Create a very long string and overcome runtime

Question

Can anyone help to reduce time to run this code?

It's not an actual code, it's just a part of one of my project:

import java.util.*;

class testing{
    public static void main(String[] args) {
        String s="";

        for(long i=0L;i<10000000000L;i++)
            s+=String.valueOf(Character.toChars((int)(Math.random()*26+97)));

        System.out.println(s.length()+"\n"+s);
    }
}

How about just replacing the string building with a bunch of `System.out.print` calls? — Andy Turner, Aug 16 '18 at 19:22
@HarshVardhan why do you want to create such a string? What are you trying to do with it? — Andy Turner, Aug 16 '18 at 19:29
If you want it that big, dump it to a stream... then it can be any size you want. — Micromuncher, Aug 16 '18 at 19:29
@AndyTurner just playing with string and pushing it to limits — Harsh Vardhan, Aug 16 '18 at 19:30
Note that you can always create a more complex data-holder which stores the data in multiple objects. Much like `BigInteger` stores integers of arbitrary size using multiple `int`s. — Zabuzard, Aug 16 '18 at 19:39
You can't make a string that large. See [this answer](https://stackoverflow.com/a/41024966/3591528). — teppic, Aug 16 '18 at 21:49

davidxxx · Accepted Answer · 2018-08-17T04:50:32.770

1) Instantiate StringBuilder by setting the wished capacity :

 new StringBuilder(wishedSize)

2) You create a random double but you cast it to int :

(int)(Math.random()*26+97)

So you can use instead your own Random object before the loop and invoke nextInt().

3) As Andy underlines (I didn't notice it) you get at the end a series of lower case letters.
You could use a Random.nextInt(int) to get a number between 97 and 122 (unicode range for small letters) such as :

final int maxRange = 122 - 97;
Random random = new Random();
for (long i = 0L; i < 100L; i++) {
    int codePoint = random.nextInt(maxRange) + 97;
    // ...
}

Besides, in this way you perform only 1 arithmetic computation on the returned random value instead of 2 of them previously.

3) OP is creating a random string of lower case letters. – Andy Turner Aug 16 '18 at 19:32 — Andy Turner, Aug 16 '18 at 19:32
@Andy Turner Really nice to having seen that. – davidxxx Aug 16 '18 at 19:48 — davidxxx, Aug 16 '18 at 19:48

score 1 · Answer 2 · answered Aug 16 '18 at 19:27

You create a lot of Strings and concatenate them, which is quite slow.

Using a StringBuilder can be faster, if the expected size is known prior to generating the string:

StringBuilder sb = new StringBuilder(expectedStringLength);

for (int i = 0; i < expectedStringLength; i++)
    sb.append(Math.random() * 26 + 97);

System.out.println(sb.length() + "\n" + sb.toString());

Note however, that the length of the string cannot be bigger than Interger.MAX_VALUE, since it is backed by an array and java only supports int indices. Then again having such a big string will use a lot of memmory, so you might need to change your concept.

score 1 · Answer 3 · answered Aug 16 '18 at 19:37

To prevent the crash, bypass the string entirely and dump it directly to filesystem.

try (OutputStream out = new OutputStreamWriter(new FileOutputStream(file)), "utf-8") {
    for (int i = 0; i < 100000000L; i++) {
        out.write(Character.toChars((int)(Math.random()*26+97)));
    }
    out.flush();
}

score 0 · Answer 4 · edited Aug 16 '18 at 19:25

0

Every string you make is immutable, so adding on to strings causes a lot of churn. Use a buffer, like this

class testing{
public static void main(String[] args) {
    StringBuffer sb = new StringBuffer(EST_SIZE);

    for(long i=0L;i<COUNT;i++)
        sb.append( Integer.toString(Math.random()*26+97 ) );

System.out.println(sb.length()+"\n"+sb.toString());

edited Aug 16 '18 at 19:25

Andy Turner

137,514
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answered Aug 16 '18 at 19:24

Micromuncher

903
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3

Use `StringBuilder` instead of `StringBuffer`. – Andy Turner Aug 16 '18 at 19:25

Create a very long string and overcome runtime

4 Answers4