思路
1、把字符串转成字符数组
2、遍历字符数组
3、判断每个字符

字符串

String str = "a b cA B CD<>?</";

转成字符数组

char[] chars = str.toCharArray();

定义四个int类型的计数器

//统计a-z的个数
int azCount = 0;

//统计A-Z的个数
int AZCount = 0;

//统计其他字符的个数
int charCount = 0;

//统计空格的个数
int empty = 0;

遍历字符数组并判断

for (char aChar : chars) {

	if ('a' <= aChar && aChar <= 'z'){
         azCount++;
         
    }else if ('A' <= aChar && aChar <= 'Z'){
         AZCount++;
         
    }else if (aChar == ' '){
         empty++;
         
    }else {
         charCount++;
    }
}

输出

String str = "a b cA B CD<>?</";

System.out.println("a-z："+azCount);
System.out.println("A-Z："+AZCount);
System.out.println("空格："+empty);
System.out.println("其他字符："+charCount);

a-z：3
A-Z：4
空格：4
其他字符：5

源代码

public class Test {

    public static void main(String[] args) {

        String str = "a b cA B CD<>?</";

        int azCount = 0;
        int AZCount = 0;
        int charCount = 0;
        int empty = 0;

        char[] chars = str.toCharArray();

        for (char aChar : chars) {

            if ('a' <= aChar && aChar <= 'z'){
                azCount++;
            }else if ('A' <= aChar && aChar <= 'Z'){
                AZCount++;
            }else if (aChar == ' '){
                empty++;
            }else {
                charCount++;
            }
        }
        System.out.println("a-z："+azCount);
        System.out.println("A-Z："+AZCount);
        System.out.println("空格："+empty);
        System.out.println("其他字符："+charCount);
    }
}