如何删除List <MyObject>中没有equals/hashcode的重复对象？

Question

我必须删除列表中的重复对象.它是来自对象Blog的List,如下所示:

public class Blog {
    private String title;
    private String author;
    private String url;
    private String description;
    ...
}

复制对象是一个对象,其标题,作者,URL和描述等于其他对象.

我无法改变这个对象.我不能把新的方法放在上面.

我该怎么做呢？

Answer 1

以下是适用于此场景的完整代码:

class Blog {
    private String title;
    private String author;
    private String url;
    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getAuthor() {
        return author;
    }

    public void setAuthor(String author) {
        this.author = author;
    }

    public String getUrl() {
        return url;
    }

    public void setUrl(String url) {
        this.url = url;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    private String description;    

    Blog(String title, String author, String url, String description)
    {
        this.title = title;
        this.author = author;
        this.url = url;
        this.description = description; 
    }
    @Override
    public boolean equals(Object obj) {
        // TODO Auto-generated method stub
        if(obj instanceof Blog)
        {
            Blog temp = (Blog) obj;
            if(this.title == temp.title && this.author== temp.author && this.url == temp.url && this.description == temp.description)
                return true;
        }
        return false;

    }
    @Override
    public int hashCode() {
        // TODO Auto-generated method stub

        return (this.title.hashCode() + this.author.hashCode() + this.url.hashCode() + this.description.hashCode());        
    }
}

}

这是主要功能,将消除重复:

public static void main(String[] args) {
    Blog b1 = new Blog("A", "sam", "a", "desc");
    Blog b2 = new Blog("B", "ram", "b", "desc");
    Blog b3 = new Blog("C", "cam", "c", "desc");
    Blog b4 = new Blog("A", "sam", "a", "desc");
    Blog b5 = new Blog("D", "dam", "d", "desc");
    List<Blog> list = new ArrayList();
    list.add(b1);
    list.add(b2);
    list.add(b3);
    list.add(b4);       
    list.add(b5);

    //Removing Duplicates;
    Set<Blog> s= new HashSet<Blog>();
    s.addAll(list);         
    list = new ArrayList<Blog>();
    list.addAll(s);        
    //Now the List has only the identical Elements
}

}

Answer 2

如果你不能编辑类的源代码(为什么不呢？),那么你需要迭代列表并根据提到的四个标准("标题,作者,网址和描述")比较每个项目.

要做到这一点的性能方法,我会创建一个新的类,像BlogKey它包含那些四种元素和其正确实施equals()和hashCode().然后,您可以遍历原始列表,BlogKey为每个列表构建一个并添加到HashMap:

Map<BlogKey, Blog> map = new HashMap<BlogKey, Blog>();
for (Blog blog : blogs) {
     BlogKey key = createKey(blog);
     if (!map.containsKey(key)) {
          map.put(key, blog);
     }
}
Collection<Blog> uniqueBlogs = map.values();

然而,最简单的方法是编辑原始源代码,Blog以便正确实现equals()和hashCode().

Answer 3

确保Blog有方法equals(Object)和hashCode()定义,addAll(list)然后到a new HashSet(),或者new LinkedHashSet()顺序是否重要.

更好的是,从一开始就使用a Set而不是a List,因为你显然不想要重复,所以你的数据模型更好地反映了它,而不是必须在事后删除它们.

Answer 4

使用集：

yourList = new ArrayList<Blog>(new LinkedHashSet<Blog>(yourList));

这将创建没有重复的列表，元素顺序将与原始列表相同。

只是不要忘记为您的班级博客实现hashCode()和equals()。