Why am i getting this Visualforce page error in my custom controller?

Question

What I am trying to do is make a page where if i click on button then all accounts with type "prospect" to be shown. below is all my code :

Controller Class

Public class q4{

List<account> acts;

public list<account> getacts(){

return null;
}


public pagereference type_prospect(){
acts = [Select name from account where type = 'Prospect'];
return null;
}

}

VF page code

<apex:page Controller="q4" >

     <apex:form >
     <apex:pageBlock title="Opportunity Record Display">
                <apex:commandButton value="type_prospect" action="{ ! type_prospect }"/>
                  <apex:pageblockSection>

                        <apex:pageBlockTable value="{! acts }" var="acc" title="Account table">

                            <apex:column value="{! acc.type }"/>

                            <apex:column value="{! acc.Name }"/>

                            <apex:column value="{! acc.Industry }"/>


                        </apex:pageBlockTable>


                  </apex:pageblockSection>        

      </apex:pageBlock>

    </apex:form>

</apex:page>

The page first get loaded up with "Opportunity Record Display" title and below the "type_prospect" button.

when i click on the button i expect it to acts to be filled with soql query and then table display the values of acts in each row.

but on clicking it shows

Visualforce Error:

Formula Expression is required on the action attributes.

Answer 1

try this

public with sharing class q4
{ 
    public list<account> acts {get;set;}

    public pagereference type_prospect(){
    acts = [Select name,Type,Industry  from account where type = 'Prospect'];
    return null;
    }  
}

Page

<apex:page Controller="q4" >

     <apex:form >
     <apex:pageBlock title="Opportunity Record Display">
                <apex:commandButton value="type_prospect" action="{!type_prospect}" rerender="accountBlockID"/>
                  <apex:pageblockSection id="accountBlockID">

                        <apex:pageBlockTable value="{! acts }" var="acc" title="Account table">

                            <apex:column value="{! acc.type }"/>

                            <apex:column value="{! acc.Name }"/>

                            <apex:column value="{! acc.Industry }"/>


                        </apex:pageBlockTable>


                  </apex:pageblockSection>        

      </apex:pageBlock>

    </apex:form>

</apex:page>

Answer 2

Please try with the below code where i have removed the getter method and included get set in same line.

Public class q4{

List<account> acts{get;set;}

public pagereference type_prospect(){
acts = [Select name from account where type = 'Prospect'];
return null;
}

}

Answer 3

I think you need to remove the space between the { and the !.

Try rewriting the action as:

action="{!type_prospect}"/>

Nitish has a good point too - my code should fix your immediate problem, but I think that you won't get your list of accounts until you implement the getter/setter combo as suggested by Nitish.