Populating hashmap with a loop in java

Question

Hi i have a problem populating hashmap with a loop, i need to store objects as a value and string as a key and if another element wants to be added to hashmap it should be checked against, if the value already exist, if so it should increment the the string key by 1 for example:

("JButton", JButtonObject);

another element wants to be added if again JButton it should be

("JButton1", JButtonObject);
...
...
...

my code: Thanks guys

private void CreateInstance(java.awt.event.ActionEvent evt) {                                
    // add code here

    Object object = null;
    if (evt.getSource() == CreateInstance)
    {
        int[] selectedIx = ClassList.getSelectedIndices();
        for (int i=0; i<selectedIx.length; i++) {
                Object sel = ClassList.getModel().getElementAt(selectedIx[i]);
                try {
                      Class classDefinition = Class.forName(sel.toString());
                      object = classDefinition.newInstance();
                      //create name
                      String data = sel.toString();
                      String substring = data.substring(12);
                      //check if name is unique

                      //add to map
                      hm.put(substring, object);----- HERE IS THE PROBLEM
                      System.out.println();
                } 
                catch (InstantiationException e) {
                      System.out.println(e);
                  } 
                catch (IllegalAccessException e) {
                      System.out.println(e);
                  } 
                catch (ClassNotFoundException e) {
                      System.out.println(e);
             }
             if(object instanceof java.awt.Component){
                DesignWindow.add((java.awt.Component)object);
                DesignWindow.validate();
             }
             else{
                System.out.println("Error");
             }
        }
    }
}

卢声远 Shengyuan Lu · Accepted Answer · 2010-08-25 07:53:14Z

2

Define a structure:

Map<String,LinkedHashMap<String,Object>> map=new LinkedHashMap<String,LinkedHashMap<String,Object>>();

It looks like:

"Jbutton"--->#####################
             #"Jbutton" , object0#
             #"Jbutton1", object1#
             #"Jbutton2", object2#
             #####################

"JBox"  ---->##################
             #"JBox" , object0#
             #"JBox1", object1#
             #"JBox2", object2#
             ##################

Then use it in your method 'CreateInstance':

LinkedHashMap<String,Object> selectedMap=map.get(substring);
if(selectedMap==null){//First put in this kind of element
    selectedMap=new LinkedHashMap<String,Object>();
    map.put(substring, selectedMap);
}else{
    selectedMap.put(substring+ (selectedMap.size()==0? "":selectedMap.size()), object);
}

edited Aug 25, 2010 at 7:53

answered Aug 25, 2010 at 5:47

卢声远 Shengyuan Lu

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1 Comment

Hermann Hans Over a year ago

If the second structure is just the name plus an index that's incremented it's not necessary to use a second HashMap. An ArrayList would suffice since when doing the look up you will have to parse out string and index anyway...

Will A · Accepted Answer · 2010-08-25 04:51:17Z

1

Maintain a second HashMap of 'preferred key' (e.g. "JButton") to an integer, and then use the integer to determine the next suffix for the actual key you'd then use.

answered Aug 25, 2010 at 4:51

Will A

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Comments

helios · Accepted Answer · 2010-08-25 07:03:35Z

Maybe you should use another structure as everybody says. But for doing what you want to:

String substring = data.substring(12);
//check if name is unique

// add this line
String uniqueKey = findUniqueKey(hm, substring);

//add to map with the calculated value
hm.put(uniqueKey, object);----- HERE IS THE PROBLEM

and...

private String findUniqueKey(Map<String, ?> map, String prefix)
{
   if (!map.containsKey(prefix))
      return prefix;
   int i = 1;
   while(true) { // or check i against a maximum
      String candidate = prefix + i;
      if (!map.containsKey(candidate))
         return candidate;
      i++;
   }
}

This way you separate the logic for deciding the key from the rest of the code.

Sidenote: Try to use Map references. Only use HashMap for creating the instance. So your code is not linked to a particular implementation of what you need: a Map.

Andreas Dolk · Accepted Answer · 2010-08-25 06:51:32Z

I don't know your actual requirement. But maybe this alternative approach to store multiple objects for a single key works too, and if it does, it will make life much easier. I wouldn't construct different keys but use one key and a list of instances for that key:

HashMap<String, List<Object>> hm = new HashMap<String, List<Object>>();

// ...

try {
  Class classDefinition = Class.forName(sel.toString());
  object = classDefinition.newInstance();
  //create name
  String data = sel.toString();
  String substring = data.substring(12);

  // new code added here:
  List<Object> objects = hm.get(substring);
  if (obejcts == null) {
    objects = new ArrayList<Object>();
    hm.put(substring, objects);
  }
  objects.add(object);

// continue with catches

It will be a bit different to use, but you may be able to do all you wanted to do:

if (hm.get(substring) == null) { // no object stored for key substring }
int numberOfObjects = hm.get(substring).size();
Object firstObject = hm.get(substring).get(0);  // the first object for substring
List<Object> objects = hm.get(substring);  // all objects

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