Cross function of development team
In Scrum guide, a development team is described as cross-functional, but in my scrum team, a UI designer is impossible to write Java code, is it necessary for development team to be cross functional?
Have a re-read of the Scrum Guide and take note of the Developers' commitments. In your situation, are the Developers sufficiently cross-functional to ensure that their joint commitments are met?
In the Scrum Guide, the Scrum Team is described as cross-functional and, as a whole, "the members have all the skills necessary to create value each Sprint". It doesn't say that individual members are cross-functional.
I will add that it is a good idea to have redundancy across your skills. If you only have a single UI designer, for example, you run into risks around how to handle a large amount of work that requires this skillset or what happens if the person with these skill is not available. However, the Scrum framework doesn't call out skill redundancy like this specifically.
I like to imagine the Scrum Team as a space shuttle crew. As a crew, they need to have all of the skills necessary to complete their missions (and to keep the crew alive). This doesn't mean that everyone knows every job, but, as Thomas mentions, it is a good idea to have redundancy across the various skills should they run into any challenges along the way or find themselves down a crew member.
As the guys correctly mentioned above, it's the "Development Team" that should be cross-functional rather than the individual. But as @Thomas highlighted, in practical terms, it might be useful for the team to see whether they could benefit from sharing skills to improve redundancy, reduce risk and possibly reduce any bottlenecks that may exist in the flow of work through the Sprint.
In the Scrum guide, a development team is indeed described as cross-functional, meaning that it possesses all the necessary skills to deliver a potentially releasable product increment. However, it's important to note that the concept of cross-functionality doesn't imply that every team member needs to be proficient in every single skill required for the project.
Let's take an example of a ChatGPT integration project team. In such a team, cross-functionality might involve having members with expertise in different areas, such as software development, user interface (UI) design, natural language processing (NLP), and project management. Each team member brings their specialized skills to the table, contributing to the overall success of the project.
In the case of your Scrum team, where the UI designer finds it difficult to write Java code, it doesn't necessarily mean that the team lacks cross-functionality. The key aspect is whether the team collectively possesses all the essential skills to deliver a releasable product increment.