Java interview question: In Java 8 count frequency of chars in a String. In this post we’ll show two implementations – using Map.merge() and Streams and Collectors.
foreach with Java 8 Map.merge()
The simplest way to count frequency of chars in a String is to use the standard foreach loop for char array traversal and doing counting with fantastic Map.merge() available since Java 8:
String s = "abcaba";
Map<Character, Integer> freqs = new HashMap<>();
for (char c : s.toCharArray()) {
freqs.merge(c, // key = char
1, // value to merge
Integer::sum); // counting
}
System.out.println("Frequencies:\n" + freqs);
Isn’t that cool? :-) Map.merge(key, value, BiFunction) is doing the tedious update of the Map if a char exists there or not.
The above code prints:
Frequencies:
{a=3, b=2, c=1}
Java 8 Stream and Collector
Another approach could be using Stream of chars and using some Collector to count and store values, but the solution is a bit cumbersome due to typing:
String s = "abcaba";
Map<Character, Integer> frequencies = s.chars().boxed()
.collect(toMap(
// key = char
k -> Character.valueOf((char) k.intValue()),
v -> 1, // 1 occurence
Integer::sum)); // counting
System.out.println("Frequencies:\n" + frequencies);
The String.chars() method returns IntStream of characters, so we have to box them to Integer to resolve type problems in Collector.
Frequencies:
{a=3, b=2, c=1}
References:
- Java 8 Map.merge() explained
- Check out other Java Tutorials
