如何创建只能设置一次但在Java中不是最终变量的变量

让我建议您一点更优雅的决定。第一个变体（不引发异常）：

public class Example {    private Long id;    // Constructors and other variables and methods deleted for clarity    public long getId() {        return id;    }    public void setId(long id) {        this.id = this.id == null ? id : this.id;    }}

第二种变体（引发异常）：

     public void setId(long id)  {         this.id = this.id == null ? id : throw_();     }     public int throw_() {         throw new RuntimeException("id is already set");     }