How to compare a list with another list

Question

I'm trying to compare the picklist (multi-select) values to a List

below is my code:

List<selectOption> options = new List<selectOption>(); 
Schema.sObjectType sobject_type = Account.getSObjectType(); 
Schema.DescribeSObjectResult sobject_describe = sobject_type.getDescribe(); 
Map<String, Schema.SObjectField> field_map = sobject_describe.fields.getMap(); 
List<Schema.PicklistEntry> pick_list_values = field_map.get('Language__c').getDescribe().getPickListValues(); 

List<string> picklistValues = new List<string>();

for (Schema.PicklistEntry a : pick_list_values) { //for all values in the picklist list 
    system.debug(a.getLabel());
    picklistValues.add(a.getValue()); //getValue
}

//picklistValues.sort();

list<string> lang = new list<string>();
lang.add('English');
lang.add('French'); 

for(integer i = 0; i<picklistValues.size(); i++)
{
   if(picklistValues.contains(string.valueof(lang[i])))
   {
      system.debug('//matched:');
   }
}

getting the following error:

Method does not exist or incorrect signature: [List].contains(String)

Answer 1

You cannot do a contains on a List of strings, only a set.

You could do

Set<string> lang = new Set<string>();
lang.add('English');
lang.add('French'); 

lang.retainAll(picklistValues);

system.debug('The matching values are: ' + lang);

The above removes all the values from the lang set that are NOT in the pick list values. You can change things around as you need depending on what you are comparing

To modify your code to just compare like you are trying to do (changes commented with //*******) :

List<selectOption> options = new List<selectOption>(); 
Schema.sObjectType sobject_type = Account.getSObjectType(); 
Schema.DescribeSObjectResult sobject_describe = sobject_type.getDescribe(); 
Map<String, Schema.SObjectField> field_map = sobject_describe.fields.getMap(); 
List<Schema.PicklistEntry> pick_list_values = field_map.get('Language__c').getDescribe().getPickListValues(); 

List<string> picklistValues = new List<string>();

for (Schema.PicklistEntry a : pick_list_values) { //for all values in the picklist list 
    system.debug(a.getLabel());
    picklistValues.add(a.getValue()); //getValue
}

//picklistValues.sort();

Set<string> lang = new Set<string>(); //********Change to set
lang.add('English');
lang.add('French'); 

for(integer i = 0; i<picklistValues.size(); i++)
{
   if(lang.contains(picklistvalues[i]) //*******see if value is in both places
   {
      system.debug('//matched:');
   }
}

Answer 2

I just wanted to provide an updated answer to this for anybody who visits this question in 2018 & beyond and mention that the upcoming version of Salesforce (Spring '18) does add the contains() method to List: Spring '18 Release Notes: New and Changed Classes

contains(listElement)

Returns true if the list contains the specified element.