I want a class that I can create instances of with one variable unset (the id
), then initialise this variable later, and have it immutable after initialisation. Effectively, I'd like a final
variable that I can initialise outside of the constructor.
Currently, I'm improvising this with a setter that throws an Exception
as follows:
public class Example {
private long id = 0;
// Constructors and other variables and methods deleted for clarity
public long getId() {
return id;
}
public void setId(long id) throws Exception {
if ( this.id == 0 ) {
this.id = id;
} else {
throw new Exception("Can't change id once set");
}
}
}
Is this a good way of going about what I'm trying to do? I feel like I should be able to set something as immutable after it's initialised, or that there is a pattern I can use to make this more elegant.
Let me suggest you a little bit more elegant decision. First variant (without throwing an exception):
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;
}
}
Second variant (with throwing an exception):
public void setId(long id) {
this.id = this.id == null ? id : throw_();
}
public int throw_() {
throw new RuntimeException("id is already set");
}