如何大写Java中的每个字符串？

Question

I'm working on this java method trying to capitalize the nth word in a string and got stuck on not being able to return a value for retVal variable

class MyTesting
{
public static void main (String[] args) throws java.lang.Exception
{
    capitalizeEveryNthWord("this is a String", 3, 3);
}

// Take a single-spaced <sentence>, and capitalize every <n> word starting   with <offset>.

public static String capitalizeEveryNthWord(String sentence, Integer offset,     Integer n) {
String[] parts = sentence.split(" ");
String retVal = "";
for (int idx = 0; idx < offset; idx++)
{
    retVal.concat(parts[idx] + " ");
}
for (int idx = offset; idx < parts.length; idx++)
{
    if (idx - offset % n == 0)
    {
        retVal.concat(parts[idx] + "-");
    }
    else
    {
        retVal.concat(parts[idx] + " ");
    }
}
System.out.println(retVal);
return retVal;
}
}

Answer 1

concat() returns a value, it doesn't modify the string on which you're calling the method. You need to use it as retVal = retVal.concat(...) or simply retVal += ...

Answer 2

Java's String class is immutable. String.concat() will return the concatenation as a new String object.

You can either use retVal = retVal.concat(...) , or use a StringBuilder .

The following works:

class MyTesting
{
  public static void main (String[] args) throws java.lang.Exception
  {
    capitalizeEveryNthWord("this is a sentence that is being tested", 3, 3);
  }

  // Take a single-spaced <sentence>, and capitalize every <n> word starting   with <offset>.
  public static String capitalizeEveryNthWord(String sentence, Integer offset, Integer n) {
    String[] parts = sentence.split(" ");
    String retVal = "";
    for (int idx = 0; idx < offset; idx++)
    {
        retVal += parts[idx] + " ";
    }
    for (int idx = offset; idx < parts.length; idx++)
    {
        if ((idx - offset) % n == 0) // added parantheses
        {
            retVal += Character.toUpperCase(parts[idx].charAt(0)) + parts[idx].substring(1) + " "; // make the first character uppercase.
        }
        else
        {
            retVal += parts[idx] + " ";
        }
    }
    System.out.println(retVal);
    return retVal;
  }
}

A more efficient approach would be something like:

public static String capitalizeEveryNthWord(String sentence, Integer offset, Integer n) {
  StringBuilder sb = new StringBuilder(sentence);
  int wordIdx = 0;
  boolean newWord = true;
  for (int i = 0; i < sb.length(); i++) {
    char c = sb.charAt(i);
    if (c == ' ') {
      wordIdx++; // assumes single space between words.
      newWord = true;
    } else if (newWord) {
      if (wordIdx >= offset && (wordIdx - offset) % n == 0) {
        sb.setCharAt(i, Character.toUpperCase(c));
      }
      newWord = false;
    }
  }
  return sb.toString();
}

This second approach only allocates one buffer which is then modified in-place to capitalize words. The previous approach allocates new String objects with every call to += (this can occasionally be optimized away by compilers but it's not guaranteed, as far as I know).

Answer 3

使用toUpperCase()方法并使用返回值 ：

String retVal = retVal.concat(...).toUpperCase();