Java How To

›Short versionOpen if you only have a minute.

Each recipe below is the happy path. The note under it is the input that still compiles and gives the wrong answer.
int addition wraps. int division drops the remainder. A sort writes into the array the caller still holds.
Math.random() never returns 1. A cast of Math.random() * 100 stops at 99.
A single pass finds a maximum. Sorting the array to read the last index is a different cost, and it reorders the data.

Short programs for a job you already know how to name. Read the snippet, then the bound it assumes: the type, the empty input, and whether the method mutates what you passed in.

Strings

Reverse a string

java

String original = "Hello";
String reversed = new StringBuilder(original)
        .reverse()
        .toString();
System.out.println(reversed); // olleH

reverse swaps char values. A character stored as a surrogate pair, including many emoji, comes back with the two halves exchanged. Building the reverse with += copies the whole string on every character.

Count words

java

String text = "Java is fun";
String[] words = text.trim().split("\\s+");
System.out.println(words.length); // 3

split(" ") counts the empty token between two spaces. "".split("\\s+") still returns one element, so an all-blank string has to return 0 before the split.

Numbers

Even or odd

java

int num = 10;
if (num % 2 == 0) {
    System.out.println("Even");
} else {
    System.out.println("Odd");
}

Zero is even. In Java -3 % 2 is -1, so n % 2 == 1 misses every negative odd int. Test odd with n % 2 != 0.

A random int from 0 through 100

java

int randomNum = (int) (Math.random() * 101);
System.out.println(randomNum);

Math.random() is in [0.0, 1.0). Multiplying by 100 and casting never returns 100. The 101 is the inclusive width.

Arrays

Largest value

java

int[] nums = {3, 7, 2, 9};
int max = nums[0];
for (int n : nums) {
    if (n > max) {
        max = n;
    }
}
System.out.println(max); // 9

Starting max at 0 returns 0 for {-5, -1, -8}, and 0 is not in the array. An empty array has no first element. Sorting just to read the last index is O(n log n) and reorders the array a linear scan would have left alone.

Sort

java

import java.util.Arrays;

int[] nums = {5, 1, 3};
int[] ordered = nums.clone();
Arrays.sort(ordered);
System.out.println(Arrays.toString(ordered)); // [1, 3, 5]
System.out.println(Arrays.toString(nums));    // [5, 1, 3]

Arrays.sort on an int[] writes into that array. Clone first when the caller still needs the original order. println on the array itself prints a type and a hash, not the elements.

Why is the multiplier 101, not 100, for a roll from 0 through 100?

Math.random() * 100 lands in [0.0, 100.0). The cast to int keeps 0 through 99.

Math.random() * 101 lands in [0.0, 101.0), so the cast keeps 0 through 100. Random.nextInt(101) documents that exclusive bound without the double.