Java CheatSheet for Developers

In this post I will keep some useful Java hacks and one-liners which are useful in every day programming

Multiple Each Item in a List by 2

IntStream.range(1, 10).map(i -> i * 2);

Sum a List of Numbers

IntStream.range(1, 100).sum();

Verify if String exists in a List

List wordList = Arrays.asList("Maven", "jdk", "JBoss", "java");
String tweet = "This is a JBoss tutorial";
wordList.stream().anyMatch(str::contains);

Create a Map where keys are “true” if > 50

Map<Boolean, List<Integer>> passedFailedMap = Stream.of(29, 18, 76, 82, 50, 88).collect(Collectors.partitioningBy(i -> i > 50));
System.out.println(passedFailedMap);
//{false=[29, 18, 50], true=[76, 82, 88]}

Find minimum (or maximum) in a List:

IntStream.of(24, 45, -3, 26, 18).min();
Arrays.asList(24, 45, -3, 26, 18).stream().min(Integer::compare);
Arrays.asList(24, 45, -3, 26, 18).stream().reduce(Integer::min);
Collections.min(Arrays.asList(24, 45, -3, 26, 18));

Sorted List by HashMap keys:

SortedSet keys = new TreeSet(myHashMap.keySet());

Sorted List by HashMap values:

SortedSet values = new TreeSet(myHashMap.values());

Sort a Map by values

Stream<Map.Entry<K,V>> sorted =
    map.entrySet().stream()
       .sorted(Map.Entry.comparingByValue());

Sorting ArrayList in ascending Order

Collections.sort(list);

Sort an ArrayList using a custom Object’s property

Collections.sort(Database.arrayList, 
                        (o1, o2) -> o1.getStartDate().compareTo(o2.getStartDate()));

How to generate a Random String

String uuid = UUID.randomUUID().toString();

Parallel Processing

List dataList = null;
dataList.parallelStream().map(line -> processItem(line));

Read a File in one line

String fileText = new String(Files.readAllBytes(Paths.get("data.txt")));
List fileLines = Files.readAllLines(Paths.get("data.txt"));

Initialization of an ArrayList in one line

List<String> places = Arrays.asList("Cat", "Dog", "Mouse");

How do I determine whether an array contains a particular value in Java?

Arrays.asList(yourArray).contains(yourValue)

String[] values = {"AB","BC","CD","AE"};
boolean contains = Arrays.stream(values).anyMatch("s"::equals);

How to print the content of an Array

String[] array = new String[] {"Fred", "John", "Bob"};
System.out.println(Arrays.toString(array));

Create ArrayList from array

new ArrayList<>(Arrays.asList(array));

How do I read / convert an InputStream into a String in Java?

#Option 1
String result = org.apache.commons.io.IOUtils.toString(inputStream, StandardCharsets.UTF_8);

#Option 2
Scanner s = new Scanner(inputStream).useDelimiter("\\A");
String result = s.hasNext() ? s.next() : "";

#Option 3
ByteArrayOutputStream result = new ByteArrayOutputStream();
byte[] buffer = new byte[1024];
for (int length; (length = inputStream.read(buffer)) != -1; ) {
 result.write(buffer, 0, length);
}
return result.toString("UTF-8");

How do I iterate over each entry in a Java Map?

Map<String, String> map = ...
for (Map.Entry<String, String> entry : map.entrySet()) {
    System.out.println(entry.getKey() + "/" + entry.getValue());
}

#Java 10+:

for (var entry : map.entrySet()) {
    System.out.println(entry.getKey() + "/" + entry.getValue());
}

Java CheatSheet – by Mastertheboss (2022). Read more CheatSheets.