The this Keyword

public class Rectangle { private double width; private double height;

public Rectangle(double width, double height) { this.width = width; // 'this.width' is the field this.height = height; // 'height' alone is the parameter }

public void resize(double width, double height) { this.width = width; this.height = height; } }

public class QueryBuilder { private String table; private String whereClause; private String orderBy; private int limit;

public QueryBuilder select(String table) { this.table = table; return this; // Return current object for chaining }

public QueryBuilder where(String condition) { this.whereClause = condition; return this; }

public QueryBuilder orderBy(String column) { this.orderBy = column; return this; }

public QueryBuilder limit(int count) { this.limit = count; return this; }

public String build() { StringBuilder sql = new StringBuilder("SELECT * FROM " + table); if (whereClause != null) sql.append(" WHERE " + whereClause); if (orderBy != null) sql.append(" ORDER BY " + orderBy); if (limit > 0) sql.append(" LIMIT " + limit); return sql.toString(); } }

// Beautiful chaining thanks to 'return this' String query = new QueryBuilder() .select("users") .where("age > 18") .orderBy("name") .limit(10) .build(); // Result: "SELECT * FROM users WHERE age > 18 ORDER BY name LIMIT 10"

public class Connection { private String host; private int port; private int timeout; private boolean ssl;

// Full constructor public Connection(String host, int port, int timeout, boolean ssl) { this.host = host; this.port = port; this.timeout = timeout; this.ssl = ssl; }

// Chains to full constructor with defaults public Connection(String host, int port) { this(host, port, 30, true); // this() calls another constructor }

// Chains with more defaults public Connection(String host) { this(host, 443); // Calls the 2-parameter constructor } }

public class EventEmitter { private String name;

public EventEmitter(String name) { this.name = name; }

public void register(EventManager manager) { // Pass THIS object to be registered manager.addListener(this); } }

class TaskManager: def __init__(self, name): self.name = name self.tasks = [] self.completed = []

def add_task(self, task): self.tasks.append(task) return self # Method chaining works in Python too!

def complete_task(self, task): if task in self.tasks: self.tasks.remove(task) self.completed.append(task) return self

def status(self): print(f"\n{self.name}'s Tasks:") print(f" Pending: {len(self.tasks)}") print(f" Done: {len(self.completed)}") return self

manager = TaskManager("Alice") manager.add_task("Write report") \ .add_task("Send emails") \ .add_task("Code review") \ .complete_task("Write report") \ .status()

#include <iostream> #include <string>

class Vector2D { private: double x, y;

public: Vector2D(double x, double y) : x(x), y(y) {}

// Return reference for chaining Vector2D& add(const Vector2D& other) { this->x += other.x; this->y += other.y; return *this; // Dereference the pointer to return the object }

Vector2D& scale(double factor) { this->x *= factor; this->y *= factor; return *this; }

void print() const { std::cout << "(" << x << ", " << y << ")" << std::endl; }

// Using this to compare with another object bool equals(const Vector2D& other) const { return this->x == other.x && this->y == other.y; } };

int main() { Vector2D v(1, 2); v.add(Vector2D(3, 4)).scale(2).print(); // (8, 12) return 0; }

public class Example { private int value;

public Example(int value) { this.value = value; }

// ERROR: this() must be first statement public Example() { System.out.println("Creating..."); // Can't be before this() this(0); // COMPILE ERROR! }

// CORRECT: public Example() { this(0); // Must be first! System.out.println("Created with default value"); }

// Cannot use 'this' in static context public static void utility() { // this.value = 5; // ERROR! No 'this' in static methods } }