<apex:page controller="Item_Master_Controller" sidebar="false" showHeader="true">
        <apex:sectionHeader title="Item Master" subtitle="" description="" ></apex:sectionHeader>
        <apex:form >
            <apex:pageBlock title="">
                <apex:outputPanel id="out1">
                   <apex:actionstatus id="status1" startText="starting...">
                        <apex:facet name="stop">
                                <apex:outputPanel >
                                        <h1>
                                        <apex:outputText style="font-style:bold" value="{!message}"></apex:outputText>
                                        <p/>
                                        </h1>
                                </apex:outputPanel>
                        </apex:facet>
                    </apex:actionstatus>
                </apex:outputPanel>
                <p/>
                
                    <h1><apex:outputText style="font-style:bold" value="Select the Category"></apex:outputText></h1>                    
                    
                    <apex:selectList value="{!selectedCategory}" multiselect="false" size="1">
                            <apex:selectOptions value="{!category}"/>
                            <apex:actionSupport event="onchange" rerender="changeSubCategory"/>
                    </apex:selectList><p/>
                   <apex:outputPanel id="changeSubCategory">
                        <h1><apex:outputText style="font-style:bold" value="Select the Sub Category"></apex:outputText></h1>                    
                        <apex:selectList value="{!selectedSubCategory}" multiselect="false" size="1">
                            <apex:selectOptions value="{!subCategory}"/>
                        </apex:selectList><p/>
                   </apex:outputPanel>     
              
                
                <apex:pageblockButtons >
                    <apex:commandButton value="Submit" action="{!save}" rerender="out1" status="status1"></apex:commandButton>
                </apex:pageblockButtons>  
              
              </apex:pageBlock> 
        </apex:form>
</apex:page>

public class Item_Master_Controller{
  
        String[] selectedCategory= new String[]{};
        String[] selectedSubCategory= new String[]{};
        String message=null;
        String itemName=null;
        String itemDesc=null;
        String upc=null;
        Integer itemNumber;
        
        List<Category_Master__c> category;  
        List<Sub_Category_Master__c> subCategory;         
               
        public PageReference save() {
            Boolean flag=true;
            //this.setMessage(String.valueOf(selectedCategory.size()));
            this.setMessage('in save method...');
            
            return null;
        }
        
        public void setMessage(String message){
            this.message=message;
        }
        public String getMessage(){
            return this.message;
        }
                
        public List<SelectOption> getCategory() {
                List<SelectOption> options = new List<SelectOption>();
                Integer i=0;
                for(Category_Master__c category : [select Category_Id__c,Category_Name__c from Category_Master__c])
                {
                    options.add(new SelectOption(String.valueOf(category.Category_Id__c),category.Category_Name__c));
                }
                return options;
        }
        public List<SelectOption> getSubCategory() {
                List<SelectOption> options = new List<SelectOption>();
                Integer catId=-1;
                if(selectedCategory.size()>=1){
                    catId=Integer.valueOf(selectedCategory[0]);
                }
                Integer i=0;
                for(Sub_Category_Master__c subCategory : [select Sub_Category_Id__c,Sub_Category_Name__c from Sub_Category_Master__c where Category_Id__c =: catId])
                {
                    i++;
                    options.add(new SelectOption(String.valueOf(subCategory.Sub_Category_Id__c),subCategory.Sub_Category_Name__c));
                }
                if(i==0){
                    options.add(new SelectOption('Zero',String.valueOf(catId)));
                }else{
                    options.add(new SelectOption('Not Zero',String.valueOf(catId)));
                }
                return options;
        }
        public String[] getSelectedCategory() {
                return selectedCategory;
        }
  
        public void setSelectedCategory(String[] selectedCategory) {
                this.selectedCategory= selectedCategory;
        }
        
        public String[] getSelectedSubCategory() {
                return selectedSubCategory;
        }
  
        public void setSelectedSubCategory(String[] selectedSubCategory) {
                this.selectedSubCategory= selectedSubCategory;
        }
      /*
        public Boolean isPresent(String agentId,String ruleId) {
            Boolean flag=false;
            for(AgentRule__c agRule : [select id from AgentRule__c where LP_Agent_Id__c = : agentId and LP_Rule_Id__c =: ruleId])
            {
                flag=true;
            }
            return flag;
        }
        */
  }