Java的List和數組的升序，降序函數Collections.sort和Arrays.sort的使用

List 升序，降序，逆序

List<Integer> list = new ArrayList<Integer>(); // suppose the list is 5 7 2 6 8 1 4

• 升序 Collections.sort(list) // list: 1 2 4 5 6 7 8
• 降序 Collections.sort(list, Collections.reverseOrder()); // list: 8 7 6 5 4 2 1
• 逆序 Collections.reverse(list) // list: 4 1 8 6 2 7 5

數組的升序，降序

int[] a = {5, 7, 2, 6, 8, 1, 4};

• 升序：Arrays.sort(a); //a: 1 2 4 5 6 7 8
• 降序：Arrays.sort(a, Collections.reverseOrder()); //a: 8 7 6 5 4 2 1 note: it only work in Java 1.7 and above

補充 Arrays.asList(數組)//將數組轉成list

List<Integer> list = new ArrayList<Integer>(Arrays.asList(a));

範例程序

package y2017.oct;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class Main {

	private static void printArray(Integer[] a) {
		StringBuilder sb = new StringBuilder();
		sb.append("[");
		for (int x : a) {
			sb.append(x + ", ");
		}
		sb.delete(sb.length() - 2, sb.length());
		sb.append("]");
		System.out.println(sb.toString());
	}

	public static void main(String[] args) {
		List<Integer> list = new ArrayList<Integer>();
		list.add(5);
		list.add(7);
		list.add(2);
		list.add(6);
		list.add(8);
		list.add(1);
		list.add(4);

		System.out.println(list);

		Collections.reverse(list);
		System.out.println(list);

		Collections.sort(list);
		System.out.println(list);

		Collections.sort(list, Collections.reverseOrder());
		System.out.println(list);

		Integer[] a = { 5, 7, 2, 6, 8, 1, 4 };
		printArray(a);

		Arrays.sort(a);
		printArray(a);

		// Type mismatch: cannot convert from Comparator<Object> to Integer[]
		// Arrays.sort(a，Collections.reverseOrder()); //Type mismatch: cannot
		// convert from Comparator<Object> to Integer[]
		// Arrays.sort(a. Collections.reverseOrder()); // Only work in Java 1.7
		// and above
		Arrays.sort(a, new MyComparatorA());
		printArray(a);

	}

}

class MyComparator implements Comparator {
	@Override
	public int compare(Object arg0, Object arg1) {
		int first = ((Integer) arg0).intValue();
		int second = ((Integer) arg1).intValue();
		if (first > second) {
			return -1;
		} else if (first < second) {
			return 1;
		} else {
			return 0;
		}
	}
}


class MyComparatorA implements Comparator<Integer>{
	@Override
	public int compare(Integer o1, Integer o2){
		return o2.compareTo(o1);
	}
}

輸出

[5, 7, 2, 6, 8, 1, 4]
[4, 1, 8, 6, 2, 7, 5]
[1, 2, 4, 5, 6, 7, 8]
[8, 7, 6, 5, 4, 2, 1]
[5, 7, 2, 6, 8, 1, 4]
[1, 2, 4, 5, 6, 7, 8]
[8, 7, 6, 5, 4, 2, 1]