What common design patterns appear in Java?

Question

Accepted Answer

**Design patterns** are reusable solutions to recurring software-design problems. Java's OOP nature and ecosystem make several patterns especially common — knowing them helps you recognize, communicate, and apply proven structures. They fall into creational, structural, and behavioral categories.

## Singleton (creational) — one shared instance

```java
public class Config {
    private static final Config INSTANCE = new Config();   // the single instance
    private Config() {}                                     // private constructor
    public static Config getInstance() { return INSTANCE; }
}
// (enum singletons are an even safer idiom). Used for shared config, connection pools.
```

## Factory (creational) — encapsulate object creation

```java
public static Shape createShape(String type) {
    return switch (type) {                 // hides the concrete class from the caller
        case "circle" -> new Circle();
        case "square" -> new Square();
        default -> throw new IllegalArgumentException();
    };
}
// caller depends on Shape, not concrete classes — decoupling.
```

## Builder (creational) — construct complex objects step by step

```java
User user = new User.Builder()
    .name("Ann")
    .email("ann@x.com")
    .age(30)
    .build();    // readable construction; avoids huge telescoping constructors
```

## Strategy (behavioral) — interchangeable algorithms

```java
interface PaymentStrategy { void pay(double amount); }
class CreditCard implements PaymentStrategy { public void pay(double a) {...} }
class PayPal implements PaymentStrategy { public void pay(double a) {...} }

// swap behavior at runtime — lambdas make this trivial in modern Java
processor.setStrategy(new PayPal());
```

## Observer (behavioral) — publish/subscribe

```java
// subjects notify registered observers of changes (events, listeners)
button.addActionListener(e -> handleClick());   // GUI events use this everywhere
```

## Decorator (structural) — wrap to add behavior

```java
// Java's I/O streams ARE the decorator pattern:
BufferedReader r = new BufferedReader(new FileReader("f.txt"));
// BufferedReader wraps FileReader, adding buffering
```

## Where patterns appear in Java itself

```text
Singleton  → Runtime.getRuntime()
Factory    → Calendar.getInstance(), DriverManager
Builder    → StringBuilder, Stream.Builder
Observer   → event listeners, java.util.Observer
Decorator  → java.io streams (BufferedReader wrapping FileReader)
Iterator   → the Iterator interface / Collections
Strategy   → Comparator, functional interfaces
```

## A caution against overuse

```text
Patterns are tools, not goals. Applying them where they don't fit ADDS complexity.
Use them when they genuinely solve your problem — many can be simpler with lambdas now.
```

## Why it matters

Design patterns provide a shared vocabulary and proven solutions for common design problems, which is valuable for communication ("use a Strategy here"), recognizing structures in existing code, and avoiding reinventing flawed solutions.

They appear throughout Java's own libraries (I/O decorators, factory methods, builders, observer-based events), so understanding them helps you use and extend the standard library effectively.

Knowing the major patterns — Singleton, Factory, Builder, Strategy, Observer, Decorator — and where each fits is a hallmark of experienced developers and a frequent interview topic.

Equally important is the judgment to apply them appropriately (solving real problems) rather than forcing them in, especially as modern features like lambdas simplify many classic patterns.