向量IndexOutOfBoundsException

Question

I am successfully adding the first student but when i add a second one i get
Exception in thread " main " java.lang.ArrayIndexOutOfBoundsException : Array index out of range: 11

at java.util.Vector.get(Unknown Source)  
    at business.StudentCollection.UseArray(StudentCollection.java:58  
    at business.Application.main(Application.java:30) 

Segments of code

 public class StudentCollection {  
private Vector<Student> collection;  
private int count;  

public StudentCollection ()
{  
collection=new Vector<Student>(10,2);  
count=0;  
for( int i=0;i< collection.capacity(); i++) //i read that object cannot be added to 
vectors if empty  
collection.add(i,new Student(0,"No Student",0));

}  

public void addStud(int ID,String name,int Credits)
   {    

for(int i=0;i< collection.capacity();i++)  
 if(collection.get(i)==null)  // new Error
collection.add(i,new Student(0,"No Student",0)); //making sure vector new index are   filled

collection.add(count,new Student(ID,name,Credits));  
count++;  

  }  
public Student UseArray(int x){  \\ Error here line 58
return collection.get(x);  

                      }

 public int getlengthh(){  
    return collection.capacity();  
                }  
}  
 public static void main (String [] args ){  
 StudentCollection C=new StudentCollection();  


        System.out.println("Enter Student's ID");  
        x=scan.nextInt();  
        for (int i=0;i< C.getlengthh();i++){    
if(C.UseArray(i).getID()==x){  // Error here
        System.out.println("A student with this ID already exists.Do you want to overwrite the existing student?yes/no");  
        scan.nextLine();  
        ans=scan.nextLine();  

        if (ans.equalsIgnoreCase("yes")){
            C.delete(x);
        continue;
        }
        else {
            System.out.println("Enter Student's ID");
        x=scan.nextInt();
        }
            }
        }

        System.out.println("Enter Student's name");
        Str=scan.nextLine();
        Str=scan.nextLine()+Str;
        System.out.println("Enter number of credits");
        y=scan.nextInt();
        C.addStud(x,Str,y);

    }

Answer 1

Modify to

 public Student UseArray(int x){  \\ Error here line 58
     if(collection.size() > x)
        return collection.get(x); 
     return null; 

    }

There is a difference between capacity and size. Capacity returns length of array created by Vector to hold the present and the incoming elements. While size is the number of elements already put into the vector. Having said that, While checking for existence of elements don't use capacity use size as below:

 public int getlengthh(){  
    return collection.size();  
                } 

Even if capacity is bigger than index still add can throw exception. See here

Answer 2

The whole point of collection classes like Vector is that you don't need to manually index into them like an array. You don't need to maintain a count variable, either -- when you want to know how many students you have, simply call size() on the Vector. Now, unless you need Vector's thread-safety, I would go with ArrayList instead, but they are both implementations of List, which means all you need to do is call add(Student) .

I would take a good look at the Java Collections Trail before continuing.

Also, on a stylistic note, clean up your source formatting. Having inconsistent indentation will make it very hard to review your code for bugs.