在JAVA中实现对象的排序可以使用Comparable接口，也可以使用Comparator接口．

• Comparable : 需要自己实现比较的方法compareTo，是一个内部排序器
• Comparator : 可以使用其默认的方法和lambda表达式来实现排序，是一个外部排序器，更像一个工具接口

一、Comparable接口

• 表明实现的对象是一个可排序的类

1.一个需要排序的类，需要实现Comparable接口中的compareTo方法

class ArticleInfo implements Comparable<ArticleInfo> {
    private Integer articleId;
    private String title;
    private Integer views;
    private Integer comments;
    private Long createTime;
    public ArticleInfo(Integer articleId, String title, Integer views, Integer comments, Long createTime) {
        this.articleId = articleId;
        this.title = title;
        this.views = views;
        this.comments = comments;
        this.createTime = createTime;
    }
    public Integer getArticleId() {
        return articleId;
    }
    public void setArticleId(Integer articleId) {
        this.articleId = articleId;
    }
    public String getTitle() {
        return title;
    }
    public void setTitle(String title) {
        this.title = title;
    }
    public Integer getViews() {
        return views;
    }
    public void setViews(Integer views) {
        this.views = views;
    }
    public Integer getComments() {
        return comments;
    }
    public void setComments(Integer comments) {
        this.comments = comments;
    }
    public Long getCreateTime() {
        return createTime;
    }
    public void setCreateTime(Long createTime) {
        this.createTime = createTime;
    }
    @Override
    public int compareTo(ArticleInfo info) {
        if (this.views > info.getViews()) {
            return -1;
        } else if (this.views.equals(info.getViews())) {
            return 0;
        } else {
            return 1;
        }
    }
    @Override
    public String toString() {
        return "ArticleInfo{" +
                "articleId=" + articleId +
                ", title='" + title + '\'' +
                ", views=" + views +
                ", comments=" + comments +
                ", createTime=" + createTime +
                '}';
    }
}

注：在实现compareTo方法时，一定要判断当两个对象的值相等的情况，不然，可能在集合对象比较时出错

2.调用排序方法

针对集合对象，可以使用

• 工具类Collections.sort方法
• 使用集合对象的sort方法

无论是Collections.sort方法还是集合对象的sort方法都可以传入一个排序方式的参数

• Comparator.naturalOrder() : 正序
• Comparator.reverseOrder() : 倒序

注：正序和倒序是相对于实现的Comparable接口的compareTo接口而言的，实现的就是正序，reverseOrder就是针对实现的CompareTo的结果再进行倒序

List<ArticleInfo> articleInfoList = new ArrayList<ArticleInfo>(10);
Long now = System.currentTimeMillis();
articleInfoList.add(new ArticleInfo(1001, "第一篇文章", 10, 2, now));
articleInfoList.add(new ArticleInfo(1002, "第二篇文章", 29, 3, now+1000));
articleInfoList.add(new ArticleInfo(1003, "第三篇文章", 9, 1, now+5000));
/**
* 1.通过Collections工具类来排序
*/
Collections.sort(articleInfoList);
System.out.println(articleInfoList);
Collections.sort(articleInfoList, Comparator.reverseOrder());
System.out.println(articleInfoList);
/**
* 2.通过List等对象的sort方法
*/
articleInfoList.sort(Comparator.reverseOrder());
System.out.println(articleInfoList);

二、Comparator接口

1.实现compare抽象方法

• 定义普通的实体类Article

class Article {
    private Integer articleId;
    private String title;
    private Integer views;
    private Integer comments;
    private Long createTime;
    public Article(Integer articleId, String title, Integer views, Integer comments, Long createTime) {
        this.articleId = articleId;
        this.title = title;
        this.views = views;
        this.comments = comments;
        this.createTime = createTime;
    }
    public Integer getArticleId() {
        return articleId;
    }
    public void setArticleId(Integer articleId) {
        this.articleId = articleId;
    }
    public String getTitle() {
        return title;
    }
    public void setTitle(String title) {
        this.title = title;
    }
    public Integer getViews() {
        return views;
    }
    public void setViews(Integer views) {
        this.views = views;
    }
    public Integer getComments() {
        return comments;
    }
    public void setComments(Integer comments) {
        this.comments = comments;
    }
    public Long getCreateTime() {
        return createTime;
    }
    public void setCreateTime(Long createTime) {
        this.createTime = createTime;
    }
    @Override
    public String toString() {
        return "Article{" +
                "articleId=" + articleId +
                ", title='" + title + '\'' +
                ", views=" + views +
                ", comments=" + comments +
                ", createTime=" + createTime +
                '}';
    }
}

• 实现compare方法

class ArticleComparator implements Comparator<Article> {
    @Override
    public int compare(Article a1, Article a2) {
        return a1.getViews().compareTo(a2.getViews());
    }
}

• 通过实现的比较器进行排序

List<Article> articleList = new ArrayList<Article>(10);
Long current = System.currentTimeMillis();
articleList.add(new Article(1001, "第一篇文章", 10, 2, current));
articleList.add(new Article(1002, "第二篇文章", 29, 3, current+1000));
articleList.add(new Article(1003, "第三篇文章", 9, 1, current+5000));
articleList.sort(new ArticleComparator());
System.out.println(articleList);

2.静态方法

• naturalOrder : 正序排列(一般使用于自定义的Comparator接口或Comparable接口)
• reverseOrder : 倒序排列(一般使用于自定义的Comparator接口或Comparable接口)
• comparing : 比较(传入一个排序函数)
• comparingInt : 比较int值
• comparingLong : 比较long型值
• comparingDouble : 比较double型值

/**
* 按views排序
*/
articleList.sort(Comparator.comparing(Article::getViews));
System.out.println(articleList);
/**
* 按comments排序
*/
articleList.sort(Comparator.comparing(Article::getComments));
System.out.println(articleList);
articleList.sort(Comparator.comparingInt(Article::getComments));

3.默认方法

• reversed : 倒序(将之前的对象进行逆向排列)
• thenComparing　：　多重比较

List<Article> articleList = new ArrayList<Article>(10);
Long current = System.currentTimeMillis();
articleList.add(new Article(1001, "第一篇文章", 10, 5, current));
articleList.add(new Article(1002, "第二篇文章", 29, 3, current+1000));
articleList.add(new Article(1003, "第三篇文章", 9, 1, current+5000));
articleList.add(new Article(1004, "第四篇文章", 9, 2, current+5000));
/**
* 按views倒序排列
*/
articleList.sort(Comparator.comparing(Article::getViews).reversed());
System.out.println(articleList);
/**
* 先按views倒序，再按comments倒序
*/
articleList.sort(Comparator.comparingInt(Article::getViews).reversed().
                thenComparing(Comparator.comparingInt(Article::getComments).reversed()));
System.out.println(articleList);
//[Article{articleId=1002, title='第二篇文章', views=29, comments=3, createTime=1548830876084}, Article{articleId=1001, title='第一篇文章', views=10, comments=5, createTime=1548830875084}, Article{articleId=1004, title='第四篇文章', views=9, comments=2, createTime=1548830880084}, Article{articleId=1003, title='第三篇文章', views=9, comments=1, createTime=1548830880084}]

• 通过thenComparing进行多重条件排序时，只要当前一个条件相同的情况下，才会按下一个指标进行排序(比较适合多个维度先优先级排序的场景)