public class CharAsciiFilter {

    // 用最少的内存过滤出小写字母
    static final String str = "Aa1b2c3d4e5f6f7";

    // 内存开销更低
    static void good() {
        char[] chars = str.toCharArray();
        int x = 0;
        for (int i = 0; i < chars.length; i++) {
            char c = chars[i];
            if (c >= 97 && c <= 122) {
                chars[x] = c;
                x = x + 1;
            }
        }
        System.out.println(new String(chars, 0, x));
    }

    // 常规解法
    static void general() {
        char[] chars = str.toCharArray();
        StringBuilder sb = new StringBuilder();
        for (char c : chars) {
            // Character.isLetter(c)
            if (c >= 97 && c <= 122) {
                sb.append(c);
            }
        }
        System.out.println(sb);
    }

}