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Java HashMap Operations
basicjava
Background
HashMap provides O(1) average time complexity for put, get, and containsKey operations in Java.
Problem
Write a Java method that counts character frequencies in a string using HashMap and returns the most frequent character.
Input
A string of characters.
Output
The most frequent character and its count, or null for empty input.
Sample tests
Sample 1
Input: "aabbbcccc"
Expected: c (4 times)
Your solution
Learning objective
Learn HashMap operations, the merge method for counting, and entrySet iteration.