Map Entry s functions

User · Answer

A Map is a collection of Key   Value pairs  which is visualized like this     fooKey fooValue  barKey barValue   quxKey quxValue     The Map interface allows a few options for accessing this collection  The Key set  fooKey  barKey quxKey   the Value set  fooValue  barValue  quxValue  and finally entry Set  fooKey fooValue  barKey barValue   quxKey quxValue    Entry set is simply a convenience to iterate over the key value pairs in the map  the Map Entry is the representation of each key value pair  An equivalent way to do your last loop would be   for  String buttonKey  listbouton keySet          this add listbouton get buttonKey         or  for  JButton button  listbouton values          this add button

User · Answer

Map Entry is a key and its value combined into one class   This allows you to iterate over Map entrySet   instead of having to iterate over Map keySet    then getting the value for each key   A better way to write what you have is   for  Map Entry lt String  JButton gt  entry   listbouton entrySet        String key   entry getKey      JButton value   entry getValue       this add value       If this wasn t clear let me know and I ll amend my answer

User · Answer

This code is better rewritten as   for  Map Entry me   entrys entrySet           this add   Component  me getValue          and it is equivalent to   for  Component comp   entrys getValues           this add  comp        When you enumerate the entries of a map  the iteration yields a series of objects which implement the Map Entry interface  Each one of these objects contains a key and a value    It is supposed to be slightly more efficient to enumerate the entries of a map than to enumerate its values  but this factoid presumes that your Map is a HashMap  and also presumes knowledge of the inner workings  implementation details  of the HashMap class  What can be said with a bit more certainty is that no matter how your map is implemented   whether it is a HashMap or something else   if you need both the key and the value of the map  then enumerating the entries is going to be more efficient than enumerating the keys and then for each key invoking the map again in order to look up the corresponding value

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Hash-Map stores the  key value  pair as the Map Entry Type As you know that Hash-Map uses Linked Hash-Map In case Collision occurs   Therefore each Node in the Bucket of Hash-Map is of Type Map Entry  So whenever you iterate through the Hash-Map you will get Nodes of Type Map Entry                                                             Now in your example when you are iterating through the Hash-Map  you will get Map Entry Type Which is Interface   To get the Key and Value from this Map Entry Node Object  interface provided methods like getValue    getKey   etc  So as per the code  In your Object you are adding all operators JButtons viz    -

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Map Entry interface helps us iterating a Map class  Check this simple example   public class MapDemo       public static void main String   args         Map lt Integer String gt  map new HashMap         map put 1   Kamran            map put 2   Ali            map put 3   From            map put 4   Dir            map put 5   Lower            for Map Entry m map entrySet             System out println m getKey        m getValue

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A Map consists of key value pairs   For example  in your code  one key is  Add  and the associated value is JButton         A Map Entry is a single key value pair contained in the Map   It s two most-used methods are getKey   and getValue     Your code gets all the pairs into a Set   Set entrys   listbouton entrySet       and iterates over them   Now  it only looks at the value part using me getValue   and adds them to your PanneauCalcul  this add  Component  me getValue          don t understand   Often this type of loop  over the Map Entry  makes sense if you need to look at both the key and the value   However  in your case  you aren t using the keys  so a far simpler version would be to just get all the values in your map and add them   e g   for  JButton jb listbouton values        this add jb       One final comment   The order of iteration in a HashMap is pretty random   So the buttons will be added to your PanneauCalcul in a semi-random order   If you want to preserve the order of the buttons  you should use a LinkedHashMap

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x000D   x000D  public HashMap lt Integer Obj gt  ListeObj  new HashMap lt  gt      public void addObj String param1  String param2  String param3     Obj newObj   new Obj param1  param2  param3     this ListObj put newObj getId    newObj      public ArrayList lt Integer gt  searchdObj  int idObj     ArrayList lt Integer gt  returnList   new ArrayList lt  gt       for  java util Map Entry lt Integer  Obj gt  e   this ListObj entrySet         if e getValue   getName   equals idObj           returnList add e getKey               return returnList    x000D   x000D   x000D

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Note that you can also create your own structures using a Map Entry as the main type  using its basic implementation AbstractMap SimpleEntry  For instance  if you wanted to have an ordered list of entries  you could write   List lt Map Entry lt String  Integer gt  gt  entries   new ArrayList lt  gt     entries add new AbstractMap SimpleEntry lt String  Integer gt  myStringValue  myIntValue      And so on  From there  you have a list of tuples  Very useful when you want ordered tuples and a basic Map is a no-go

[java] Map.Entry: How to use it?

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