Cheatsheet - Java

Compilation & Execution

# Compile and run
$ javac Main.java && java Main

# Single-file source code execution (Java 11+)
$ java Main.java

# Run packaged executable JAR
$ java -jar app.jar

# JVM options
$ java -Xms512m -Xmx4g -XX:+UseG1GC -jar app.jar

Primitive Types & Numbers

// Primitives: byte (8-bit), short (16-bit), int (32-bit), long (64-bit)
long largeNum = 1_000_000_000L;
float f = 3.14f;
double d = 3.1415926535;
boolean flag = true;
char ch = 'A';                          // 16-bit Unicode

// Binary & Hex literals
int bin = 0b1010;                       // 10
int hex = 0x7F;                         // 127

// Bitwise Operations
int shifted = 1 << 4;                   // 16 (left shift)
int unsignedShift = -1 >>> 1;           // 2147483647 (zero-fill right shift)
int bitAnd = 0b1100 & 0b1010;          // 0b1000 (8)
int bitOr  = 0b1100 | 0b1010;          // 0b1110 (14)
int bitXor = 0b1100 ^ 0b1010;          // 0b0110 (6)

// Parsing & Limits
int maxInt = Integer.MAX_VALUE;         // 2,147,483,647
int parsed = Integer.parseInt("123");   // primitive int
Integer obj = Integer.valueOf("123");   // boxed Integer
String hexStr = Integer.toHexString(255); // "ff"

Strings & Text Blocks

String s = "  Java 21  ";
s.strip();                              // "Java 21" (Unicode-aware trim)
s.substring(0, 4);                      // "Java"
s.startsWith("  J");                    // True
s.contains("21");                       // True
s.replace("21", "22");                  // Literal replacement
s.split("\\|");                         // Regex split (escape special chars)

// Multiline Text Blocks (Java 15+)
String json = """
    {
      "name": "Java",
      "version": 21
    }
    """;

// Formatting
String formatted = String.format("User: %s, Score: %.2f", "Alice", 98.765);

Collections & Streams

List, Set, and Map Creation

// Immutable Collections (Java 9+)
List<String> list = List.of("A", "B", "C");
Set<Integer> set = Set.of(1, 2, 3);
Map<String, Integer> map = Map.of("k1", 1, "k2", 2);

// Mutable Collections
List<String> mutableList = new ArrayList<>(list);
mutableList.add("D");
mutableList.removeIf(s -> s.startsWith("A"));

Map<String, Integer> mutableMap = new HashMap<>(map);
mutableMap.put("k3", 3);
mutableMap.getOrDefault("k4", 0);       // 0
mutableMap.computeIfAbsent("k5", k -> k.length());

Stream Pipeline Operations

List<String> names = List.of("charlie", "alice", "bob", "alice");

// Filter, map, sort, and collect to List (Java 16+ .toList())
List<String> filtered = names.stream()
    .filter(n -> n.length() > 3)
    .map(String::toUpperCase)
    .distinct()
    .sorted()
    .toList();                          // ["ALICE", "CHARLIE"]

// Joining Strings
String csv = names.stream().collect(Collectors.joining(", "));

// Grouping By
Map<Integer, List<String>> byLength = names.stream()
    .collect(Collectors.groupingBy(String::length));

Modern Control Flow & Pattern Matching

Switch Expressions (Java 14+)

String dayType = switch (day) {
    case MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY -> "Weekday";
    case SATURDAY, SUNDAY -> "Weekend";
};

// Pattern Matching for switch (Java 21+)
static String formatValue(Object obj) {
    return switch (obj) {
        case Integer i -> String.format("int: %d", i);
        case Long l    -> String.format("long: %d", l);
        case String s when !s.isEmpty() -> String.format("non-empty string: %s", s);
        case null      -> "null value";
        default        -> obj.toString();
    };
}

Record Types (Java 16+)

Concise immutable data carrier classes with auto-generated constructor, getters, equals(), hashCode(), and toString():

public record User(long id, String name, String email) {}

User user = new User(1L, "Alice", "[email protected]");
System.out.println(user.name());        // "Alice"

Date & Time (java.time)

import java.time.*;
import java.time.format.DateTimeFormatter;

// Instant & Timestamps
Instant now = Instant.now();
LocalDate today = LocalDate.now();      // 2024-05-18
LocalTime time = LocalTime.now();       // 14:30:00
LocalDateTime dt = LocalDateTime.now(); // 2024-05-18T14:30:00
ZonedDateTime utc = ZonedDateTime.now(ZoneOffset.UTC);

// Parsing & Formatting
DateTimeFormatter fmt = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm");
String str = dt.format(fmt);
LocalDate parsed = LocalDate.parse("2024-05-18");

// Duration & Period
LocalDate nextWeek = today.plusDays(7);
Duration duration = Duration.ofMinutes(90);
Period period = Period.between(today, nextWeek);

Virtual Threads & Concurrency (Java 21+)

// Spawn a lightweight virtual thread
Thread.startVirtualThread(() -> {
    System.out.println("Running on virtual thread: " + Thread.currentThread());
});

// ExecutorService with virtual threads per task
try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    IntStream.range(0, 10_000).forEach(i -> {
        executor.submit(() -> {
            Thread.sleep(Duration.ofMillis(100));
            return i;
        });
    });
} // Automatically awaits completion of all virtual threads