Java内置方法 - 在字符串中出现char

Question

Is there a java built-in method to count occurrences of a char in a string ? for example: s= "Hello My name is Joel" the number of occurrences of l is 3

Thanks

Answer 1

There is no such method, but you can do:

String s = "Hello My name is Joel";
int counter = 0;
for( int i=0; i<s.length(); i++ ) {
    if( s.charAt(i) == 'l' ) {
        counter++;
    } 
}

(code from Simple way to count character occurrences in a string )

Answer 2

If you want to count the number of times multiple characters have appeared in a particular string, then mapping of characters with their number of occurrences in the string will be a good option. Here's how one would achieve the solution in that case:

import java.util.HashMap;
import java.util.Map;

public class CharacterMapper
{
  private Map<Character, Integer> charCountMap;

  public CharacterMapper(String s)
  {
    initializeCharCountMap(s);
  }

  private void initializeCharCountMap(String s)
  {
    charCountMap = new HashMap<>();
    for (int i = 0; i < s.length(); i++)
    {
      char ch = s.charAt(i);
      if (!charCountMap.containsKey(ch))
      {
        charCountMap.put(ch, 1);
      }
      else
      {
        charCountMap.put(ch, charCountMap.get(ch) + 1);
      }
    }
  }

  public int getCountOf(char ch)
  {
    if (charCountMap.containsKey(ch))
      return charCountMap.get(ch);
    return 0;
  }

  public static void main(String[] args)
  {
    CharacterMapper ob = new CharacterMapper("Hello My name is Joel");
    System.out.println(ob.getCountOf('o')); // Prints 2
  }
}