REFERENSI CEPAT JAVA
OOP, collection, stream, penanganan exception
Dasar
Hello World
public class Main {
public static void main(String[] args) {
System.out.println("Hello, World!");
}
}
Compile & Jalankan
javac Main.java # compile
java Main # run
java Main.java # single-file (Java 11+)
Konvensi Penamaan
| ClassName | PascalCase untuk class dan interface |
| methodName | camelCase untuk method dan variabel |
| CONSTANT_NAME | UPPER_SNAKE untuk konstanta |
| com.example.pkg | Domain terbalik huruf kecil untuk package |
Tipe Data
Primitif
| byte | 8-bit bertanda (-128 hingga 127) |
| short | 16-bit bertanda |
| int | 32-bit bertanda (integer default) |
| long | 64-bit bertanda (sufiks L) |
| float | 32-bit IEEE-754 (sufiks f) |
| double | 64-bit IEEE-754 (desimal default) |
| boolean | true / false |
| char | Karakter Unicode 16-bit |
String
String s = "hello";
String joined = s + " world"; // concatenation
int len = s.length();
String sub = s.substring(0, 3); // "hel"
boolean eq = s.equals("hello"); // content equality
Type Casting
int i = (int) 3.14; // narrowing cast
double d = i; // widening (auto)
int n = Integer.parseInt("42"); // string to int
String s = String.valueOf(42); // int to string
Array
int[] nums = {1, 2, 3};
String[] names = new String[5];
int[][] matrix = new int[3][4];
Arrays.sort(nums);
Alur Kontrol
If / Else
if (x > 0) {
System.out.println("positive");
} else if (x == 0) {
System.out.println("zero");
} else {
System.out.println("negative");
}
Switch
// Traditional
switch (day) {
case "Mon": doWork(); break;
case "Sat": case "Sun": rest(); break;
default: routine();
}
// Switch expression (Java 14+)
String type = switch (day) {
case "Sat", "Sun" -> "weekend";
default -> "weekday";
};
Perulangan
for (int i = 0; i < 10; i++) { }
for (String s : list) { } // enhanced for
while (condition) { }
do { } while (condition);
Method
Definisi
public static int add(int a, int b) {
return a + b;
}
Varargs & Overloading
static int sum(int... nums) {
int total = 0;
for (int n : nums) total += n;
return total;
}
// sum(1, 2) sum(1, 2, 3) both work
Access Modifier
| public | Dapat diakses dari mana saja |
| protected | Package yang sama + subclass |
| (default) | Package yang sama saja (tanpa keyword) |
| private | Class yang sama saja |
Class & Objek
Definisi Class
public class User {
private String name;
private int age;
public User(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() { return name; }
}
Record (Java 16+)
public record Point(double x, double y) {
// auto: constructor, getters, equals, hashCode, toString
public double distance() {
return Math.sqrt(x * x + y * y);
}
}
Static & Final
| static | Milik class, bukan instance |
| final field | Tidak bisa diubah setelah inisialisasi |
| final method | Tidak bisa di-override |
| final class | Tidak bisa di-subclass |
Pewarisan
Extends
public class Animal {
public void speak() { System.out.println("..."); }
}
public class Dog extends Animal {
@Override
public void speak() { System.out.println("Woof!"); }
}
Abstract Class
public abstract class Shape {
abstract double area();
public void describe() {
System.out.println("Area: " + area());
}
}
Konsep Utama
| super | Panggil constructor atau method parent |
| @Override | Pemeriksaan override saat compile |
| instanceof | Pemeriksaan tipe saat runtime |
| sealed (17+) | Batasi class mana yang bisa extends |
Interface
Definisi
public interface Printable {
void print(); // abstract
default String format() { // default method
return toString();
}
static Printable of(String s) { // static method
return () -> System.out.println(s);
}
}
Implementasi
