fix code

hutool-captcha/src/main/java/cn/hutool/captcha/generator/AbstractGenerator.java

@@ -13,9 +13,9 @@ public abstract class AbstractGenerator implements CodeGenerator {
 	private static final long serialVersionUID = 8685744597154953479L;
 	
 	/** 基础字符集合，用于随机获取字符串的字符集合 */
-	protected String baseStr;
+	protected final String baseStr;
 	/** 验证码长度 */
-	protected int length;
+	protected final int length;
 	
 	/**
 	 * 构造，使用字母+数字做为基础

hutool-captcha/src/main/java/cn/hutool/captcha/generator/MathGenerator.java

@@ -42,12 +42,11 @@ public class MathGenerator implements CodeGenerator {
 		number1 = StrUtil.padAfter(number1, this.numberLength, CharUtil.SPACE);
 		number2 = StrUtil.padAfter(number2, this.numberLength, CharUtil.SPACE);
 
-		final String code = StrUtil.builder()//
+		return StrUtil.builder()//
 				.append(number1)//
 				.append(RandomUtil.randomChar(operators))//
 				.append(number2)//
 				.append('=').toString();
-		return code;
 	}
 
 	@Override