Convert a string to int in Java without allocating a String

Question

I have thousands of lines of text file, similar to this:

0000:0010:1111:3000
0003:0010:1113:3000
0004:0010:1188:3009

and so on, where the data is to be parsed as ints.

The most obvious way to do it is by using Integer.parseInt(String) . However, it needs a String , and since String s are immutable, we have to allocate again and again String s just to convert them to ints.

It is to be run in a mobile device where allocating a String and letting the GC run takes a considerable effort.

I am hoping to be able to have a method like parseInt(char[] chars, int offset, int length) so that we don't need to do allocations. Is that possible? Is there any implementation in standard Java/JDK/Android library that can already do that?

Answer 1

You could use a Scanner and call useDelimiter(String) to set your separator(s). Something like,

String str = "0000:0010:1111:3000\n" + "0003:0010:1113:3000\n"
        + "0004:0010:1188:3009";
Scanner sc = new Scanner(str);
sc.useDelimiter("[:|\\s]+"); // <-- one or more colon or whitespace
while (sc.hasNextInt()) {
    System.out.printf("%04d%n", sc.nextInt()); // <-- format to 4 digits with
                                               //     leading zeros.
}

Answer 2

It's quite simple to parse int from char array, this is demo code:

public static int parseInt(char[] chars, int offset, int length) {
    int r = 0;
    for (int i = offset; i < offset + length; ++i) {
        r *= 10;
        r += chars[i] - '0';
    }

    return r;
}

You can add some security check for array length and make sure the char is really digits if you like.

Answer 3

If you really care that much about performance, you'll have to write your own code to parse the ints. eg

public static int parseInt(String s, int startPos, int len) {
  // code omitted, it would be simple
}

Where the String s is your entire file (or perhaps one line thereof). Search for end of lines and colons and get your numbers. Or use a char[] as in your idea. I don't know of a built in Java class to do this but the code is simple enough - something you learn in Java 1 at school.