public class Report implements Printable, Serializable {
@Override
public void print() {
System.out.println("Report");
}
}
Functional Interface
| Runnable | () -> void |
| Supplier<T> | () -> T |
| Consumer<T> | T -> void |
| Function<T,R> | T -> R |
| Predicate<T> | T -> boolean |
| Comparator<T> | (T, T) -> int |
Collection
List
List list = new ArrayList<>();
list.add("a");
list.get(0); // "a"
list.size(); // 1
List immutable = List.of("a", "b", "c");
Map
Map map = new HashMap<>();
map.put("key", 42);
map.getOrDefault("key", 0); // 42
map.containsKey("key"); // true
map.forEach((k, v) -> { });
Set
Set set = new HashSet<>();
set.add("a");
set.contains("a"); // true
Set immutable = Set.of("a", "b", "c");
Implementasi Umum
| ArrayList | Array resizable, akses acak cepat |
| LinkedList | Doubly-linked, insert/remove cepat |
| HashMap | Hash table, O(1) get/put |
| TreeMap | Diurutkan berdasarkan key, O(log n) |
| HashSet | Elemen unik, O(1) lookup |
| LinkedHashMap | HashMap dengan urutan penyisipan |
Penanganan Exception
Try / Catch / Finally
try {
int result = 10 / 0;
} catch (ArithmeticException e) {
System.err.println(e.getMessage());
} finally {
// always executes
}
Try-with-Resources
try (var reader = new BufferedReader(new FileReader(path))) {
String line = reader.readLine();
} // auto-closes reader
Hierarki Exception
| Throwable | Root dari semua error dan exception |
| Error | Masalah serius (OutOfMemoryError) |
| Exception | Checked exception (harus ditangani) |
| RuntimeException | Unchecked (NullPointer, IndexOutOfBounds) |
Custom Exception
public class AppException extends Exception {
public AppException(String msg) { super(msg); }
public AppException(String msg, Throwable cause) {
super(msg, cause);
}
}
Stream & Lambda
Sintaks Lambda
Comparator byLen = (a, b) -> a.length() - b.length();
Runnable task = () -> System.out.println("run");
Function len = String::length; // method ref
Pipeline Stream
List result = names.stream()
.filter(n -> n.length() > 3)
.map(String::toUpperCase)
.sorted()
.collect(Collectors.toList());
Operasi Stream Umum
| .filter(pred) | Simpan elemen yang cocok dengan predicate |
| .map(func) | Transformasi setiap elemen |
| .flatMap(func) | Map dan ratakan stream bersarang |
| .sorted() | Urutkan (natural atau dengan Comparator) |
| .distinct() | Hapus duplikat |
| .limit(n) | Ambil n elemen pertama |
| .collect() | Terminal: kumpulkan ke dalam collection |
| .forEach() | Terminal: lakukan aksi pada setiap elemen |
| .reduce() | Terminal: gabungkan menjadi satu nilai |
| .count() | Terminal: hitung elemen |
Generics
Class & Method Generic
public class Box {
private T value;
public Box(T value) { this.value = value; }
public T get() { return value; }
}
public static List listOf(T... items) {
return List.of(items);
}
Tipe Terbatas & Wildcard
| <T extends Number> | T harus Number atau subclass |
| <T extends A & B> | Batasan ganda (class + interface) |
| <?> | Tipe tidak diketahui (read-only) |
| <? extends T> | Upper bound wildcard (producer) |
| <? super T> | Lower bound wildcard (consumer) |
Optional & Java Modern
Optional
Optional opt = Optional.ofNullable(getValue());
String result = opt.orElse("default");
opt.ifPresent(v -> System.out.println(v));
String upper = opt.map(String::toUpperCase).orElse("");
Text Block (Java 15+)
String json = """
{ "name": "Alice", "age": 30 }
""";
Utilitas Berguna
| var (10+) | Inferensi tipe variabel lokal |
| record (16+) | Class pembawa data immutable |
| sealed (17+) | Hierarki class terbatas |
| pattern matching (21+) | instanceof dengan auto-cast |
| virtual threads (21+) | Thread ringan via Thread.ofVirtual() |