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

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<String, JButton> listbouton = new HashMap<String, JButton>() ;

    public PanneauCalcul() {
        for(int i = 0; i < 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.

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) ;
}

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.

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<Map.Entry<String, Integer>> entries = new ArrayList<>();
entries.add(new AbstractMap.SimpleEntry<String, Integer>(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.

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 (+,-,/,*,=).

Map.Entry interface helps us iterating a Map class

Check this simple example:

public class MapDemo {
    public static void main(String[] args) {
     Map<Integer,String> 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());
        }
    }
}

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.