getting a null pointing exception in eclipse java -

when try call addgrade method in student class, uses addgrade in grades class nullpointerexception. says have add more details i'm not sure why.

class 1 is

public class grades { private double qualpts; private int numcreds;  public grades() {  }    /**  * returns gpa  */ public double getgpa() {     if (numcreds>0)     return qualpts/numcreds;     return 0; }  /**  * adds new grade's credits  * weights newly added grade  */ public void addgrade(int creds, double grade) {  numcreds+=creds;  qualpts+=creds*grade; } /**  * returns number of credits  */ public int getnumcred() {     return numcreds; }  } 

class 2 is

public class student  { private string name; private string bnumber; private grades grades;  public student(string name, string bnumber) { this.name=name; this.bnumber=bnumber; }    /**  * adds new grade's credits  * weights new grade  */ public void addgrade(int creds, double grade) {     grades.addgrade(creds,grade);  }    

}

in constructor in student class, need create instance of grades.

public student(string name, string bnumber) { this.name=name; this.bnumber=bnumber; this.grades = new grades(); }  

otherwise this.grades null, because never set value.