# Java Quick Reference

*OOP, collections, streams, exception handling essentials*

> Source: Oracle Java Documentation (docs.oracle.com/javase) · MIT

## Basics

### Hello World

```
public class Main {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}
```

### Compile & Run

```
javac Main.java      # compile
java Main             # run
java Main.java        # single-file (Java 11+)
```

### Naming Conventions

| Command | Description |
|---------|-------------|
| `ClassName` | PascalCase for classes and interfaces |
| `methodName` | camelCase for methods and variables |
| `CONSTANT_NAME` | UPPER_SNAKE for constants |
| `com.example.pkg` | Lowercase reverse domain for packages |

## Data Types

### Primitives

| Command | Description |
|---------|-------------|
| `byte` | 8-bit signed (-128 to 127) |
| `short` | 16-bit signed |
| `int` | 32-bit signed (default integer) |
| `long` | 64-bit signed (suffix `L`) |
| `float` | 32-bit IEEE-754 (suffix `f`) |
| `double` | 64-bit IEEE-754 (default decimal) |
| `boolean` | `true` / `false` |
| `char` | 16-bit Unicode character |

### Strings

```
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
```

### Arrays

```
int[] nums = {1, 2, 3};
String[] names = new String[5];
int[][] matrix = new int[3][4];
Arrays.sort(nums);
```

## Control Flow

### 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";
};
```

### Loops

```
for (int i = 0; i < 10; i++) { }
for (String s : list) { }         // enhanced for
while (condition) { }
do { } while (condition);
```

## Methods

### Definition

```
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 Modifiers

| Command | Description |
|---------|-------------|
| `public` | Accessible from anywhere |
| `protected` | Same package + subclasses |
| `(default)` | Same package only (no keyword) |
| `private` | Same class only |

## Classes & Objects

### Class Definition

```
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; }
}
```

### Records (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

| Command | Description |
|---------|-------------|
| `static` | Belongs to class, not instance |
| `final field` | Cannot be reassigned after init |
| `final method` | Cannot be overridden |
| `final class` | Cannot be subclassed |

## Inheritance

### Extends

```
public class Animal {
    public void speak() { System.out.println("..."); }
}
public class Dog extends Animal {
    @Override
    public void speak() { System.out.println("Woof!"); }
}
```

### Abstract Classes

```
public abstract class Shape {
    abstract double area();
    public void describe() {
        System.out.println("Area: " + area());
    }
}
```

### Key Concepts

| Command | Description |
|---------|-------------|
| `super` | Call parent constructor or method |
| `@Override` | Compile-time override check |
| `instanceof` | Runtime type check |
| `sealed (17+)` | Restrict which classes can extend |

## Interfaces

### Definition

```
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);
    }
}
```

### Implementation

```
public class Report implements Printable, Serializable {
    @Override
    public void print() {
        System.out.println("Report");
    }
}
```

### Functional Interfaces

| Command | Description |
|---------|-------------|
| `Runnable` | `() -> void` |
| `Supplier<T>` | `() -> T` |
| `Consumer<T>` | `T -> void` |
| `Function<T,R>` | `T -> R` |
| `Predicate<T>` | `T -> boolean` |
| `Comparator<T>` | `(T, T) -> int` |

## Collections

### List

```
List<String> list = new ArrayList<>();
list.add("a");
list.get(0);                  // "a"
list.size();                  // 1
List<String> immutable = List.of("a", "b", "c");
```

### Map

```
Map<String, Integer> map = new HashMap<>();
map.put("key", 42);
map.getOrDefault("key", 0);   // 42
map.containsKey("key");       // true
map.forEach((k, v) -> { });
```

### Set

```
Set<String> set = new HashSet<>();
set.add("a");
set.contains("a");            // true
Set<String> immutable = Set.of("a", "b", "c");
```

### Common Implementations

| Command | Description |
|---------|-------------|
| `ArrayList` | Resizable array, fast random access |
| `LinkedList` | Doubly-linked, fast insert/remove |
| `HashMap` | Hash table, O(1) get/put |
| `TreeMap` | Sorted by key, O(log n) |
| `HashSet` | Unique elements, O(1) lookup |
| `LinkedHashMap` | Insertion-ordered HashMap |

## Exception Handling

### 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
```

### Exception Hierarchy

| Command | Description |
|---------|-------------|
| `Throwable` | Root of all errors and exceptions |
| `Error` | Serious problems (OutOfMemoryError) |
| `Exception` | Checked exceptions (must handle) |
| `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);
    }
}
```

## Streams & Lambdas

### Lambda Syntax

```
Comparator<String> byLen = (a, b) -> a.length() - b.length();
Runnable task = () -> System.out.println("run");
Function<String, Integer> len = String::length;  // method ref
```

### Stream Pipeline

```
List<String> result = names.stream()
    .filter(n -> n.length() > 3)
    .map(String::toUpperCase)
    .sorted()
    .collect(Collectors.toList());
```

### Common Stream Operations

| Command | Description |
|---------|-------------|
| `.filter(pred)` | Keep elements matching predicate |
| `.map(func)` | Transform each element |
| `.flatMap(func)` | Map and flatten nested streams |
| `.sorted()` | Sort (natural or with Comparator) |
| `.distinct()` | Remove duplicates |
| `.limit(n)` | Take first n elements |
| `.collect()` | Terminal: gather into collection |
| `.forEach()` | Terminal: perform action on each |
| `.reduce()` | Terminal: combine into single value |
| `.count()` | Terminal: count elements |

## Generics

### Generic Class & Method

```
public class Box<T> {
    private T value;
    public Box(T value) { this.value = value; }
    public T get() { return value; }
}
public static <T> List<T> listOf(T... items) {
    return List.of(items);
}
```

### Bounded Types & Wildcards

| Command | Description |
|---------|-------------|
| `<T extends Number>` | T must be Number or subclass |
| `<T extends A & B>` | Multiple bounds (class + interfaces) |
| `<?>` | Unknown type (read-only) |
| `<? extends T>` | Upper bound wildcard (producer) |
| `<? super T>` | Lower bound wildcard (consumer) |

## Optional & Modern Java

### Optional

```
Optional<String> opt = Optional.ofNullable(getValue());
String result = opt.orElse("default");
opt.ifPresent(v -> System.out.println(v));
String upper = opt.map(String::toUpperCase).orElse("");
```

### Text Blocks (Java 15+)

```
String json = """
        { "name": "Alice", "age": 30 }
        """;
```

### Useful Utilities

| Command | Description |
|---------|-------------|
| `var (10+)` | Local variable type inference |
| `record (16+)` | Immutable data carrier class |
| `sealed (17+)` | Restricted class hierarchies |
| `pattern matching (21+)` | `instanceof` with auto-cast |
| `virtual threads (21+)` | Lightweight threads via `Thread.ofVirtual()` |