java：TreeSet 集合和可比较接口
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java : TreeSet Collection and Comparable interface
提问by mcacorner
I have following code : I am trying to insert Item object in TreeSet and I am not getting desire output.
我有以下代码： 我试图在 TreeSet 中插入 Item 对象，但没有得到想要的输出。
public class Main
{
public static void main(String a[])
{
Item i1=new Item(1,"aa");
Item i2=new Item(5,"bb");
Item i3=new Item(10,"dd");
Item i4=new Item(41,"xx");
Item i5=new Item(3,"x5");
TreeSet t=new TreeSet();
t.add(i1);
t.add(i2);
t.add(i3);
t.add(i4);
t.add(i5);
System.out.println(t);
}
}
class Item implements Comparable<Item>
{
String nm;
int price;
public Item(int n,String nm)
{
this.nm=nm;
price=n;
}
public int compareTo(Item i1)
{
if(price==i1.price)
return 0;
else if(price>=i1.price)
return 1;
else
return 0;
}
public String toString()
{
return "\nPrice "+price+" Name : "+nm;
}
}
Output :
输出 ：
[ Price 1 Name : aa,
Price 5 Name : bb,
Price 10 Name : dd,
Price 41 Name : xx ]
[价格1名称：aa，
价格5名称：bb，
价格10名称：dd，
价格41名称：xx]
Item i5=new Item(3,"x5");is not Inserted why?
Why I can do to insert in TreeSet.
Item i5=new Item(3,"x5");没有插入为什么？
为什么我可以在 TreeSet 中插入。
回答by Andrew Logvinov
You haven't implemented compareTo()correctly. Here's an extract from javadoc:
你没有compareTo()正确实施。这是javadoc的摘录：
Compares this object with the specified object for order. Returns a negative integer,
zero, or a positive integer as this object is less than, equal to, or greater than
the specified object.
Your implementation doesn't return -1in case price of current object is less than price of object you compare with.
-1如果当前对象的价格低于您比较的对象的价格，则您的实现不会返回。
回答by Muel
In your compareTomethod you should have else return -1;!
在你的compareTo方法中你应该有else return -1;！
回答by Mordechai
In compareToreplace:
在compareTo替换：
else
return 0;
with:
和：
else
return -1;
回答by onon15
A class implementing Comparablemust comply with the contract: if a.compareTo(b) < 0then b.compareTo(a) > 0. Yours does not comply.
一个类实现Comparable必须遵守契约： if a.compareTo(b) < 0then b.compareTo(a) > 0。你的不符合。
回答by Rais Alam
Main problem is in your compareTo method. You have implemented a wrong logic. I have modified your class and tested at my eclipse it giving desired output. look at below code.
主要问题出在您的 compareTo 方法中。你实现了错误的逻辑。我已经修改了您的课程并在我的日食中进行了测试，它提供了所需的输出。看看下面的代码。
import java.util.TreeSet;
public class Main
{
public static void main(String a[])
{
Item i1=new Item(1,"aa");
Item i2=new Item(5,"bb");
Item i3=new Item(10,"dd");
Item i4=new Item(41,"xx");
Item i5=new Item(3,"x5");
TreeSet<Item> t=new TreeSet<Item>();
t.add(i1);
t.add(i2);
t.add(i3);
t.add(i4);
t.add(i5);
System.out.println(t);
}
}
class Item implements Comparable<Item>
{
String nm;
int price;
public Item(int n,String nm)
{
this.nm=nm;
price=n;
}
public int compareTo(Item i1)
{
// Objects equal so no need to add
if(price==i1.price)
{
return 0;
}
// Object are greater
else if(price>i1.price)
{
return 1;
}
// Object is lower
else
{
return -1;
}
}
public String toString()
{
return "\nPrice "+price+" Name : "+nm;
}
}
