Salesforce::Override save method of Opportunity

Apex Class
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public class OpportunityEditControllerExtension {

public String status;
public String getStatus() {return status;}

public opportunity oppo;
private final ApexPages.StandardController controller;

//initializes the private member variables
public OpportunityEditControllerExtension(ApexPages.StandardController stdController)
{
controller= stdController;
this.oppo= (opportunity)controller.getRecord();
}


public PageReference mysave()
{
try{
update oppo;
}
catch(DmlException ex){
ApexPages.addMessages(ex);
}
status='Updated...';
//PageReference sc = new ApexPages.StandardController(oppo).view();
//return sc;
return null;
}
}
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Apex Page
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<apex:page standardcontroller="opportunity" extensions="OpportunityEditControllerExtension" >
<apex:form >
<apex:pageblock >
<apex:pageBlockButtons >
<apex:commandbutton value="Save" action="{!mysave}" rerender="resultsPanel" status="status"/>

<apex:inputfield value="{!opportunity.name}"/>
<apex:inputfield value="{!opportunity.amount}"/>
<apex:inputfield value="{!opportunity.closedate}"/>

<apex:actionStatus id="status" startText="Fetching map..."/>
<apex:outputPanel id="resultsPanel">