1. DMLException:

DML Exceptions are exceptions that occur whenever a DML operation fails. This may happen due to many reasons, the most common one is inserting a record without a required field.

Ex: try

{

    Position__c pos = new Position__c();

    insert pos;

} 

catch(DmlException e)

{

System.debug(‘The following exception has occurred: ‘ + e.getMessage()); 

}

2. ListException:

ListException catches any type of run time error with a list. This list can be of any data type such as integer, string, or sObject.

Ex: try

{

    List<String> stringList = new List<String>();

    stringList.add(‘Bhavna’);

    // This list contains only one element,

    // but we will attempt to access the second element

    // from this zero-based list.

    String str1 = stringList[0]; //this will execute fine

    String str2 = stringList[1]; // Causes a ListException

}

catch(ListException le)

{

System.debug(‘The following exception has occurred: ‘ + le.getMessage());

}

3. NullPointerException:

NullPointer Exception catches exceptions that occur when we try to reference a null variable. Use this exception whenever you are referencing a variable that might turn out to be null.

Ex: try

{

String stringVariable;

Boolean boolVariable = stringVariable.contains(‘John’);

// Causes a NullPointerException

}

catch(NullPointerException npe)

{

System.debug(‘The following exception has occurred: ‘ + npe.getMessage());

}

4. QueryException:

QueryException catches any run time errors with SOQL queries. QueryException occurs when there is a problem in SOQL queries such as assigning a query that returns no records or more than one record to a single sObject variable.

Ex: try {

    // This statement doesn’t cause an exception,

    // if we don’t have a problem in SOQL Queries.

    // The list will just be empty.

List<Position__c> positionList = [SELECT Name FROM Position__c WHERE Name=’Salesforce Developer’];

    // positionList.size() is 0 

    System.debug(positionList.size());

// However, this statement causes a QueryException because 

// we’re assiging the query result to a Position sObject varaible

       // but no Position record is found

Position__c pos = [SELECT Name FROM Position__c WHERE Name=’Salesforce Developer’ LIMIT 1];

}

catch(QueryException ge) {

System.debug(‘The following exception has occurred: ‘ + ge.getMessage());    

}

5. Generic Exception:

This exception type can catch any type of exception; that’s why it’s called generic Exception type. This is used when you are not sure which exception type to use and what exception might occur in the code.

Ex: try {

List<Position__c> positionList = [SELECT Name FROM Position__c WHERE Name=’Salesforce Developer’];

     // positionList.size() is 0 

     System.debug(positionList.size());

Position__c pos = [SELECT Name FROM Position__c WHERE Name=’Salesforce Developer’ LIMIT 1];

} catch(Exception ex) {

System.debug(‘The following exception has occurred: ‘ + ex.getMessage());    

}

Common Exception Methods:

All Exception types are Exception classes and these classes have methods that can provide a lot of information about the exception and error that occurred.

1. getTypeName(): Returns the type of exception, such as DmlException, ListException, and QueryException. 
2. getMessage(): Returns the error message and displays for the user.
3. getCause(): Returns the cause of the exception as an exception object.
4. getLineNumber(): Returns the line number from where the exception was thrown.
5. getStackTraceString():Returns the stack trace as a string. 

Ex: try {

    Position__c pos = new Position();

    // Causes an QueryException because

    // we are inserting record without required fields

    insert pos;

} 

catch(Exception e) 

{

    System.debug(‘Exception type caught: ‘ + e.getTypeName());    

    System.debug(‘Message: ‘ + e.getMessage());    

    System.debug(‘Cause: ‘ + e.getCause());    // returns null

    System.debug(‘Line number: ‘ + e.getLineNumber());

}

USER_DEBUG|[6]|DEBUG|Exception type caught: System.QueryException

USER_DEBUG|[7]|DEBUG|Message: List has no rows for assignment to SObject

USER_DEBUG|[8]|DEBUG|Cause: null

USER_DEBUG|[9]|DEBUG|Line number: 2

USER_DEBUG|[10]|DEBUG|Stack trace: AnonymousBlock: line 2, column 1