用Java从文件中读取大量数据 [英] Read large amount of data from file in Java

问题描述

我有一个包含 1 000 002 个数字的文本文件，格式如下:

I've got text file that contains 1 000 002 numbers in following formation:

123 456
1 2 3 4 5 6 .... 999999 100000

现在我需要读取该数据并将其分配给 int 变量(前两个数字)和所有其余的(1 000 000 个数字)到一个数组int[].

Now I need to read that data and allocate it to int variables (the very first two numbers) and all the rest (1 000 000 numbers) to an array int[].

这不是一项艰巨的任务，但是 - 这太慢了.

It's not a hard task, but - it's horrible slow.

 Scanner stdin = new Scanner(new File("./path"));
 int n = stdin.nextInt();
 int t = stdin.nextInt();
 int array[] = new array[n];

 for (int i = 0; i < n; i++) {
     array[i] = stdin.nextInt();
 }

它正常工作，但执行需要大约 7500 毫秒.我需要在数百毫秒内获取该数据.

It works as excepted but it takes about 7500 ms to execute. I need to fetch that data in up to several hundred of milliseconds.

使用 BufferedReader.readLine() 和 String.split() 我在大约 1700 毫秒 内得到了相同的结果，但它仍然是很多.

Using BufferedReader.readLine() and String.split() I got the same results in about 1700 ms, but it's still too many.

我怎样才能在不到 1 秒的时间内读取这么多数据?最终结果应该等于:

How can I read that amount of data in less that 1 second? The final result should be equal to:

int n = 123;
int t = 456;
int array[] = { 1, 2, 3, 4, ..., 999999, 100000 };

根据垃圾神的回答:

StreamTokenizer 解决方案很快(大约需要 1400 毫秒)但它仍然太慢:

According to trashgod answer:

StreamTokenizer solution is fast (takes about 1400 ms) but it's still too slow:

StreamTokenizer st = new StreamTokenizer(new FileReader("./test_grz"));
st.nextToken();
int n = (int) st.nval;

st.nextToken();
int t = (int) st.nval;

int array[] = new int[n];

for (int i = 0; st.nextToken() != StreamTokenizer.TT_EOF; i++) {
    array[i] = (int) st.nval;
}

附注.无需验证.我 100% 确定 ./test_grz 文件中的数据是正确的.

PS. There is no need for validation. I'm 100% sure that data in ./test_grz file is correct.

推荐答案

感谢您的每一个回答，但我已经找到了符合我的条件的方法:

Thanks for every answer but I've already found a method that meets my criteria:

BufferedInputStream bis = new BufferedInputStream(new FileInputStream("./path"));
int n = readInt(bis);
int t = readInt(bis);
int array[] = new int[n];
for (int i = 0; i < n; i++) {
    array[i] = readInt(bis);
}

private static int readInt(InputStream in) throws IOException {
    int ret = 0;
    boolean dig = false;

    for (int c = 0; (c = in.read()) != -1; ) {
        if (c >= '0' && c <= '9') {
            dig = true;
            ret = ret * 10 + c - '0';
        } else if (dig) break;
    }

    return ret;
}

读取 100 万个整数只需要大约 300 毫秒！

It requires only about 300 ms to read 1 mln of integers!

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