Java集合排序

List 集合

存储元素不为 null，且排序字段不为 null

public static void main(String[] args) {

	List<UserDTO> userDTO = new ArrayList<>();
	userDTO.add(new UserDTO(new BigInteger("1"), "c", "s"));
	userDTO.add(new UserDTO(new BigInteger("2"), "d", "ss"));
	userDTO.add(new UserDTO(new BigInteger("3"), "a", "ss"));
	
	// 匿名内部类排序
	Collections.sort(userDTO, new Comparator<UserDTO>() {
		@Override
	    public int compare(UserDTO o1, UserDTO o2) {
			return o1.getUsername().compareTo(o2.getUsername());
		}
	});
	
	// 匿名内部类简化为 Lambda 排序
	Collections.sort(userDTO, (UserDTO u1, UserDTO u2) -> u1.getUsername().compareTo(u2.getUsername()));
	
	// Java8中，List 类的 sort() 方法排序，简化为
	userDTO.sort((UserDTO u1, UserDTO u2) -> u1.getUsername().compareTo(u2.getUsername()));
	
	// Java8 Lambda 类型推断简化排序
	userDTO.sort((u1, u2) -> u1.getUsername().compareTo(u2.getUsername()));
	
	// 静态方法抽取公共 Lambda 表达式
	userDTO.sort(UserDTO::compareByUsername);
	
	// Comparator 类 comparing() 方法，将 Function 参数作为比较元素
	userDTO.sort(Comparator.comparing(UserDTO::getUsername));
	
	// 多条件排序: Java8 使用 comparing()方法 和 thenComparing() 方法
	userDTO.sort(Comparator.comparing(UserDTO::getUsername).thenComparing(UserDTO::getId));
	
	// Java8 Stream sorted()排序
	userDTO = userDTO.stream().sorted(Comparator.comparing(UserDTO::getUsername)).collect(Collectors.toList());
	
	// 调转排序判断 倒序
	userDTO.sort((u1, u2) -> u2.getUsername().compareTo(u1.getUsername()));
	
	// Comparator reversed() 方法倒序
	Comparator<UserDTO> comparator = (u1, u2) -> u1.getUsername().compareTo(u2.getUsername());
	userDTO.sort(comparator.reversed());
	
	// Comparator comparing() 方法倒序
	userDTO.sort(Comparator.comparing(UserDTO::getUsername, Comparator.reverseOrder()));
	
	// Stream Comparator.reverseOrder()倒序
	userDTO.stream().sorted(Comparator.comparing(UserDTO::getUsername, Comparator.reverseOrder())).collect(Collectors.toList());

	// 遍历打印
	userDTO.forEach(userDTO1 -> System.out.println(userDTO1));
	
}

存储元素为 null，且排序字段不为 null

public static void main(String[] args) {

	List<UserDTO> userDTO = new ArrayList<>();
	userDTO.add(new UserDTO(new BigInteger("1"), "c", "s"));
	userDTO.add(null);
	userDTO.add(new UserDTO(new BigInteger("2"), "d", "ss"));
	userDTO.add(new UserDTO(new BigInteger("3"), "a", "ss"));
	
	// Comparator.nullsLast() 排序,null 在结尾 集合存空对象,排序条件的字段不能为 null
    userDTO.sort(Comparator.nullsLast(Comparator.comparing(UserDTO::getUsername)));

	// Comparator.nullsFirst() 排序,null 在开头 集合存空对象,排序条件的字段不能为 null
    userDTO.sort(Comparator.nullsFirst(Comparator.comparing(UserDTO::getUsername)));

	// 遍历打印
	userDTO.forEach(userDTO1 -> System.out.println(userDTO1));

}

存储元素为 null，且排序字段为 null

public static void main(String[] args) {

    List<UserDTO> userDTO = new ArrayList<>();
	userDTO.add(new UserDTO(new BigInteger("1"), "c", "s"));
	userDTO.add(null);
	userDTO.add(new UserDTO(new BigInteger("2"), null, "ss"));
	userDTO.add(new UserDTO(new BigInteger("3"), "a", "ss"));

	// 排序条件的字段是 null，Comparator 组合
    userDTO.sort(Comparator.nullsLast(Comparator.nullsLast(Comparator.comparing(UserDTO::getUsername, Comparator.nullsLast(String::compareTo)))));

	// 遍历打印
	userDTO.forEach(userDTO1 -> System.out.println(userDTO1));
	
}

借鉴网址

使用 Lambda 表达式实现超强的排序功能（原创网址）