Java Map sort by value
Solution 1
In your compare function, when the values are equal, you should then compare the keys. This will ensure that different keys having the same value will not be "merged", because it disambiguates entries that would otherwise compare equal.
For example:
@Override
public int compare(String a, String b) {
Integer x = base.get(a);
Integer y = base.get(b);
if (x.equals(y)) {
return a.compareTo(b);
}
return x.compareTo(y);
}
(you'll need to modify the code above to match your policy for null values)
Note that your approach of sorting on values is pretty fragile, though. Your "sorted" map will not support addition of new entries, which could be pretty confusing.
Solution 2
base.get(a) == base.get(b)
This code compares boxed Integer
s by reference.
Change it to base.get(a).equals(base.get(b))
and it should work.
Solution 3
Try this ...
HashMap<String, Integer> h = new HashMap<String, Integer>();
h.put("z",30);
h.put("e",10);
h.put("b",20);
h.put("c",20);
List<Map.Entry> a = new ArrayList<Map.Entry>(h.entrySet());
Collections.sort(a,
new Comparator() {
public int compare(Object o1, Object o2) {
Map.Entry e1 = (Map.Entry) o1;
Map.Entry e2 = (Map.Entry) o2;
return ((Comparable) e1.getValue()).compareTo(e2.getValue());
}
});
for (Map.Entry e : a) {
System.out.println(e.getKey() + " " + e.getValue());
}
Ouptut:
e 10
b 20
c 20
z 30
Solution 4
Try this:
return base.get(a).compareTo(base.get(b));
To demonstrate that you are incorrect about auto-unboxing:
Integer a = new Integer(2);
Integer b = new Integer(2);
boolean isEqual = ( a == b );
System.out.println("equal: " + isEqual);
System.out.println("a: " + a);
System.out.println("b: " + b);
My output is:
equal: false
a: 2
b: 2
![Terry Li](https://i.stack.imgur.com/4DoCf.jpg?s=256&g=1)
Terry Li
Updated on May 05, 2020Comments
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Terry Li about 4 years
I was looking for ways of sorting
Map<String, Integer>
by values. I found this post, which solved my sorting problem, but not exactly. According to the post, I wrote the following code:import java.util.*; public class Sort { static class ValueComparator implements Comparator<String> { Map<String, Integer> base; ValueComparator(Map<String, Integer> base) { this.base = base; } @Override public int compare(String a, String b) { if (base.get(a) >= base.get(b)) { return 1; } else { return -1; } } } public static void main(String[] args) { HashMap<String, Integer> map = new HashMap<String, Integer>(); ValueComparator vc = new ValueComparator(map); TreeMap<String, Integer> sorted = new TreeMap<String, Integer>(vc); map.put("A", 1); map.put("B", 2); sorted.putAll(map); for (String key : sorted.keySet()) { System.out.println(key + " : " + sorted.get(key)); // why null values here? } System.out.println(sorted.values()); // But we do have non-null values here! } }
Output:
A : null B : null [1, 2] BUILD SUCCESSFUL (total time: 0 seconds)
As you can see from the output, the
get
method always returnsnull
. The reason is myValueComparator.compare()
method never returns0
, which I've figured out by making this post.Someone suggested in that post the following to solve the
null
value problem:public int compare(String a, String b) { if (base.get(a) > base.get(b)) { return 1; }else if(base.get(a) == base.get(b)){ return 0; } return -1; }
I've tested this piece of code and it introduces a key merging problem. In other words, when the values are equal their corresponding keys are merged.
I've also tried the following:
public int compare(String a, String b) { if (a.equals(b)) return 0; if (base.get(a) >= base.get(b)) { return 1; } else return -1; }
It does not work either. Some of the values are still
null
. Besides, this workaround might potentially have logical problems.Anyone can propose a fully working solution to my problem? I'd like the sort by value function to work and the
get
method to work at the same time. -
Terry Li over 11 yearsI don't think it's the real problem :)
-
Terry Li over 11 yearsIt has the same key merging problem. By the way, you can compare
Integer
objects directly using==
. -
Terry Li over 11 yearsGood point! I didn't know that. But my problem persists even with the correct comparison method :(
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Terry Li over 11 yearsThank you! Solved my problem.
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SLaks over 11 years@TerryLi: No; the
==
operator does not autounbox.