主函数

public class SetPractise {    public static void main(String[] args) {        List<Message> received = new ArrayList<>();        received.add(new Message(1, "Hello!"));        received.add(new Message(2, "去哪吃饭？"));        received.add(new Message(2, "去哪吃饭？"));        received.add(new Message(3, "回家？"));        received.add(new Message(3, "回家？"));        received.add(new Message(4, "Over!"));        List<Message> displayMessages = process(received);        for (Message message : displayMessages) {            System.out.println(message.text);        }    }    static List<Message> process(List<Message> received) {        // TODO: 按sequence去除重复消息        return new ArrayList<>(new HashSet<>(received));    }}

Message实体类

public class Message {    public final int sequence;    public final String text;    public Message(int sequence, String text) {        this.sequence = sequence;        this.text = text;    }    @Override    public int hashCode() {        return Objects.hash(sequence, text);    }    @Override    public boolean equals(Object obj) {        if (obj instanceof Message) {            Message message = (Message) obj;            return message.sequence == this.sequence && Objects.equals(message.text, this.text);        }        return false;    }}