design-patterns

Design Patterns Skill

Practical design patterns reference for Java with modern examples.

When to Use

• User asks to implement a specific pattern
• Designing extensible/flexible components
• Refactoring rigid code structures
• Code review suggests pattern usage

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Quick Reference: When to Use What

Problem	Pattern
Complex object construction	Builder
Create objects without specifying class	Factory
Multiple algorithms, swap at runtime	Strategy
Add behavior without changing class	Decorator
Notify multiple objects of changes	Observer
Ensure single instance	Singleton
Convert incompatible interfaces	Adapter
Define algorithm skeleton	Template Method

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Creational Patterns

Builder

Use when: Object has many parameters, some optional.

// ❌ Telescoping constructor antipattern
public class User {
    public User(String name) { }
    public User(String name, String email) { }
    public User(String name, String email, int age) { }
    public User(String name, String email, int age, String phone) { }
    // ... explosion of constructors
}

// ✅ Builder pattern
public class User {
    private final String name;      // required
    private final String email;     // required
    private final int age;          // optional
    private final String phone;     // optional
    private final String address;   // optional

    private User(Builder builder) {
        this.name = builder.name;
        this.email = builder.email;
        this.age = builder.age;
        this.phone = builder.phone;
        this.address = builder.address;
    }

    public static Builder builder(String name, String email) {
        return new Builder(name, email);
    }

    public static class Builder {
        // Required
        private final String name;
        private final String email;
        // Optional with defaults
        private int age = 0;
        private String phone = "";
        private String address = "";

        private Builder(String name, String email) {
            this.name = name;
            this.email = email;
        }

        public Builder age(int age) {
            this.age = age;
            return this;
        }

        public Builder phone(String phone) {
            this.phone = phone;
            return this;
        }

        public Builder address(String address) {
            this.address = address;
            return this;
        }

        public User build() {
            return new User(this);
        }
    }
}

// Usage
User user = User.builder("John", "[email protected]")
    .age(30)
    .phone("+1234567890")
    .build();