java - 优化输入流读取速度更快/提高串联速度

Question

目前我正在读取 100,0000+/- 行输入(例如: http://pastebin.com/zhuHLcvA )，其以单个“;”结尾。

使用此代码在我的电脑上读取需要 20 秒以上(这太长了):

public static String readFromStandardIO() {

    String returnValue = "";

    try {
        BufferedReader reader = new BufferedReader(new InputStreamReader(
                System.in));
        String userInput;
        // System.out.println("Enter polynomials:\n");
        while (true) {
            userInput = reader.readLine();
            // System.out.println(userInput);
            returnValue = returnValue.concat(userInput);
            if (userInput.equals(";")) {
                break;
            }
        }

    } catch (Exception e) {

    }

    return returnValue;

}

使用计时方法我得到奇怪的结果:

long start = System.nanoTime();
pol = readFromStandardIO();
long elaspedTime = System.nanoTime() - start;
System.out.println("reading stream took: " + elaspedTime);      

它输出:

reading stream took: 1914854722

行连接似乎减慢了一切:

returnValue = returnValue.concat(userInput)

没有它，一切都是瞬时的。

如何提高串联速度？

Answer 1

public static String readFromStandardIO() {
   StringBuilder returnValue = new StringBuilder(9999999);
   try {
       BufferedReader reader = new BufferedReader(System.in);
       String userInput;
       while ((userInput = reader.readLine()) != null) {
          returnValue.append(userInput);
       }
       reader.close();
   } catch (Exception e) {

   }
   return returnValue.toString();
}

或者这个更快(如果你的jdk支持*.nio):

public static String readFromStandardNIO() {
    java.nio.ByteBuffer buffer = java.nio.ByteBuffer.allocate(20480000);
    try {
        ReadableByteChannel channel = Channels.newChannel(System.in);
        channel.read(buffer);
        channel.close();
    } catch (Exception e) {

    }
    return new String(buffer.array());
}