Java使用筆記之stream和sorted使用

//對象類型stream排序
List<User> users = new ArrayList<User>(){
    {
        add(new User("a", "1983/12/24", 36));
        add(new User("a", "1993/12/24", 26));
        add(new User("a", "1973/12/24", 46));
        add(new User("a", "1995/12/24", 24));
        add(new User("a", "1981/12/24", 38));
        add(new User("a", "1991/12/24", 28));
    }
};

//升序
List<User> ascUsers = users.stream().sorted(Comparator.comparing(User :: getAge)).collect(Collectors.toList());

//降序
List<User> descUsers = users.stream().sorted(Comparator.comparing(User :: getAge).reversed()).collect(Collectors.toList());

用戶類：必須繼承Comparable接口，否則排序調用Comparator.comparing的時候會報錯，這點很重要。

public class User implements Comparable<User> {

    @NotNull
    private String name;

    @Format(value="YYYY-MM-DD", message = "字符串格式不正確")
    private String birthday;

    private int age;

    public User(){
    }

    public User(String name, String birthday, int age){
        this.name = name;
        this.birthday = birthday;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getBirthday() {
        return birthday;
    }

    public void setBirthday(String brithday) {
        this.birthday = birthday;
    }

    /**
     * 兩個相同類型的對象，比較多屬相是否相等的時候
     * 可以通過重寫equals方法來實現
     * @param object
     * @return
     */

    @Override
    public boolean equals(Object object) {
        if (object instanceof User) {
            return this.name.equals(((User) object).getName()) && this.age == ((User) object).getAge();
        } else {
            return false;
        }
    }

    /**
     * 重寫 equals 就應當重寫 hashcode
     * 因為當兩個對象hashcode 值相等，但是 equals 比較為 false，
     * 那么在 HashMap 中會產生鏈表，影響查詢性能
     * @return
     */
    @Override
    public int hashCode() {
        return  this.name.hashCode() + age;
    }

    @Override
    public int compareTo(User user) {
        return name.compareTo(user.getName());
    }
}