如何对字符串中的字母进行计数和排序

我想对用户输入的字母进行排序，并打印出用户输入的字符串中每个字母的数量。这就是我到目前为止所拥有的，我想知道这是否是正确的方法。我对java比较陌生，所以请让事情尽可能简单。根据我使用循环而不是大量 if else 构造的建议，我对我的代码做了一些调整。这就是我所拥有的：

public class Assignment9
{
public static void main( String [] args )
{
    String user_string = Input.getString( "Please enter a string" );
    int length = user_string.length();
    int char_number = 1;
    int alphabet[] = new int[26];
    for( int repeats = 0 , repeats <= length , repeats++ )
    {
        char letter = user_string.charAt( char_number );
        char to_be_tested = Character.toLowerCase( letter );
        int subscript = 0;
        for(int letter_number = 97 , letter_number <= 122 , letter_number++ )
        {
            char tester = (char) letter_number;

            if( to_be_tested == tester )
            {
                alphabet[subscript]++;
                subscript++;
            }
        }
        char_number++;
    }
    display( alphabet );
}
public static void display( int alphabet[] )
{
    int letter = 65;
    for( int a = 0; a < alphabet.length; a++ )
    {
        char character = ( char )letter;
        System.out.println ( "letter " + character + " count is " + alphabet[a] );
        letter++;
    }
}
}

我收到这些错误

编译错误 https://ideone.com/rlwDOH:

test.java:9: error: ';' expected
            for( int repeats = 0 , repeats <= length , repeats++ )
                                          ^
test.java:9: error: illegal start of expression
            for( int repeats = 0 , repeats <= length , repeats++ )
                                           ^
test.java:9: error: ';' expected
            for( int repeats = 0 , repeats <= length , repeats++ )
                                             ^
test.java:9: error: illegal start of expression
            for( int repeats = 0 , repeats <= length , repeats++ )
                                                     ^
test.java:9: error: ')' expected
            for( int repeats = 0 , repeats <= length , repeats++ )
                                                      ^
test.java:9: error: illegal start of expression
            for( int repeats = 0 , repeats <= length , repeats++ )
                                                                 ^
test.java:14: error: ';' expected
                    for(int letter_number = 97 , letter_number <= 122 , letter_number++ )
                                                              ^
test.java:14: error: illegal start of expression
                    for(int letter_number = 97 , letter_number <= 122 , letter_number++ )
                                                               ^
test.java:14: error: ';' expected
                    for(int letter_number = 97 , letter_number <= 122 , letter_number++ )
                                                                 ^
test.java:14: error: illegal start of expression
                    for(int letter_number = 97 , letter_number <= 122 , letter_number++ )
                                                                      ^
test.java:14: error: ')' expected
                    for(int letter_number = 97 , letter_number <= 122 , letter_number++ )
                                                                       ^
test.java:14: error: illegal start of expression
                    for(int letter_number = 97 , letter_number <= 122 , letter_number++ )
                                                                                        ^
12 errors

这是一种忽略大小写的更简单的字母计数方法：

final int[] chars = new int[26];
for (char c : value.toLowerCase().toCharArray()) {
  if ((c >= 'a') && (c<= 'z')) {
     chars[c - 'a']++;
  }
}

Demo: https://repl.it/CmYR/0 https://repl.it/CmYR/0