Java List Cheatsheet

Published: Oct 17, 2021
Updated: Nov 1, 2021
Table of Contents

While at the hospital for the birth of our second child, I was reading Java by Comparison, and liked their teaching style. Fast-forward to our last day there, I was taking a quick, scalding-hot shower, and thought, “Hey, it would be cool if there was a cheatsheet on how to do common list operations in Java”. The thing you’re reading is my go at that.

Rules #

  1. Only the Java API can be used. No external libraries allowed
  2. All code samples must work with Java 11

Run it #

These samples are also available in src/App.java. You can run them with:

javac App.java && java App

Boilerplate #

To reduce boilerplate, the following functions will be used:

static void log(String format, Object... args) {
  System.out.println(String.format(format, args));
}

static List<String> createLetters() {
  return Arrays.asList("a", "b", "c", "c");
}

static List<Integer> createNumbers() {
  return Arrays.asList(1, 2, 3);
}

Linear search a list # 

List<String> letters = createLetters();

int index = IntStream.range(0, letters.size())
  .filter(i -> letters.get(i).equals("c"))
  .findFirst()
  .orElse(-1);

log("The first index of c in %s is %s", letters, index);
// The first index of c in [a, b, c, c] is 2

Binary search a list # 

List<String> letters = createLetters();

// Reminder that binarySearch requires a sorted list.
// For this sample, letters is already sorted.
// Collections.sort(letters);

int index = Collections.binarySearch(letters, "c");

log("The first index of c in %s is %s", letters, index);
// The first index of c in [a, b, c, c] is 2

Sort a list # 

List<String> letters = createLetters();

List<String> reversed = letters
  .stream()
  .sorted(Comparator.reverseOrder())
  .collect(Collectors.toList());

log("When %s is reversed it becomes %s", letters, reversed);
// When [a, b, c, c] is reversed it becomes [c, c, b, a]

Filter a list # 

List<Integer> numbers = createNumbers();

List<Integer> evenNumbers = numbers
  .stream()
  .filter(number -> number % 2 == 0)
  .collect(Collectors.toList());

log("The even numbers in %s are %s", numbers, evenNumbers);
// The even numbers in [1, 2, 3] are [2]

Transform a list # 

List<String> letters = createLetters();

List<String> uppercasedLetters = letters
  .stream()
  .map(String::toUpperCase)
  .collect(Collectors.toList());

log("When %s is uppercased it becomes %s", letters, uppercasedLetters);
// When [a, b, c, c] is uppercased it becomes [A, B, C, C]

Remove duplicate values from a list # 

List<String> letters = createLetters();

List<String> uniqueLetters = letters
  .stream()
  .distinct()
  .collect(Collectors.toList());

log("These are duplicated %s but these are unique %s", letters, uniqueLetters);
// These are duplicated [a, b, c, c] but these are unique [a, b, c]

Count how many times a value appears in a list # 

List<String> letters = createLetters();

 long count = letters
  .stream()
  .filter(letter -> letter.equals("c"))
  .count();

log("%s appears %s times in %s", "c", count, letters);
// c appears 2 times in [a, b, c, c]

Sum a list # 

List<Integer> numbers = createNumbers();

int sum = numbers
  .stream()
  .reduce(0, Integer::sum);

log("The sum of %s is %s", numbers, sum);
// The sum of [1, 2, 3] is 6

Find the max value in a list # 

List<Integer> numbers = createNumbers();

int max = numbers
  .stream()
  .mapToInt(Integer::valueOf)
  .max()
  .orElseThrow();

log("The max of %s is %s", numbers, max);
// The max of [1, 2, 3] is 3

Find the min value in a list # 

List<Integer> numbers = createNumbers();

int min = numbers
  .stream()
  .mapToInt(Integer::valueOf)
  .min()
  .orElseThrow();

log("The min of %s is %s", numbers, min);
// The min of [1, 2, 3] is 1

Does any list value meet the condition? # 

List<Integer> numbers = createNumbers();

boolean result = numbers
  .stream()
  .anyMatch(number -> number % 2 == 0);

log("Some numbers in %s are even? %s", numbers, result);
// Some numbers in [1, 2, 3] are even? true

Do all list values meet the condition? # 

List<Integer> numbers = createNumbers();

boolean result = numbers
  .stream()
  .allMatch(number -> number % 2 == 0);

log("All numbers in %s are even? %s", numbers, result);
// All numbers in [1, 2, 3] are even? false