how to cover Id!=null part in test Class

Question

public class democlass{

public string invoiceid{get;set;}

PUBLIC DEMOCLASS(apexpages.standardcontroller controller){

invoiceid=apexpages.currentpage().getparameters().get('id');
**if(invoiceid!=null)
 {
  invoice=[select id,name,net__c,Policy_Status__c,Amount__c,Period__c from invoice_policy__c where id=:invoiceid];

//some code here
}**else{
  invoice=new invoice_policy__C();
}
}
public pagereference saveIn(){
    newinvoicepolicy=new invoice_policy__c();
   **if(invoiceid!=null){
//some code
}**else{
insert newinvoicepolicy;
  p=new Pagereference('/'+newinvoicepolicy.Id);
}
p.setredirect(true);
return p;
}

}

My Test class is

public static testmethod void policyTest(){

        Invoice_policy__c newinvoice = new invoice_policy__c(Name='Invoice',internal_alerts__c=true,American_express__c = true);
        insert newinvoice;
apexpages.currentpage().getparameters().put('invoiceid',newinvoice.id);
apexpages.standardcontroller controller;
        democlass policy = new democlass(controller);
        policy.saveIn();
}

Here I can't cover the invoiceid!=null part.wat should I do to achieve it

Answer 1

Add the below code before calling initializing the demoClass.

Apexpages.standardcontroller controller = new Apexpages.standardcontroller(newinvoice); democlass policy = new democlass(controller); policy.saveIn();

Answer 2

You have a mistake in your test class

apexpages.currentpage().getparameters().put('invoiceid',newinvoice.id);  

It needs to be

apexpages.currentpage().getparameters().put('id',newinvoice.id);