我创建了一个jframe登录。它连接到sql。我遇到的问题是，如果我运行它并输入错误的凭证，else语句将执行3次。这是我的代码：

String username = txtUsername.getText();
String password = txtPassword.getText();
String sql = "select * from users where username ='"+username+"' and password='"+password+"'";
ResultSet res = st.executeQuery(sql);

for (int i=0; i<3;i++){
    if (res.next()){
        dispose();
        up.show(); 
    }
    else{
        txtUsername.setText("");
        txtPassword.setText("");
        JOptionPane.showMessageDialog(this, "Incorect inputs "+ i+ " attempts left");
    }
}