您可以使用Java8提供的流。

public static void main(String[] args) {
    List<String> arr = Arrays.asList("the", "in", "the", "only", "the", "was", "the", "it", "the", "was", "in", "only", "the",
            "was", "the", "was", "in");
    Map<String, Long> numOccurrences = arr.stream().collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
    System.out.println(numOccurrences);
}

输出：

{the=7, in=3, was=4, only=2, it=1}

如果正确地创建了频率Map，则在迭代Map时不需要内部循环。
演示：

import java.util.HashMap;
import java.util.Map;

class Main {
    public static void main(String[] args) {
        String[] arr = { "the", "in", "the", "only", "the", "was", "the", "it", "the", "was", "in", "only", "the",
                "was", "the", "was", "in" };

        // Create the frequency map
        Map<String, Integer> map = new HashMap<>();
        for (String s : arr) {
            map.merge(s, 1, Integer::sum);
        }

        // Display
        for (Map.Entry<String, Integer> entry : map.entrySet()) {
            System.out.println(entry.getKey() + ": " + entry.getValue());
        }
    }
}

输出：

the: 7
in: 3
was: 4
only: 2
it: 1