30 Seconds of Java

Java CI with Gradle

Inspired by 30 seconds of code, this is a collection of reusable tested Java 11 compatible code snippets that you can understand in 30 seconds or less.

How to contribute

In general we are interested in well implemented standalone code snippets. There are some tasks that we would like to see implemented in the issue list. Please raise a new issue if there's an interesting snippet you would like to see in this library but can't implement yourself.

For new snippets the general implementation steps are:

  • Update Library.java with the new code snippet.
  • Add a test for the new code snippet in LibraryTest.java.
  • Finally insert the new snippet into this README.md.
  • Submit pull request against master branch.

Table of Contents

Algorithm

Array

File

Math

Media

Networking

String

Class

I/O

Algorithm

Quicksort

    public static void quickSort(int[] arr, int left, int right) {
        var pivotIndex = left + (right - left) / 2;
        var pivotValue = arr[pivotIndex];
        var i = left;
        var j = right;
        while (i <= j) {
            while (arr[i] < pivotValue) {
                i++;
            }
            while (arr[j] > pivotValue) {
                j--;
            }
            if (i <= j) {
                var tmp = arr[i];
                arr[i] = arr[j];
                arr[j] = tmp;
                i++;
                j--;
            }
            if (left < i) {
                quickSort(arr, left, j);
            }
            if (right > i) {
                quickSort(arr, i, right);
            }
        }
    }

Bubblesort

  public static void bubbleSort(int[] arr) {
    var lastIndex = arr.length - 1;

    for(var j = 0; j < lastIndex; j++) {
      for(var i = 0; i < lastIndex - j; i++) {
        if(arr[i] > arr[i + 1]) {
          var tmp = arr[i];
          arr[i] = arr[i + 1];
          arr[i + 1] = tmp;
        }
      }
    }
  }

Selectionsort

    public static void selectionSort(int[] arr) {
        var len = arr.length;
        
        for (var i = 0; i < len - 1; i++) {
            var minIndex = i;
        
            for (var j = i + 1; j < len; j++) {
                if(arr[j] < arr[minIndex])
                  minIndex = j;
            }
        
            var tmp = arr[minIndex];
            arr[minIndex] = arr[i];
            arr[i] = tmp;
        }
    }

Array

Generic two array concatenation

    public static <T> T[] arrayConcat(T[] first, T[] second) {
        var result = Arrays.copyOf(first, first.length + second.length);
        System.arraycopy(second, 0, result, first.length, second.length);
        return result;
    }

Generic N array concatenation

    public static <T> T[] nArrayConcat(T[] first, T[]... rest) {
        var totalLength = first.length;
        for (var array : rest) {
            totalLength += array.length;
        }
        var result = Arrays.copyOf(first, totalLength);
        var offset = first.length;
        for (var array : rest) {
            System.arraycopy(array, 0, result, offset, array.length);
            offset += array.length;
        }
        return result;
    }

Check if all elements of array are equal

  public static <T> boolean allEqual(T[] arr) {
    return Arrays.stream(arr).distinct().count() == 1;
  }

Find maximum integer from the array

   public static int findMax(int[] arr) {
      return Arrays.stream(arr).reduce(Integer.MIN_VALUE, Integer::max);
    }

File

List directories

    public static File[] listDirectories(String path) {
        return new File(path).listFiles(File::isDirectory);
    }

List files in directory

    public static File[] listFilesInDirectory(final File folder) {
        return folder.listFiles(File::isFile);
    }

List files in directory recursively

    public static List<File> listAllFiles(String path) {
        var all = new ArrayList<File>();
        var list = new File(path).listFiles();
        if (list != null) {  // In case of access error, list is null
            for (var f : list) {
                if (f.isDirectory()) {
                    all.addAll(listAllFiles(f.getAbsolutePath()));
                } else {
                    all.add(f.getAbsoluteFile());
                }
            }
        }
        return all;
    }

Read lines from file to string list

    public static List<String> readLines(String filename) throws IOException {
        return Files.readAllLines(new File(filename).toPath());
    }

Zip file

    public static void zipFile(String srcFilename, String zipFilename) throws IOException {
        var srcFile = new File(srcFilename);
        try (
            var fileOut = new FileOutputStream(zipFilename);
            var zipOut = new ZipOutputStream(fileOut);
            var fileIn = new FileInputStream(srcFile);
        ) {
            var zipEntry = new ZipEntry(srcFile.getName());
            zipOut.putNextEntry(zipEntry);
            final var bytes = new byte[1024];
            int length;
            while ((length = fileIn.read(bytes)) >= 0) {
                zipOut.write(bytes, 0, length);
            }
        }
    }

Zip multiple files

    public static void zipFiles(String[] srcFilenames, String zipFilename) throws IOException {
        try (
            var fileOut = new FileOutputStream(zipFilename);
            var zipOut = new ZipOutputStream(fileOut);
        ) {
            for (var i=0; i<srcFilenames.length; i++) {
                var srcFile = new File(srcFilenames[i]);
                try (var fileIn = new FileInputStream(srcFile)) {
                    var zipEntry = new ZipEntry(srcFile.getName());
                    zipOut.putNextEntry(zipEntry);
                    final var bytes = new byte[1024];
                    int length;
                    while ((length = fileIn.read(bytes)) >= 0) {
                        zipOut.write(bytes, 0, length);
                    }
                }
            }
        }
    }

Math

Fibonacci

    public static int fibonacci(int n) {
        if (n <= 1) return n;
        else return fibonacci(n-1) + fibonacci(n-2);
    }

