diff --git a/CharAsciiFilter.java b/CharAsciiFilter.java
new file mode 100644
index 0000000..9c455dc
--- /dev/null
+++ b/CharAsciiFilter.java
@@ -0,0 +1,33 @@
+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);
+    }
+
+}