Design patterns 是对重复出现的 software-design 问题的可重用解决方案。Java 的 OOP 特性和生态系统使得几种模式特别常见——了解它们可以帮助你识别、交流和应用已验证的结构。它们分为 creational、structural 和 behavioral 三个类别。
{
();
{}
Config { INSTANCE; }
}
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.
User user = new User.Builder()
.name("Ann")
.email("[email protected]")
.age(30)
.build(); // readable construction; avoids huge telescoping constructors
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());
// subjects notify registered observers of changes (events, listeners)
button.addActionListener(e -> handleClick()); // GUI events use this everywhere
// Java's I/O streams ARE the decorator pattern:
BufferedReader r = new BufferedReader(new FileReader("f.txt"));
// BufferedReader wraps FileReader, adding buffering
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
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.
Design patterns 为常见的设计问题提供了共同的词汇表和已验证的解决方案，这对于交流（"在这里使用 Strategy"）、识别现有代码中的结构以及避免重新发明有缺陷的解决方案很有价值。
它们在 Java 的标准库中随处可见（I/O decorators、factory methods、builders、observer-based events），因此理解它们可以帮助你有效地使用和扩展标准库。
了解主要模式——Singleton、Factory、Builder、Strategy、Observer、Decorator——以及每个模式适用的场景，是经验丰富的开发者的标志，也是常见的面试话题。
同样重要的是判断力，要恰当地应用这些模式（解决真实问题），而不是强行使用它们，尤其是当像 lambdas 这样的现代特性简化了许多经典模式的时候。
一个包含详细解答的 IT 面试题库——从初级到高级。
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