Here is a guide to convert decimal to binary in detail.Try this.
Apply the same theory for octal (use 8 to divide decimal numbers) and hexadecimal (use 16).Or

Once you have binary number say 10011011 its really easy to convert it into any form.
Say octal pair three digits from right to left
(when short of digits use 0's):
010 011 011
2 3 3

Say hex pair four digits right to left :
(when short of digits use 0's)
1001 1011
9 B

^ the method to convert binary will only work on bases with a power of 2... Works for octal, hex, and base-64 (armored ascii)... won't work for decimal.

but other than the details, your methods are correct. :)

public class StudentListings{
private String name; // key field
private int ID;
private double GPA;
private int next;
private int size;
private StudentListing[] data;
public StudentListings(){
this.name= name;
this.id= ID;
this.gpa=GPA;
}
public StudentListings(){
next=0;
data= new StudentListings[Size];
size= Size;
} // end of constructor
...