Java code examples · runnable

20 runnable Java code examples.

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How to convert a string to an int in JavaConvert a String to an int in Java with Integer.parseInt, know when Integer.valueOf and its cache matter, catch NumberFormatException cleanly, check whether a string is numeric first, and reach for Long.parseLong or BigInteger when the number is too big.How to iterate over a map in JavaIterate over a Java Map with a for-each loop over entrySet(), plus Map.forEach((k, v) -> …), keySet() and values(), why keySet() + get() does extra lookups, the iteration order of HashMap, LinkedHashMap and TreeMap, removing entries safely, and streams over entries.How to convert an array to a list in JavaConvert a Java array to a List with new ArrayList<>(Arrays.asList(arr)), the fixed-size Arrays.asList view, List.of (Java 9+) and a stream for int[], then turn it into a Set or back into an array with toArray(new String[0]) and mapToInt.How to generate a random number in a range in JavaGenerate a random int between min and max in Java with ThreadLocalRandom.current().nextInt(min, max + 1), avoid the exclusive-bound off-by-one, seed Random for repeatable numbers, get doubles and streams of random numbers, and build secure tokens and random alphanumeric strings with SecureRandom.How to check if an array contains a value in JavaCheck if a Java array contains a value with Arrays.asList(array).contains() for object arrays, Arrays.stream().anyMatch() for int[] and other primitives, a plain loop with equals() or Objects.equals(), a field test for arrays of objects, and a HashSet or Arrays.binarySearch() for many lookups.How to initialize a list in one line in JavaInitialize a Java List, Set or Map in one line with List.of, Set.of and Map.of (Java 9+), wrap them in new ArrayList<>(…) or new HashMap<>(…) when you need to add, and know where Arrays.asList, Stream.toList(), Map.ofEntries and a static final map fit. Plus why double-brace initialization is a trap.How to declare and initialize an array in JavaDeclare and initialize a Java array with the {…} initializer, new int[] {…} in-line, or new int[n] with default values, then fill it with Arrays.fill, Arrays.setAll or IntStream. Also covers 2-D and jagged arrays, length, and index bounds.How to print an array in JavaPrint a Java array with Arrays.toString, see why println(array) shows [I@ and a hash, print 2-D arrays with Arrays.deepToString, choose your own separator with String.join or Collectors.joining, and learn why Lists and char[] print fine on their own.How to convert a string to an enum in JavaConvert a string to an enum in Java with valueOf(), and handle what it throws: a case-insensitive lookup that returns Optional, a static map for custom codes, values() to loop, name() vs toString(), == vs equals() and switch on an enum.How to break out of nested loops in JavaBreak out of nested loops in Java with a labeled break outer;, skip to the next outer iteration with continue outer;, move the loops into a method and return, use a boolean flag, or replace a search with anyMatch and findFirst. Plus why a plain break only leaves the innermost loop.How to split a string in JavaSplit a string in Java with split(), which takes a regex: escape . | and $ or use Pattern.quote, keep trailing empty strings with a limit of -1, split once with a limit of 2, split on whitespace, and turn the pieces into a List.How to sort a Map by value in JavaSort a Java Map by value: stream entrySet() with Map.Entry.comparingByValue() into a LinkedHashMap, descending order and tie-breaks, a sorted List of entries and top N, why a TreeMap sorts by key, and HashMap vs LinkedHashMap vs TreeMap.How to read a file into a String in JavaRead a whole file into a String with Files.readString (Java 11+), get a List of lines with readAllLines, stream a large file line by line with Files.lines or a BufferedReader, turn an InputStream into a String with readAllBytes, and handle missing files and the wrong charset.How to round to 2 decimal places in JavaRound a number to 2 decimal places in Java with BigDecimal.setScale(2, RoundingMode.HALF_UP), see why Math.round(x * 100.0) / 100.0 turns 1.005 into 1.0, format with String.format("%.2f") or DecimalFormat, pick a RoundingMode, drop trailing zeros and fix the BigDecimal.divide ArithmeticException.How to concatenate two lists in JavaJoin two lists in Java by copying the first into a new ArrayList and calling addAll, or with Stream.concat(...).toList() on Java 16+. Also covers many lists with flatMap, the UnsupportedOperationException from List.of and Arrays.asList, merging without duplicates, and concatenating arrays.How to sort a list of objects by property in JavaSort a Java list of objects by a property with list.sort(Comparator.comparing(Person::name)): comparingInt to avoid boxing, reversed() for descending, thenComparing for ties, case-insensitive and null-safe keys, Comparable, and a sorted copy with stream().sorted().toList().How to compare strings in JavaCompare strings in Java with equals(), and see why == only seems to work on literals. Plus null-safe checks with "literal".equals(x) and Objects.equals, equalsIgnoreCase, compareTo and Collator for ordering, contentEquals, isEmpty vs isBlank, and switch on strings.How to join a list into a string in JavaJoin a Java List<String> into one string with String.join(", ", list), use Collectors.joining for numbers, objects, prefixes and suffixes, handle null elements, build strings with StringJoiner or a StringBuilder loop without a trailing comma, and repeat a string with String.repeat.How to remove items from a list while iterating in JavaRemove items from a Java list while looping without a ConcurrentModificationException: removeIf (Java 8+), Iterator.remove(), a backwards index loop and a filtered stream copy, plus the remove(int) vs remove(Object) trap, maps, and why Arrays.asList and List.of throw UnsupportedOperationException.How to get the current date and time in JavaGet the current date and time in Java with LocalDateTime.now(), format it as yyyy-MM-dd HH:mm:ss.SSS with DateTimeFormatter, avoid the YYYY, hh and mm pattern traps, use ZonedDateTime and Instant for time zones and epoch millis, and convert to and from java.util.Date.

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