ยูทิลิตีของคอลเลกชัน

ภาพรวม

การดำเนินการกับข้อมูลเดิม

โค้ด: sort/reverse/shuffle

public class Main {
  public static void main(String[] args) {
    java.util.List<String> names = new java.util.ArrayList<>();
    names.add("Bob"); names.add("Ada"); names.add("Cyd");

    System.out.println("orig=" + names);
    java.util.Collections.sort(names);          // natural order (A–Z)
    System.out.println("sorted=" + names);
    java.util.Collections.reverse(names);       // reverse order
    System.out.println("reversed=" + names);
    java.util.Collections.shuffle(names);       // random order
    System.out.println("shuffled=" + names);
  }
}

ค่าต่ำสุด/สูงสุดและความถี่

โค้ด: min/max/frequency

public class Main {
  public static void main(String[] args) {
    java.util.List<Integer> nums = new java.util.ArrayList<>();
    nums.add(7); nums.add(3); nums.add(7); nums.add(1);

    int min = java.util.Collections.min(nums);
    int max = java.util.Collections.max(nums);
    int freq7 = java.util.Collections.frequency(nums, 7);

    System.out.println("min=" + min + ", max=" + max + ", count(7)=" + freq7);
  }
}

โค้ด: addAll และ unmodifiableList

public class Main {
  public static void main(String[] args) {
    java.util.List<String> a = new java.util.ArrayList<>();
    java.util.Collections.addAll(a, "red", "green");

    java.util.List<String> b = new java.util.ArrayList<>();
    java.util.Collections.addAll(b, "blue", "green");

    // merge b into a
    a.addAll(b);
    System.out.println("merged=" + a); // [red, green, blue, green]

    // create a read-only view
    java.util.List<String> readOnly = java.util.Collections.unmodifiableList(a);
    System.out.println("readonly size=" + readOnly.size());
    // readOnly.add("x"); // would throw UnsupportedOperationException if uncommented
  }
}

แบบฝึกตรวจสอบเครื่องมือ Collections

ทบทวน