## Top 100 Classes Used in Java Projects

A large portion of Java software development is using APIs from various libraries. From 10,000 open source Java projects, I extracted the frequency of API classes. The classes are either from Java standard library or from third-party libraries. Each class is counted once for each project. The list below shows the top 100.

## LeetCode – Nth Digit (Java)

Find the nth digit of the infinite integer sequence 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, … For example, given 3, output should be 3. Given 11, output should be 0. (The 11th digit of the sequence 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, … is a … Read more

## LeetCode – Binary Watch (Java)

Given a non-negative integer n which represents the number of LEDs that are currently on, return all possible times the watch could represent. Example: Input: n = 1 Return: [“1:00”, “2:00”, “4:00”, “8:00”, “0:01”, “0:02”, “0:04”, “0:08”, “0:16”, “0:32”] Accepted Java Solution public List<String> readBinaryWatch(int num) { List<String> result = new ArrayList<String>();   for(int i=0; … Read more

## LeetCode – Remove K Digits (Java)

Given a non-negative integer num represented as a string, remove k digits from the number so that the new number is the smallest possible. Note: The length of num is less than 10002 and will be â‰¥ k. The given num does not contain any leading zero. Example 1: Input: num = “1432219”, k = … Read more

## Find a Path in a Matrix

Given a 2d matrix, find a path from the top left corner to bottom right corner. Assume there exists at least one path, and you only need to find one valid path. You can move up, right, down, left at any position. For example, given [1, 0, 0, 0, 0] [1, 0, 1, 1, 1] … Read more

## LeetCode – Decode String (Java)

Given an encoded string, return it’s decoded string. For example, given “3[a2[b]]”, return “abbabbabb”. https://leetcode.com/problems/decode-string/ Java Solution The key to solve this problem is convert the string to a structured data structure and recursively form the return string. class Solution { public String decodeString(String s) { Stack<Exp> stack = new Stack<>();   Exp e = … Read more

## LeetCode – Longest Substring with At Least K Repeating Characters (Java)

Find the length of the longest substring T of a given string (consists of lowercase letters only) such that every character in T appears no less than k times. Example 1: Input: s = “aaabb”, k = 3 Output: 3 The longest substring is “aaa”, as ‘a’ is repeated 3 times. Java Solution This problem … Read more

## LeetCode – UTF-8 Validation (Java)

A character in UTF8 can be from 1 to 4 bytes long, subjected to the following rules: For 1-byte character, the first bit is a 0, followed by its unicode code. For n-bytes character, the first n-bits are all one’s, the n+1 bit is 0, followed by n-1 bytes with most significant 2 bits being … Read more