Sorted keys in a Map

[M Burke]

I want the keys to be sorted in ascending order. So I need a way to name them so it will show up this whey when I go to access the Map. I tried putting them in ascending order using a number. But when you run this, the order is wrong.

I am using this to populate a multi select combo box. Option entries can be added and removed based on other element entries, so I need to insure all the keys are unique and there origin is identifiable. So I am using the combo box ID and option ID to identify the entry. Example: select2Options1 = Combo box 2, Option Group 1.


This is a contrived example:

import java.util.Map;
import java.util.TreeMap;


public class TestTreeMap {

	public static void main(String[] args) {
		Map<String, String> mapOptions = new TreeMap<String, String>();

		for(int i = 1; i <= 30; i++){
			String key = i + "select2Options1 ";
			String value = "select2Options1:" + i;
			mapOptions.put(key, value);
		}
		
		
		for(String key: mapOptions.keySet()){
			String value = mapOptions.get(key);
			System.out.println(key + "\t" + value);
			
		}
		
		
	}

}

[Campbell Ritchie]

Your keys are Strings so they are not sorted by number order but by what is jocularly called asciibetical order. 19a does not come before 20a but before 1a. Try 000001select2Optional instead. You can get that maybe most easily with the String#format method.

[Greg Charles]

It would be easier if you could just use an Integer key. I know; you said you need the name of the selector and option group, but do you, really?

Another option is passing in your own Comparator class to the TreeMap. I know there are "smart" string comparators you could find that would compare strings with numbers in them in a more consistent way, and if you can't find one already built, it's not hard to write one yourself. Padding the number as Campbell suggested seems simpler in this case, but keep custom Comparators in mind. They can be a useful tool.

[M Burke]

Yes, I need the keys the way they are. I tried this and it works.

import java.util.Comparator;
import java.util.Map;
import java.util.TreeMap;


public class TestTreeMap {

	public static void main(String[] args) {
		Map<String, String> mapOptions = new TreeMap<String, String>(new CBIDSorter());

		for(int i = 1; i <= 30; i++){
			String key = i + "-select2Options1:" + i;
			String value = "select2Options1" + i;
			System.out.println(key + "\t" + value);
			mapOptions.put(key, value);
		}
		
		System.out.println("---------");
		
		for(String key: mapOptions.keySet()){
			String value = mapOptions.get(key);
			System.out.println(key + "\t" + value);
			
		}
		
		
	}
	
	
	

}

class CBIDSorter implements Comparator<String> {

	@Override
	public int compare(String sarg0, String sarg1) {
		int rc = 0;
		
		sarg0 = sarg0.substring(0, sarg0.indexOf('-'));
		sarg1 = sarg1.substring(0, sarg1.indexOf('-'));
		
		Integer iarg0 = Integer.parseInt(sarg0);
		Integer iarg1 = Integer.parseInt(sarg1);
		
		if(iarg0.compareTo(iarg1) > 0){
			rc = 1;
		}else if(iarg0.compareTo(iarg1) == 0){
			rc = 0;
		}else {
			rc = -1;
		}
		
		return rc;
	}
	
	
	
}

[Campbell Ritchie]

You can probably reduce that compare method to

@Override
public int compare(String sarg0, String sarg1) {
    int rc = 0;
         
    sarg0 = sarg0.substring(0, sarg0.indexOf('-'));
    sarg1 = sarg1.substring(0, sarg1.indexOf('-'));
         
    return sarg0.compareTo(sarg1);
}

Which will sort 19 before 2, or

@Override
public int compare(String sarg0, String sarg1) {
    int rc = 0;
         
    sarg0 = sarg0.substring(0, sarg0.indexOf('-'));
    sarg1 = sarg1.substring(0, sarg1.indexOf('-'));
         
    return Integer.valueOf(sarg0).compareTo(Integer.valueof(sarg1));
}

You can also use a regular expression for digits to find the number, or indexOf("select").

[Tim Holloway]

You can use a TreeMap. It has has an ordering to its keys.

I am a little worried about all you guys understanding what collating sequences are.

There are 2 options if you don't care what the code is.

1. Create a TreeMap<Integer, String> to hold your key/value pairs and increment an integer counter. Since the natural ordering of integers places 9 before 10, unlike text-based ordering where "1", "10", "11", "2",,, "9" is the rule, that's sufficient.

2. Alternatively, look at the Java formatting resources such as MessageFormat. You can build character strings in the form "001", "002", ... "009", "010", and they, too will collate properly, thanks to the leading zeroes.

[Greg Charles]

For Campbell's suggestion, just the basic String.format() could do the job. You could change your loop to:

        for(int i = 1; i <= 30; i++){
            String key = String.format("%06d%s", i, "select2Options1)";
            String value = "select2Options1:" + i;
            mapOptions.put(key, value);
        }

The keys would sort correctly without needing a custom Comparator, and probably marginally faster too.