Account picklist ,by clicking on account it displays the related contacts

code

apex page:---

<apex:page controller="picacc_con_cls" >
                    <apex:form >
                       <apex:pageBlock title="Account Name">
                               Account Name&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
                                   <apex:selectList value="{!selectedAccId}" size="1">
                                      
                                               <apex:selectOptions value="{!AccountNames}" />
                                         <apex:actionSupport event="onchange" reRender="conlist"/>

                           </apex:selectList>  <br/>  <br/> 
           
            <apex:outputPanel id="conlist"> 
            <apex:outputPanel rendered="{!ContactNames!=null && ContactNames.size>0}">
                <apex:outputLabel value="Related Contacts:"/>
               <apex:pageBlockTable value="{!ContactNames}" var="cons">
             <apex:column value="{!cons.Name}"/>
                <apex:column value="{!cons.Email}"/>
                 <apex:column value="{!cons.MobilePhone}"/>
             
             </apex:pageBlockTable>         
                     
             </apex:outputPanel>   
            </apex:outputPanel>
      </apex:pageBlock> 
            </apex:form>
</apex:page>



Apex Controller:

public class picacc_con_cls {
  
public String selectedAccId{get;set;}
public String selectedConId{get;set;}
   
      public picacc_con_cls()
    {
      
    }
public List<SelectOption> getAccountNames() {
List<SelectOption> accOptions= new List<SelectOption>();
       accOptions.add( new SelectOption('','--Select--'));
        for( Account acc : [select Id,name from Account ] ) {
              accOptions.add( new SelectOption(acc.Id,acc.name)); /*SelectOption list takes two parameters one is value and other               one is label .In this case account name as a label and Id is the value .*/
        }
      return accOptions;
    }
    public List<Contact> getContactNames() {
      if(selectedAccId!=Null && selectedAccId!=''){
        
		    	List<Contact> conlist = new List<contact>();
		    	conlist = [select id,Name,Email,MobilePhone,Accountid from contact where Accountid=:selectedAccId];
        	
    	  	return conlist; 
             
     }
    return Null;
   
  }


}