實現Comparable介面，並實現該介面下的compareTo()方法， 再使用Collections.sort()重新排序，就可以得到自己想要的順序。

public class UserInfo implements Comparable<UserInfo> {


	private String userName;
	private int userAge;


	public UserInfo(String userName, int userAge) {
		this.userName = userName;
		this.userAge = userAge;
	}


	public String getUserName() {
		return userName;
	}


	public void setUserName(String userName) {
		this.userName = userName;
	}


	public int getUserAge() {
		return userAge;
	}


	public void setUserAge(int userAge) {
		this.userAge = userAge;
	}


	@Override
	public int compareTo(UserInfo o) {
		return this.userAge - o.getUserAge();
	}


}


	public static void main(String[] args) {


		UserInfo user2 = new UserInfo("22", 22);
		UserInfo user1 = new UserInfo("20", 20);
		UserInfo user3 = new UserInfo("24", 24);


		ArrayList<UserInfo> userList = new ArrayList<UserInfo>();
		userList.add(user1);
		userList.add(user2);
		userList.add(user3);


		Collections.sort(userList);


		for (UserInfo userInfo : userList) {
			System.out.println(userInfo.getUserName());
		}
	}
 

需要注意的是，java.util.Collection是一個集合的介面，雖然兩個只是相差一個s，但是功能完全不一樣。

	public static <T extends Comparable<? super T>> void sort(List<T> list) {
		Object[] a = list.toArray();
		Arrays.sort(a);
		ListIterator<T> i = list.listIterator();
		for (int j = 0; j < a.length; j++) {
			i.next();
			i.set((T) a[j]);
		}
	}