两个列表比较取相同值

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Random;


public class ListCompare {

    //传入两个元素之间可相互比较的列表, 从两个列表中选择相同的元素组成新列表返回
    @SuppressWarnings("unchecked")
    public static List<Comparable<?>> getSameList(List<Comparable<?>> list1, List<Comparable<?>> list2){
        List<Comparable<?>> result = new ArrayList<Comparable<?>>();
        //先转为数组排序
        Object[] c1 = list1.toArray();
        Object[] c2 = list2.toArray();
        //O(nlog(n))
        Arrays.sort(c1);
        Arrays.sort(c2);
        int len = c1.length;
        //两个指针,当a>b时, 指向b的指针向后移;反正,当a<b时,指向a的指针向后移
        int i=0,j=0;
        //不会超过O(2n)
        for(int k=0;k<len;k++){
            Comparable a = (Comparable)c1[i];
            Comparable b = (Comparable)c2[j];
            if(a.compareTo(b)==0){
                i++;
                j++;
                result.add(a);
            }
            else if(a.compareTo(b)<0) {
                i++;
            }else {
                j++;
            }
        }
        //去掉重复的
        return new ArrayList(new LinkedHashSet(result));
    }

    static Random random = new Random();
    //获取随机字符串,长度1~5，由小写字母组成
    private static String getRandomString(){
        int len = random.nextInt(5)+1;
        char[] chs = new char[len];
        for(int i=0;i<len;i++){
            chs[i]=(char)('a'+random.nextInt(26));
        }
        return new String(chs);
    }

    //打印列表
    private static void printList(List<?> list){
        if(list == null){
            System.out.println("null");
        }
        for(Object object :list){
            System.out.print(object+"\t");
        }
        System.out.println();
    }

    //生成由随机字符串组成的列表
    private static List<Comparable<?>> generateRandomList(int len){
        List<Comparable<?>> l1 = new ArrayList<Comparable<?>>();
        for(int i=0;i<len;i++){
            l1.add(getRandomString());
        }
        return l1;
    }


    //太大的不打印,写到文件自己查看吧
    @SuppressWarnings("unchecked")
    private static void writeListToTxt(List list,String path){
        Object[] arr = list.toArray();
        Arrays.sort(arr);
        int len = arr.length;
        StringBuilder sb = new StringBuilder(len);
        for(int i=0; i<len;i++){
            sb.append(arr[i]).append("\t");
        }
        File file = new File(path);
        OutputStreamWriter writer = null;
        try {
            if(!file.exists()){
                file.createNewFile();
            }
            writer = new OutputStreamWriter(new FileOutputStream(file));
            writer.write(sb.toString());
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }finally{
            if(writer!=null){
                try {
                    writer.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
    //超过1000行的就不打印了，大家随意
    private static boolean isPrintable(int len){
        return len<=1000;
    }

    @SuppressWarnings("unchecked")
    public static void main(String[] args) {
        int len = 100000;
        List<Comparable<?>> l1 = generateRandomList(len);
        List<Comparable<?>> l2 = generateRandomList(len);

        if(isPrintable(len)){
            System.out.println("the two list are here");
            printList(l1);
            printList(l2);
        }

        writeListToTxt(l1, "C:\\l1.txt");
        writeListToTxt(l2, "C:\\l2.txt");



        long start = System.currentTimeMillis();
        List<Comparable<?>> sameList = getSameList(l1, l2);
        long end = System.currentTimeMillis();
        System.out.println("spends "+(end-start)+" ms to make it done. "
                +"\nthe sameList's size is "+sameList.size()
                +"\nand the sameList elements are:");
        printList(sameList);
        //写到文件
        writeListToTxt(sameList, "C:\\same.txt");
    }
}