เมธอดและคลาสเจเนอริก

ภาพรวม

เหตุใดจึงต้องใช้คลาสเจเนอริก

โค้ด: Pair<T,U>

public class Main {
  // Simple generic data holder with two type parameters
  static class Pair<T, U> {
    private final T first;
    private final U second;
    Pair(T first, U second) { this.first = first; this.second = second; }
    T first() { return first; }
    U second() { return second; }
    public String toString() { return "Pair(" + first + ", " + second + ")"; }
  }

  public static void main(String[] args) {
    Pair<String, Integer> p = new Pair<>("age", 21);
    System.out.println(p);
    System.out.println(p.first() + " -> " + p.second());
  }
}

เมธอดเจเนอริก: ลายเซ็น

โค้ด: swap แบบเจเนอริก

public class Main {
  // Generic utility method: swap two indices of an array of any reference type
  static <T> void swap(T[] arr, int i, int j) {
    T tmp = arr[i]; arr[i] = arr[j]; arr[j] = tmp;
  }

  public static void main(String[] args) {
    String[] names = { "Ada", "Bob", "Cyd" };
    swap(names, 0, 2); // works with String[]
    for (int k = 0; k < names.length; k = k + 1) {
      System.out.print(names[k] + (k < names.length - 1 ? " " : "\n"));
    }
  }
}

โค้ด: โรงงานเจเนอริกแบบสแตติก

public class Main {
  static class Box<T> {
    private final T value;
    private Box(T v) { this.value = v; }
    public T get() { return value; }
    // Static generic factory method inside a generic class
    public static <T> Box<T> of(T v) { return new Box<T>(v); }
  }

  public static void main(String[] args) {
    // Type inferred by diamond and method generic parameter
    Box<Integer> bi = Box.of(42);
    Box<String> bs = Box.of("hi");
    System.out.println(bi.get() + 1);
    System.out.println(bs.get().toUpperCase());
  }
}

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