Java Design Patterns You Should Learn: A Practical Guide for Developers
Meta Title: The Java Design Patterns to Know (And Their Examples)
Meta Description: Learn about the Java design patterns that are essential for programming and maintenance. Read some useful descriptions and examples of the singleton pattern, factory pattern, builder pattern, observer pattern, strategy pattern, and much more.
Primary Keyword: Java Design Patterns You Should Learn
Secondary Keywords: Java design patterns, design patterns in Java, creational patterns, behavioral patterns, writing better Java code, Singleton pattern, Factory pattern
There comes a point when every Java programmer faces the same situation. It is working, but adding one minor feature requires changes in ten classes. Bugs start to appear in completely unrelated areas. It takes a new team member a whole week just to understand the codebase. That is where design patterns come in.
This article provides you with a look at the Java design patterns you should learn first, an explanation why each is important, and some information about their real-life use cases.
What Are Java Design Patterns?
A design pattern is a proven and reusable solution to a design problem that recurs. It is neither a library nor some code that you paste into your program. It is rather a template for solving a particular type of problem. There are three main categories of Java patterns:
• Creation patterns regulate object creation process.
• Structural patterns regulate composition of classes and objects into more complex structure.
• Behavioral patterns regulate interaction between objects.
You will be getting two major things out of learning design patterns: better ways to solve commonly occurring problems and a common vocabulary. "Let's implement a strategy here" will substitute lengthy discussion of some ideas on the whiteboard.
Why Should You Learn Java Design Patterns?
• Clean Code: Design patterns make you strive for loose coupling and single responsibility.
• Easier maintainability: The well-formed structure of code makes it easy to extend without causing any trouble.
• Communication: The pattern name is an acronym that every experienced developer knows.
• Awareness of frameworks: The design patterns are plenty in Spring, Hibernate, and even in JDK.
• Interview skills: The design patterns form part of every Java interview question.
Creational Patterns
1. Singleton
Problem: Need just a single instance of a class like Configuration Manager or Logger.
Solution: The constructor needs to be made private, and there needs to be a single point of entry.
public enum AppConfig {
INSTANCE;
public String getEnvironment() {
return "production";
}
}
Enum is the most straightforward way of doing so in Java, which is thread safe. Watch out: Excessive use of singletons will lead to many global variables complicating testing.
2. Factory Method
Problem: You need to instantiate objects, but you don’t know what class until runtime.
Solution: Use a factory method where object creation happens inside of a method.
interface Notification {
void send(String message);
}
class EmailNotification implements Notification {
public void send(String message) { /* send email */ }
}
class SmsNotification implements Notification {
public void send(String message) { /* send SMS */ }
}
class NotificationFactory {
static Notification create(String type) {
return switch (type) {
case "email" -> new EmailNotification();
case "sms" -> new SmsNotification();
default -> throw new IllegalArgumentException("Unknown type: " +type);
};
}
}
In order to introduce push-notification channel later you just have to add another class andone case statement without changing anything in the calling code.
3. Builder
The problem is that a class may have lots of optional fields and constructors with eight parameters become incomprehensible.
The idea is to create the object gradually, and conclude with a build() method call.
User user = new User.Builder("alice")
.email("alice@example.com")
.age(29)
.build();
This pattern will be encountered everywhere in modern Java – from StringBuilder to Lombok’s @Builder.
Structural Patterns
4. Adapter Pattern
Problem: You have a class which has the required functionality, but its interface doesn’t conform to your expectations.
Solution: Implement the adapter design pattern that will provide an adaptation layer over that non-conformant class.A typical scenario is iswhen integrating legacy or third party systems. An example of the adapter pattern can be seen in the JDK's Arrays.asList().
5. Decorator Design Pattern
Problem: You want to be able to modify the behavior of an object without changing its class or by subclassing the class.
Solution: Implement a decorator design pattern to decorate the object with some more behavior.
Java's I/O library is an ideal example:
BufferedReader reader = new BufferedReader(
new InputStreamReader(new FileInputStream("data.txt")));
InputStreamReader(new FileInputStream("data.txt"))); The added layer of abstraction does not break the underlying contract.
6. Facade
The problem is that a subsystem is complicated and a client needs just a few operations.
The idea is to create a simpler interface in front of everything complicated. Service classes in layered applications are an example of a facade.
Behavioral Patterns
7. Strategy
Problem: The same action can be done differently, and the if-else construct grows in size.
Solution: Surround every algorithm by an object and select the right one at run time. interface DiscountStrategy {
double apply(double price); }
class FestivalDiscount implements DiscountStrategy {
public double apply(double price) {
return price * 0.80;
}
}
class NoDiscount implements DiscountStrategy {
public double apply(double price) {
return price;
}
}
class Checkout {
private final DiscountStrategy strategy;
Checkout(DiscountStrategy strategy) {
this.strategy = strategy;
}
double total(double price) {
return strategy.apply(price);
}
}
With the introduction of lambda expressions, Strategy pattern in Java is easier than ever andComparator is already a Strategy for you.
8. Observer
Problem: There exists an object that is being altered; other objects must react to this, but they cannot be rigidly tied to it.
Solution: The objects monitor the observer objects and update them of any change.
9. Template Method
Problem: Some number of classes implement the same general algorithm with certain differences in particular steps.
Solution: The basic structure is implemented in the base class, with specific actions left to its subclasses. It is used extensively in frameworks, e.g. JdbcTemplate and servlet lifecycle.
10. Command
Problem: It would be nice to model requests as objects which can then be queued up, logged, or even undone.
Solution: Combine the action with its arguments in the command object.
Effective Learning of Java Design Patterns
1. Start with issues, not patterns. First identify the issue, and then you will understand the pattern.
2. Get hold of a few patterns. Pay attention to Singleton, Factory, Builder, Strategy, and Observer in the first place.
3. Look at the framework code. Identify patterns in JDK and Spring sources.
4. Refactor your code. Take an example of a complex if-else block and implement strategy on it.
5. Don’t try to fit patterns unnecessarily. An application of a pattern that is complicated and doesn’t solve any problem is an anti-pattern.
Common Errors to Beware Of
• Using patterns to show off your intelligence.
• Excessive use of Singleton and creating hidden dependencies.
• Memorizing UML diagrams but failing to grasp the problem behind them.
• Not using simpler language constructs like lambdas and records instead.
Final Thoughts
The design patterns for Java that you have to know are not about rote memorization of 23 established patterns. Instead, they are about the ability to spot patterns of problems and having a trusted toolkit to solve them. Start with the ten mentioned above, work with them in small projects, and spot them in frameworks that you are already familiar with. You will be able to write better code in terms of readability, extensibility, and testability and become much more effective in design discussions and interviews.
Select one design pattern today, and apply it to a suitable place in your ongoing project. This is the fastest way to learn them.
Author:
Pooja Ghodekar
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Pooja Ghodekar
Expert trainer and consultant at SevenMentor with years of industry experience. Passionate about sharing knowledge and empowering the next generation of tech leaders.