JSF array nullPointerException

Question

I am new in working with JSF and i am trying to write a simple code for a training center

In this JSF code, the subjects array is working fine when i load page but when i submit page, it gives me a null pointer exception , i do not know why this happen.

**The Java Bean Code :**
===============================================
College.java:
===============================================
package Training_Center;

import javax.faces.bean.ManagedBean;

@ManagedBean(name = "College")

public class College {

    private int college;   // College Number HardCoded from College.xhtml page(Drop Down List)
    private String special[] = new String[4]; //specialization array (Radio Button)
    private String specialTicked;  //Variable to store the element value from special[] array

    private String subjects[] = new String[4];//Subjects[] array ( Check Boxes )

    public College() {

    }

    public int getCollege() {
        return college;
    }

    public void setCollege(int college) {
        this.college = college;
    }

    public String[] getSpecial() {

        if (getCollege() == 0) {  //  AOU Unoversity
            special[0] = "AAAA";
            special[1] = "IT";
            special[2] = "BBBB";
            special[3] = "CCCC";

        }
        return special;
    }

    public String getSpecialTicked() {
        return specialTicked;
    }

    public void setSpecialTicked(String specialTicked) {
        this.specialTicked = specialTicked;

    }

    public String[] getSubjects() {
        if ("IT".equals(specialTicked)) { 
            subjects[0] = "M105 Introduction To Java";
            subjects[1] = "M257 Java OOP";
            subjects[2] = "M275 PHP";
            subjects[3] = "M180 Data Structure";
        }

        return subjects;
    }


    public void setSubjects(String[] subjects) {
        this.subjects = subjects;
    }

}
=======================================================
College.xhtml page:
========================================================
        <h:form >
            <h:selectOneMenu value="#{College.college}" > 
                <f:selectItem itemValue="-1" itemLabel="...Select..." />
                <f:selectItem itemValue="0" itemLabel="AOU University" />
            </h:selectOneMenu> 
            <h:commandButton value="Show Subjects" action="ShowSubjects" />

        </h:form>
=====================================================
ShowSubjects.xhtml page :
=====================================================
        <h:form>
            <h:selectOneRadio  value="#{College.specialTicked}" layout="pageDirection" >
                <f:selectItems value="#{College.special}" ></f:selectItems>            
            </h:selectOneRadio>  

            <h:commandButton value="Submit" action="subjects" />
        </h:form>
========================================================
subjects..xhtml page : ( this page gives the exception when i click submit button)
======================================================

    <h:form>
        <h:selectManyCheckbox  value="#{College.subjects}" layout="pageDirection" >
            <f:selectItems value="#{College.subjects}" ></f:selectItems>            
        </h:selectManyCheckbox> 
        <h:commandButton value="Submit"/>
    </h:form>


Thanks