There will be scenarios when you want to throw an exception from your catch block, may be to propagate to the invoking class so it is handled properly.

Below is a Custom Exception class which can be used to throw different types of exceptions as needed.

public with sharing class CustomExceptions {

public class InputDataException extends Exception {}
public class RequiredException extends Exception {}
public class GovernorLimitException extends Exception {}
public class CommunityException extends Exception{}
public class RecordLockedException extends Exception{}
public class AuthorizationException extends Exception {}

}

From the Apex class: you can use the code below to throw custom exceptions

if( String.isEmpty(case.Analysis__c) ) {
throw new CustomExceptions.RequiredException(‘Analysis is missing from the Case’);
}
else {
throw new CustomExceptions.RequiredException(‘Cases can be assigned by Tier 1 or Admin Users’);
}

There must have been times when creating a map from a SOQL is required. I am showing 2 ways of doing the same.

1. Map from List ( obtained from SOQL)
   Map<Id, CustomObj__c> myMap = new Map<Id, CustomObj__c>();
   for(CustomObj__c objCS : [Select Id Name,Status__c From CustomObj__c ]){
   myMap.put(objCS.Id, objCS);                                                                                                      }
2. Map directly from SOQL

Map<Id, CustomObj__c> myMap = new Map<String, CustomObj__c>([Select   Id,Name,Status__c From CustomObj__c ]);

However, if you want to put any other field ( other than Id as key, you will have to iterate over the list and put field in the map like approach #1

Map<String, CustomObj__c> myMap = new Map<String, CustomObj__c>();
for(CustomObj__c objCS : [Select Id Name,Status__c From CustomObj__c ]){
myMap.put(objCS.Status__c, objCS);

Maps are pretty useful and can make your code more readable and efficient. In particular, maps can be instantiated in a number of ways.

Here is the standard way of instantiating a map:

Map<Id, Account> accountsById = new Map<Id, Account>();

However, if you happen to have a list of sObjects you can just pass that list in the constructor like this:

Map<Id, Account> accountsById = new Map<Id, Account>([SELECT Id, Name FROM Account LIMIT 10]);

It is easy to work with Maps in Triggers

As you can see, no loops were needed to get a set of account ids. We simply passed the list of accounts to the new map instance and then used the maps keySet() method to get a list of ids for us.

Map<Id, Account> accountsById = new Map<Id, Account>(trigger.new);
Set accountIds = accountsById.keySet();
List contacts = [SELECT Id, FirstName, LastName FROM Contact WHERE AccountId IN :accountIds];

If you have a list of sObjects you don’t have to loop through and get a set of ids. You could just pass that list of sObjects to your query like this (assuming you have a list of account records in a variable called List accounts.

[SELECT Id FROM Contact WHERE AccountId IN :accounts]

Apex is smart enough to know how to get the account ids from the list and to apply them as part of the WHERE clause.