Factorial

    public static int factorial(int number) {
        var result = 1;
        for (var factor = 2; factor <= number; factor++) {
            result *= factor;
        }
        return result;
    }

Lottery

    public static Integer[] performLottery(int numNumbers, int numbersToPick) {
        var numbers = new ArrayList<Integer>();
        for(var i = 0; i < numNumbers; i++) {
            numbers.add(i+1);
        }
        Collections.shuffle(numbers);
        return numbers.subList(0, numbersToPick).toArray(new Integer[numbersToPick]);
    }

Greatest Common Divisor

    public static int gcd(int a, int b) { 
        if (b == 0) 
            return a; 
        return gcd(b, a % b);  
    }

Prime

	public static boolean isPrime(int number) {
		if (number < 3) {
			return true;
		}
		// check if n is a multiple of 2
		if (number % 2 == 0) {
			return false;
		}
		// if not, then just check the odds
		for (var i = 3; i * i <= number; i += 2) {
			if (number % i == 0) {
				return false;
			}
		}
		return true;
	}

Media

Capture screen

    public static void captureScreen(String filename) throws AWTException, IOException {
        var screenSize = Toolkit.getDefaultToolkit().getScreenSize();
        var screenRectangle = new Rectangle(screenSize);
        var robot = new Robot();
        var image = robot.createScreenCapture(screenRectangle);
        ImageIO.write(image, "png", new File(filename));
    }

Networking

HTTP GET

    public static HttpResponse<String> httpGet(String uri) throws Exception {
        var client = HttpClient.newHttpClient();
        var request = HttpRequest.newBuilder()
                .uri(URI.create(uri))
                .build();
        return client.send(request, BodyHandlers.ofString());
    }

HTTP POST

    public static HttpResponse<String> httpPost(String address, HashMap<String,String> arguments) throws IOException, InterruptedException{
        var sj = new StringJoiner("&");
        for(var entry : arguments.entrySet()) {
            sj.add(URLEncoder.encode(entry.getKey(), "UTF-8") + "=" 
                 + URLEncoder.encode(entry.getValue(), "UTF-8"));
        }
        var out = sj.toString().getBytes(StandardCharsets.UTF_8);
        var request = HttpRequest.newBuilder()
                           .uri(URI.create(address))
                           .headers("Content-Type", "application/x-www-form-urlencoded; charset=UTF-8")
                           .POST(BodyPublishers.ofByteArray(out))
                           .build();

        return HttpClient.newHttpClient().send(request, BodyHandlers.ofString());
    }

String

Palindrome check

    public static boolean isPalindrome(String s) {
        var sb = new StringBuilder();
        for (var c : s.toCharArray()) {
            if (Character.isLetter(c)) {
                sb.append(c);
            }
        }
        var forward = sb.toString().toLowerCase();
        var backward = sb.reverse().toString().toLowerCase();
        return forward.equals(backward);
    }

Reverse string

    public static String reverseString(String s) {
        return new StringBuilder(s).reverse().toString();
    }

String to date

    public static Date stringToDate(String date, String format) throws ParseException {
        var simpleDateFormat = new SimpleDateFormat(format);
        return simpleDateFormat.parse(date);
    }

Anagram Check

public boolean isAnagram(String s1, String s2) {
	var l1 = s1.length();
	var l2 = s2.length();
	var arr1 = new int[256];
	var arr2 = new int[256];
	if (l1 != l2) {
	  return false;
	}
	for (var i = 0; i < l1; i++) {
	  arr1[s1.charAt(i)]++;
	  arr2[s2.charAt(i)]++;
	}
	return Arrays.equals(arr1, arr2);
  }

Class

Get methods name

    public static List<String> getAllMethods(final Class<?> cls) {
        var list = new ArrayList<String>();
        for (var method : cls.getDeclaredMethods()) {
          list.add(method.getName());
        }
        return list;
    }

Get fields name

  public static List<String> getAllFieldNames(final Class<?> cls) {
    return Arrays.stream(cls.getFields())
            .map(Field::getName)
            .collect(Collectors.toList());
  }

Find Levenshtein distance

  public static int findLevenshteinDistance(String word1, String word2) {
    // If word2 is empty, removing
    int[][] ans = new int[word1.length() + 1][word2.length() + 1];
    for (int i = 0; i <= word1.length(); i++) {
      ans[i][0] = i;
    }
    // if word1 is empty, adding
    for (int i = 0; i <= word2.length(); i++) {
      ans[0][i] = i;
    }
    // None is empty
    for (int i = 1; i <= word1.length(); i++) {
      for (int j = 1; j <= word2.length(); j++) {
        int min = Math.min(Math.min(ans[i][j - 1], ans[i - 1][j]), ans[i - 1][j - 1]);
        ans[i][j] = word1.charAt(i - 1) == word2.charAt(j - 1) ? ans[i - 1][j - 1] : min + 1;
      }
    }
    return ans[word1.length()][word2.length()];
  }

I/O

Read file by stream

  public static List<String> readFile(String fileName) throws FileNotFoundException {
    try (Stream<String> stream = new BufferedReader(new FileReader(fileName)).lines()) {
      return stream.collect(Collectors.toList());
    }
  }

InputStream to String

   public static String inputStreamToString(InputStream inputStream) throws IOException {
     try (var reader = new BufferedReader(new InputStreamReader(inputStream))) {
       var stringBuilder = new StringBuilder();
       var data = reader.read();
 
       while (data != -1) {
         stringBuilder.append((char) data);
         data = reader.read();
       }
       return stringBuilder.toString();
     }
   }