请参考doIt函数。
每当我将尾集存储在变量中并对其执行操作时，它也会更改ss变量的值。如何在不将其从主分拣集ss中移除的情况下移除尾集的第一个元素？
输入文件：
苹果
干酪
开胃酒
种族隔离
多瑙河
猿类
花钱少的
降低价格
应用程序
谢
丹
输出文件：苹果
ss:【苹果，奶酪】去掉【苹果】后的ss
ss:【开胃酒，苹果】去除后的ss【苹果】
党卫军：[种族隔离，苹果]党卫军在移除[苹果]
ss:[苹果，多瑙河]删除[苹果]后的ss
ss:[猿，苹果]删除[苹果]后的ss
ss:【苹果，便宜】移除后的ss【苹果】
ss:[苹果，便宜]删除[苹果]后的ss
ss:【应用程序，苹果】删除后的ss【苹果】
ss:[苹果，chea]删除[苹果]后的ss
ss:【苹果，丹】移除【苹果】后的ss

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.*;

public class Part4 {

    /**
     * 
     * @param r the reader to read from
     * @param w the writer to write to
     * @throws IOException
     */

    public static void doIt(BufferedReader r, PrintWriter w) throws IOException {
        SortedSet<String> ss = new TreeSet<String>();
        for (String line = r.readLine(); line != null; line = r.readLine()) {
            if (ss.isEmpty()) {
                ss.add(line);
                w.println(line);
            } else {
                w.println("");
                ss.add(line);
                w.println("ss: " + ss);
                SortedSet<String> tailset = new TreeSet<>();
                tailset = ss.tailSet(line);
                tailset.remove(line);
                w.println("ss after remove"+ss);

            }

        }
    }

    /**
     * The driver. Open a BufferedReader and a PrintWriter, either from System.in
     * and System.out or from filenames specified on the command line, then call
     * doIt.
     * 
     * @param args
     */
    public static void main(String[] args) {
        try {
            BufferedReader r;
            PrintWriter w;
            if (args.length == 0) {
                r = new BufferedReader(new InputStreamReader(System.in));
                w = new PrintWriter(System.out);
            } else if (args.length == 1) {
                r = new BufferedReader(new FileReader(args[0]));
                w = new PrintWriter(System.out);
            } else {
                r = new BufferedReader(new FileReader(args[0]));
                w = new PrintWriter(new FileWriter(args[1]));
            }
            long arrlirt = System.nanoTime();
            doIt(r, w);
            w.flush();
            long stop = System.nanoTime();
            System.out.println("Execution time: " + 1e-9 * (stop - arrlirt));
        } catch (IOException e) {
            System.err.println(e);
            System.exit(-1);
        }
    }
}