A HashMap has plenty of uses, so let's see how you can find keys and values, load data from a CSV file into one, and use it for sorting.
Introduction
Having explored HashMap in several previous articles (here and here), let us now learn how to search and sort a HashMap by key as well as value.
Finding Keys and Values in HashMap
Finding a key in a HashMap is quite simple. The HashMap API provides the containsKey() method, which tells you whether the key exists.
Map<String,Integer> namefreq = new HashMap<>();
namefreq.put("Petra", 14);
namefreq.put("Mario", 11);
namefreq.put("Kasandra", 23);
namefreq.put("Charity", 18);
namefreq.put("Minerva", 5);
if ( namefreq.containsKey("Charity") ) {
}
Finding a value is also easy given the method containsValue().
if ( namefreq.containsValue(10) ) {
}
Pretty simple, right? Well, what if you need to find not a
particular value but search for a general expression, such as names
starting with say “A”. Gets a bit more involved.
Loading CSV Into HashMap
The following search and sort examples make use of a name
frequency table, which is loaded from CSV using the following code. It
streams lines from a CSV file, splits the line into fields, selects
values for the “
CA” state, and stores the value as a Map of (year => count). (The data is from US Census Bureau which publishes name frequency counts for each year.)
The code creates a multi-HashMap with the structure:
(name => ((year => count),
(year => count)),
...
Pattern pattern = Pattern.compile(",");
String csvFile = "StateNames.csv";
try (BufferedReader in = new BufferedReader(new FileReader(csvFile));){
Map<String,Map<Integer,Integer>> namefreq = in
.lines()
.skip(1)
.map(x -> pattern.split(x))
.filter(x -> x[4].equals("CA"))
.collect(HashMap::new, (map, x) ->
map.compute(x[1], (k, v) -> {
if ( v == null )
v = new HashMap<Integer,Integer>();
v.put(Integer.parseInt(x[2]),
Integer.parseInt(x[5]));
return v;
}),
Map::putAll);
}
Here is a snippet of the sample data being loaded.
Id,Name,Year,Gender,State,Count
1,Mary,1910,F,AK,14
2,Annie,1910,F,AK,12
3,Anna,1910,F,AK,10
4,Margaret,1910,F,AK,8
5,Helen,1910,F,AK,7
6,Elsie,1910,F,AK,6
7,Lucy,1910,F,AK,6
8,Dorothy,1910,F,AK,5
9,Mary,1911,F,AK,12
10,Margaret,1911,F,AK,7
Search Key in HashMap
The following code searches for a key ending with “
x” and prints the matches.namefreq
.entrySet()
.stream()
.filter(e -> e.getKey().endsWith("x"))
.forEach(e -> {
System.out.println(e.getKey() + " :");
e.getValue().forEach((kk, vv) -> {
System.out.println(" " + kk + " => " + vv);
});
});
Rex :
1959 => 86
Margaux :
2003 => 8
Maxx :
2010 => 28
Search Value in HashMap
Let us now search for a value in the HashMap using the same method. Before we do that, let us total the names for each year and store it under the key
0 for each name. Something like this:Elza :
0 => 10
1993 => 5
2014 => 5
...
We compute the total value with the following code. It
computes a total of the existing mappings of (year => count) and
stores it under key
0.namefreq
.entrySet()
.stream()
.forEach(e -> {
Map<Integer,Integer> v = e.getValue();
int tot = v
.entrySet()
.stream()
.reduce(0, (x, ee) -> x + ee.getValue(),
(x, y) -> x+y);
v.put(0, tot);
});
Let us now search the HashMap for names occurring more than
1000 times.namefreq
.entrySet()
.stream()
.filter(e -> e.getValue().getOrDefault(0, 0) > 1000)
.forEach(e ->
System.out.println(e.getKey()+" : "+e.getValue().get(0)));
...
Solomon : 1967
Javier : 28472
Esther : 16974
Lenora : 1261
Sam : 6971
Lenore : 1261
Rowan : 1297
Lukas : 2888
...
The actual search is being performed by the following segment:
.filter(e -> e.getValue().getOrDefault(0, 0) > 1000)
Here, you can use an expression of arbitrary complexity to
search for exactly what you need. In the following example, we search
for key containing the string “
mon” and total value more than 1000. The results are stored in another HashMap (for further processing).Map<String,Map<Integer,Integer>> subset = namefreq
.entrySet()
.stream()
.filter(e -> e.getKey().contains("mon")&&e.getValue().getOrDefault(0, 0) > 1000)
.collect(HashMap::new,
(m, e) -> m.put(e.getKey(), e.getValue()),
Map::putAll);
subset.forEach((k, v) -> System.out.println(k + " : " + v.get(0)));
Simone : 3114
Desmond : 2498
Ramona : 8139
Raymond : 60506
...
Sort HashMap by Key
To sort a HashMap by key and print the mappings, we can do something like this.
namefreq
.entrySet()
.stream()
.sorted((x, y) -> x.getKey().compareTo(y.getKey()))
.forEach(e -> {
System.out.println(e.getKey() + " :");
e.getValue().forEach((kk, vv) -> {
System.out.println(" " + kk + " => " + vv);
});
});
Lylia :
0 => 6
2009 => 6
Lyliana :
0 => 15
2007 => 5
1998 => 5
1999 => 5
...
To store the result (sorted map), we use a LinkedHashMap (which preserves the insertion order) as follows.
Map<String,Map<Integer,Integer>> subset = namefreq
.entrySet()
.stream()
.sorted((x, y) -> x.getKey().compareTo(y.getKey()))
.collect(LinkedHashMap::new,
(m, e) -> m.put(e.getKey(), e.getValue()),
Map::putAll);
subset.forEach((k, v) -> System.out.println(k + " : " + v.get(0)));
Byanca : 90
Byanka : 79
Byran : 65
Byron : 7254
Cache : 10
Cadance : 30
Cade : 1874
...
Sort HashMap by Value
Sorting the HashMap by value is very similar to the above example. The following code sorts the name-count HashMap by total-count and prints the top 20 names.
namefreq
.entrySet()
.stream()
.sorted((x, y) ->
y.getValue().get(0).compareTo(x.getValue().get(0)))
.limit(20)
.forEach(x -> System.out.println(x.getKey() + " => " +
x.getValue().get(0)));
Michael => 422157
David => 364853
Robert => 347637
John => 310120
James => 274168
Daniel => 244229
Richard => 222633
Christopher => 215728
William => 209173
Anthony => 174064
...
Summary
Searching for a key in a HashMap involves applying a
filtering pipeline to the entries. The same method is also applicable
to search for the HashMap values. In fact, arbitrarily complex
expressions can be used with such a pipeline for search. We also learned
how to sort the HashMap by keys or values and possibly store the results in a LinkedHashMap.
See Also
- Java Collections Introduction gives an overall picture of the most important classes and interfaces.
- Java Collections – HashMap presents parsing a CSV to a HashMap as well as a multi-map.
Source: Java HashMap Search and Sort
