添加字符串“补位”方法。

src/main/java/xyz/zhouxy/plusone/commons/util/StrUtil.java

@@ -0,0 +1,34 @@
+package xyz.zhouxy.plusone.commons.util;
+
+import java.util.Arrays;
+
+public class StrUtil {
+
+    public static String fillBefore(String src, int minLength, char c) {
+        if (src.length() >= minLength) {
+            return src;
+        }
+        char[] result = new char[minLength];
+        Arrays.fill(result, c);
+        for (int i = 1; i <= src.length(); i++) {
+            result[minLength - i] = src.charAt(src.length() - i);
+        }
+        return String.valueOf(result);
+    }
+
+    public static String fillAfter(String src, int length, char c) {
+        if (src.length() >= length) {
+            return src;
+        }
+        char[] result = new char[length];
+        Arrays.fill(result, c);
+        for (int i = 0; i < src.length(); i++) {
+            result[i] = src.charAt(i);
+        }
+        return String.valueOf(result);
+    }
+
+    private StrUtil() {
+        throw new IllegalStateException("Utility class");
+    }
+}

src/test/java/xyz/zhouxy/plusone/commons/util/StrUtilTests.java

@@ -0,0 +1,23 @@
+package xyz.zhouxy.plusone.commons.util;
+
+import org.junit.jupiter.api.Test;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+class StrUtilTests {
+
+    private static final Logger log = LoggerFactory.getLogger(StrUtilTests.class);
+
+    @Test
+    void testFillZero() {
+        char c = '=';
+        log.info(StrUtil.fillBefore("1234", 6, c));
+        log.info(StrUtil.fillBefore("12345", 6, c));
+        log.info(StrUtil.fillBefore("123456", 6, c));
+        log.info(StrUtil.fillBefore("1234567", 6, c));
+        log.info(StrUtil.fillAfter("1234", 6, c));
+        log.info(StrUtil.fillAfter("12345", 6, c));
+        log.info(StrUtil.fillAfter("123456", 6, c));
+        log.info(StrUtil.fillAfter("1234567", 6, c));
+    }
+}