Is there a better way to traverse a string in java?

Question

Please bear with me. I haven't programmed in over a year and I'm currently reviewing my Java for job interviews by doing "homework questions." My function is supposed to return a string containing every second character in the given string. Is there a less awkward way to do this?

public String stringBits(String str) {
  StringBuffer tmp = new StringBuffer();
  for(int i = 0; i<str.length(); i+=2)
    tmp.append(str.charAt(i));
  String ret = new String(tmp);
  return ret;

Answer 1

I would use StringBuilder, not StringBuffer. StringBuffer is for multithreaded situations, and is therefore slower than StringBuilder, as it doesn't synchronize. I tested the four basic ways of doing this listed by various answers in this thread. Notice, though, the certain things I always do here; these should be the things your interviewer is really looking for:

• I never use String += nextCharacter; as it is much, much slower than using a StringBuilder.
• I set the initialCapacity because doing that is always faster. If you don't, if the StringBuilder gets full, it has to reallocate a new array and copy over, which is slow.

And the code:

import com.google.caliper.Runner;
import com.google.caliper.SimpleBenchmark;

import java.text.CharacterIterator;
import java.text.StringCharacterIterator;
import java.util.Random;

public class EveryOtherTest {
    public static class StringBenchmark extends SimpleBenchmark {
        private String input;

        protected void setUp() {
            Random r = new Random();
            int length = r.nextInt(1000) + 1000;
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < length; i++) {
                sb.append((char) ('A' + r.nextInt(26)));
            }
            input = sb.toString();
        }

        public String timeCharArrayForeach(int reps) {
            String output = "";
            Random r = new Random();
            for (int i = 0; i < reps; i++) {
                StringBuilder sb = new StringBuilder(input.length() / 2 + 1);
                boolean use = false;
                for (char c : input.toCharArray()) {
                    if(use) sb.append(c);
                    use = !use;
                }
                String newOutput = sb.toString();
                if (r.nextBoolean()) output = newOutput; // Trick the JIT
            }

            return output;
        }

        public String timeCharArrayPlusTwo(int reps) {
            String output = "";
            Random r = new Random();
            for (int i = 0; i < reps; i++) {
                StringBuilder sb = new StringBuilder(input.length() / 2 + 1);
                char[] charArray = input.toCharArray();
                for(int j = 0; j < input.length(); j += 2) {
                    sb.append(charArray[j]);
                }
                String newOutput = sb.toString();
                if (r.nextBoolean()) output = newOutput; // Trick the JIT
            }

            return output;
        }

        public String timeCharAt(int reps) {
            String output = "";
            Random r = new Random();
            for (int i = 0; i < reps; i++) {
                StringBuilder tmp = new StringBuilder(input.length() / 2 + 1);
                for (int j = 0; j < input.length(); j += 2) {
                    tmp.append(input.charAt(j));
                }
                String newOutput = tmp.toString();
                if (r.nextBoolean()) output = newOutput; // Trick the JIT
            }

            return output;
        }

        public String timeIterator(int reps) {
            String output = "";
            Random r  = new Random();
            for(int i = 0; i < reps; i++) {
                StringBuilder buf = new StringBuilder(input.length() / 2 + 1);
                StringCharacterIterator iterator = new StringCharacterIterator(input);
                for (char c = iterator.first(); c != CharacterIterator.DONE; c = iterator.next()) {
                    buf.append(c);
                    iterator.next();
                }
                String newOutput = buf.toString();
                if (r.nextBoolean()) output = newOutput; // Trick the JIT
            }

            return output;
        }

        public String timeRegex(int reps) {
            String output = "";
            Random r  = new Random();
            for(int i = 0; i < reps; i++) {
                String newOutput = input.replaceAll("(?<!^).(.)", "$1");
                if (r.nextBoolean()) output = newOutput; // Trick the JIT
            }

            return output;
        }
    }

    public static void main(String... args) {
        Runner.main(StringBenchmark.class, args);
    }
}

Results:

 0% Scenario{vm=java, trial=0, benchmark=CharArrayForeach} 2805.55 ns; ?=688.96 ns @ 10 trials
20% Scenario{vm=java, trial=0, benchmark=CharArrayPlusTwo} 3428.48 ns; ?=475.32 ns @ 10 trials
40% Scenario{vm=java, trial=0, benchmark=CharAt} 2138.68 ns; ?=379.44 ns @ 10 trials
60% Scenario{vm=java, trial=0, benchmark=Iterator} 3963.94 ns; ?=389.53 ns @ 10 trials
80% Scenario{vm=java, trial=0, benchmark=Regex} 58743.66 ns; ?=10850.33 ns @ 10 trials

       benchmark    us linear runtime
CharArrayForeach  2.81 =
CharArrayPlusTwo  3.43 =
          CharAt  2.14 =
        Iterator  3.96 ==
           Regex 58.74 ==============================

vm: java
trial: 0

Answer 2

There is a StringCharacterIterator class if you prefer an iterator approach.

Answer 3

You could use this regex equivalent

String newString = str.replaceAll("(?<!^).(.)", "$1");

Answer 4

What you're doing works as far as I can tell. Here's a simple test case:

package com.sandbox;

import org.junit.Test;

import static org.junit.Assert.assertEquals;

public class SandboxTest {

    @Test
    public void testMocking() {
        assertEquals("foo", stringBits("f1o2o3"));
    }

    public String stringBits(String str) {
        StringBuffer tmp = new StringBuffer();
        for (int i = 0; i < str.length(); i += 2) {
            tmp.append(str.charAt(i));
        }
        String ret = new String(tmp);
        return ret;
    }
}

I think that's a pretty straight forward way of doing it. There's probably a way to do it with a regex and a group, but I have a feeling your current code would be easier to read.

---

I'd never heard of the StringCharacterIterator until I saw @Joey's answer, but it looks like an interesting solution. Here's the code using his answer:

package com.sandbox;

import org.junit.Test;

import java.text.CharacterIterator;
import java.text.StringCharacterIterator;

import static org.junit.Assert.assertEquals;

public class SandboxTest {

    @Test
    public void testMocking() {
        assertEquals("foo", stringBits("f1o2o3"));
    }

    public String stringBits(String str) {
        StringBuilder buf = new StringBuilder();
        StringCharacterIterator iterator = new StringCharacterIterator(str);
        for (char c = iterator.first(); c != CharacterIterator.DONE; c = iterator.next()) {
            buf.append(c);
            iterator.next();
        }
        return buf.toString();
    }
}

Answer 5

No. And this is not awkward at all. For every useful task, there might be a more suitable way, but in this case you have no choice but to iterate over the string.

Answer 6

I believe this should work too, and looks more simple to me.

public String stringBits(String str) {
    String tmp = "";
    for(int i = 0; i<str.length(); i+=2)
        tmp+=str.charAt(i);
    return tmp;

I edit to say that i should be equal to 1 if you want the second, fourth, six, ... characters.

Answer 7

You could turn the string into a CharArray and use a for-each loop:

for (char c: str.toCharArray()){
}

Of course then you would probably need a counter or a flag in order to get every other character, so it's probably not less awkward.