Java中用TreeSet方法排序字符型数字怎么排序啊，求指教，谢谢

public class Demo implements Comparable{
//String str;
String str;
/*public Demo(){

}*/
public Demo(String str){
    this.str=str;
}
public void getStr(String str){
    this.str=str;
}
public String setStr(){
    return str;
}
//重写toString()方法
public String toString(){
    return  " "+this.str;
}
//比较规则
public int compareTo(Object arg0) {
    return 1;
}

}

public class SortByNum implements Comparator{

public int compare(Object arg0, Object arg1) {
    Demo d1=(Demo)arg0;
    Demo d2=(Demo)arg1;
    int temp = 0;
    String str1=d1.toString();//转换成字符串
    String str2=d2.toString();//转换成字符串
    char[] ch1=str1.toCharArray();//转换成数组
    char[] ch2=str2.toCharArray();//转换成数组
    for(int i=0;i<ch1.length;i++){
        for(int j=0;j<ch2.length;j++){
            temp=d1.compareTo(d2);
        }
    }
    //int temp=d1.compareTo(d2);
    return  temp;
}

}

public class TreeSets {
public static void main(String[] args) {
//Demo demo=new Demo();
//创建一个TreeSet集合
TreeSet tree=new TreeSet(new SortByNum());
//添加字符
tree.add(new Demo("15"));
tree.add(new Demo("19"));
tree.add(new Demo("16"));
tree.add(new Demo("99"));
System.out.println(tree);
}
}

麻烦大家看一下，就是不知道怎么做，例如：String str="8 10 15 2 5 7";

输出"2 5 7 8 10 15" 谢谢了

 /* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

class Demo implements Comparable{
    String str;

    public Demo(String str){
        this.str=str;
    }
    public void setStr(String str){
        this.str=str;
    }
    public String getStr(){
        return str;
    }
    //重写toString()方法
    public String toString(){
        return  " "+this.str;
    }
    //比较规则
    public int compareTo(Object arg0) {
        Demo d2 = (Demo)arg0;
        if (d2.getStr().length() != str.length()) return str.length() - d2.getStr().length();
        return str.compareTo(d2.getStr());
    }
}

class SortByNum implements Comparator{

    public int compare(Object arg0, Object arg1) {
        Demo d1=(Demo)arg0;
        Demo d2=(Demo)arg1;
        return d1.compareTo(d2);
    }
}

/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
    public static void main (String[] args) throws java.lang.Exception
    {
        // your code goes here
        //创建一个TreeSet集合
        TreeSet tree=new TreeSet(new SortByNum());
        //添加字符
        String s = "8 10 15 2 5 7";
        for (String s1 : s.split(" "))
            tree.add(new Demo(s1));
        System.out.println(tree);
    }
}

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