Java 17：模式匹配的切换

发布者：:Fahd Shariff inCore Java September 28th, 2021 0 Views

在Java 17中（几天前才发布），switch的模式匹配作为一个预览语言功能被引入，它允许用模式而不仅仅是常量进行案例标签。下面是一个例子，展示了如何在类型模式上进行匹配：

public static String typedPatternMatching(Object o) {
  return switch(o) {
    case null      -> "I am null";
    case String s  -> "I am a String. My value is " + s;
    case Integer i -> "I am an int. My value is " + i;
    default        -> "I am of an unknown type. My value is " + o.toString();
  };
}

// Output:
> typedPatternMatching("HELLO")
"I am a String. My value is HELLO"

> typedPatternMatching(123)
"I am an int. My value is 123"

> typedPatternMatching(null)
"I am null"

> typedPatternMatching(0.5)
"I am of an unknown type. My value is 0.5"

你也可以使用守护模式，以细化模式，使其只在某些条件下匹配，例如：

public static String guardedPattern(Collection<String> coll) {
  return switch(coll) {
    case List list && (list.size() > 10) -> 
        "I am a big List. My size is " + list.size();
    case List list -> 
        "I am a small List. My size is " + list.size();
    default -> 
        "Unsupported collection: " + coll.getClass();
  };
}

如果你有一个Sealed Class（在Java 17中成为永久的语言特性），编译器可以验证switch语句是否完整，所以不需要default 标签，比如说：

sealed interface Vehicle permits Car, Truck, Motorcycle {}
final class Car implements Vehicle {}
final class Truck implements Vehicle {}
final class Motorcycle implements Vehicle {}

public static String sealedClass(Vehicle v) {
  return switch(v) {
    case Car c -> "I am a car";
    case Truck t -> "I am a truck";
    case Motorcycle m -> "I am a motorcycle";
  };
}