Use of "this.somevariable = somevariable;

Question

GEEZ! Everytime the light goes on for one thing, it's something else!
I wrote two classes one with a constructor method and one class with a call to that method.
Student student = new Student("9999", 4.0);
student.printInfo();
was the call, and among other things
public Student(String studentid, double gpa)
this.studentid = studentid;
this.gpa = gpa;
public void printInfo()
System.out.println("The student's ID is " + studentid + "." );
System.out.println("The student's GPA is " + gpa + "." );
I tried to say:
this.studentid = id;
with "id" in the print statement.
Why didn't Java like that? Kept setting to null. I thought I could rename a variable anything I wanted
in a method...
Brad

Answer 1

I'm not entirely sure what you mean by "rename this", but I assume you mean you want the instance variable to have a different name to the parameter in the constructor, so that you can then use "id" instead of "studentid" throughout the rest of the class?
If so, then the code below demonstrates the error.
Inside the constructor, "this.studentid" refers to the instance variable named studentid (your "private String studentid"), and "studentid" refers to the parameter passed in to the constructor. The explicit "this" is needed to differentiate, because there are two variables in scope that have the same name. When you use "id" in the constructor, it refers to the instance variable named id (your "public String id"); there is no need for an explicit "this" qualifier, since there is only one accessible variable named id.
Thus the code "this.studentid = id" says "set the value of the studentid instance variable to be the same as the value of the id instance variable". But id has not been explicitely initialised, so its value is null.
To get the effect that I think you are after, just swap the statement around to "this.id = studentid", which says "set the value of the id instance variable to be the same as the value of the studentid parameter passed to the constructor" (note that the explicit "this" is no longer necessary).
The reason that you managed to get away with a compiler error is that you have an instance variable named id and one named studentid. Based on your code I would suggest that you only need one or the other, so I've deleted studentid from the code below; Now, if you try to compile it with the assignment statement back the other way around, as "this.studentid = id", you will get a compiler error, since "this.studentid" has not been declared.
class Student {
    private double hours;
    private double points;
    private double gpa;
    public String id;
    public Student(String studentid, double gpa) {
        this.id = studentid;
        this.gpa = gpa;
    public void printInfo() {
        System.out.println("The student's ID is " + id + ".");
        System.out.println("The student's GPA is " + gpa + ".");
}Hope this helps,
Matt.