sorting of HashMap in Java

Question

I have a Map, with keys and values in it

Keys:- case 1, case 2......case 10

Values:- Jan, Feb......Oct

Now when i put this map in array list (so that i can sort it)

for (String key : dataValueMap.keySet()) {
    System.out.println(key + " " + dataValueMap.get(key));
}
ArrayList<String> keys = new ArrayList<String>(
dataValueMap.keySet());
Collections.sort(keys);
for (String counter : keys) {
   System.out.println(counter); >>>>>BUT it prints like :- case 1  then  case 10  then  case 2, case 3, case 4 etc.
}

Kindly help in fixing this issue,

Thanks in advance

Well yes, you have strings, and as strings, "10" sorts before "2". If all your entries are really integers, you should treat them that way. It's not clear what this really has to do with maps though - essentially you're just populating an ArrayList<String> with the strings. The fact that they happened to be keys in a map is irrelevant. — Jon Skeet
– Jon Skeet, Commented Oct 13, 2016 at 13:10
(And if you only ever have keys 1-12, I'd suggest just using an array instead...) — Jon Skeet
– Jon Skeet, Commented Oct 13, 2016 at 13:11

d.j.brown · Accepted Answer · 2016-10-13 13:14:57Z

3

Use a custom Comparator implementation to sort your collection. This takes and compares the integers of the key, but is reliant on the format case n

Example

List<String> keys = new ArrayList<>();
keys.add("case 10");        
keys.add("case 3");        
keys.add("case 1");
keys.add("case 2");        
Collections.sort(keys, new Comparator<String>() {

    @Override
    public int compare(String s1, String s2) {
        s1 = s1.split(" ")[1];
        s2 = s2.split(" ")[1];
        return Integer.compare(Integer.parseInt(s1), Integer.parseInt(s2));
    }
});

for (String key : keys) {
    System.out.println(key);
}

answered Oct 13, 2016 at 13:14

d.j.brown

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Comments

Darshan Mehta · Accepted Answer · 2016-10-13 13:17:50Z

1

If you are certain about the key format then you can split it by space and compare the actual value with comparator, e.g.:

Map<String, Object> myMap = 
        new TreeMap<String, Object>(new Comparator<String>()
        {
            public int compare(String o1, String o2)
            {
                Integer value1 = Integer.parseInt(o1.split("\\s+")[1]);
                Integer value2 = Integer.parseInt(o2.split("\\s+")[1]);
                return value1.compareTo(value2);
            } 
});

Although it would break if the key format is not correct, we can add the handling for that depending upon the expected behavior.

answered Oct 13, 2016 at 13:17

Darshan Mehta

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Comments

Raman Sahasi · Accepted Answer · 2016-10-13 13:46:19Z

0

Well you just have to define a new custom Comparator for your version of string and can write any logic that will compare two Strings from the list.

List<String> keys = new ArrayList<>();

keys.add("case 10");    
keys.add("case 3");        
keys.add("case 1");
keys.add("case 2");

Comparator<String> comparator = new Comparator<String>(){
    @Override
    public int compare(String s1, String s2) {
        return Integer.compare(Integer.parseInt(s1.substring(5)), Integer.parseInt(s2.substring(5)));
    }  
};

Collections.sort(keys, comparator);
for (String counter : keys) {
    System.out.println(counter);
}

answered Oct 13, 2016 at 13:46

Raman Sahasi

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