java小技巧-生成重復的字符

 今天碰到個需求，根據字段個數，動態生成sql的占位符，如下：

    public static void main(String[] args) {
        System.out.println(String.join(",",Collections.nCopies(10,"?")));
    }

顯示結果：

來個實際的例子：

public class Test {

    public static void main(String[] args) {
        String tableName = "user";
        String fieldNames = "name,age,weight";
        
        System.out.println(getInsertSqlString(tableName,fieldNames));
    }
    
    private static String getInsertSqlString(String tableName,String fieldNames){
        int size = fieldNames.split(",").length;
        StringBuffer sb = new StringBuffer("");
        sb.append("insert into ").append(tableName).append("(").append(fieldNames).append(")")
        .append("values(").append(String.join(",",Collections.nCopies(size,"?"))).append(");");
        return sb.toString();
    }

}

輸入參數tableName, fieldNames, 顯示如下：