Code used in set advanced methods tutorial on SFDC Stop (https://youtu.be/SYzg37oIU8A)

apexsetadvancedmethods.cls

clear():
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Set<Integer> numbers = new Set<Integer>{30,30,30,20,20,10,40,40};
System.debug(numbers);
numbers.clear();
System.debug(numbers);

isEmpty():
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Set<Integer> numbers = new Set<Integer>{30,30,30,20,20,10,40,40};
System.debug('Is my set empty? -> ' + numbers.isEmpty());
numbers.clear();
System.debug('Is my set empty? -> ' + numbers.isEmpty());

size():
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Set<Integer> numbers = new Set<Integer>{30,30,30,20,20,10,40,40};
System.debug(numbers);
System.debug('Size of the set -> ' + numbers.size());
numbers.clear();
System.debug('Size of the set -> ' + numbers.size());

containsAll():
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-> Check the whole list or set as a part of another set

Set<Integer> numbers = new Set<Integer>{30,30,30,20,20,10,40,40};
System.debug('Set -> ' + numbers);

List<Integer> numbersList = new List<Integer>{10,20, 100};
System.debug('List -> ' + numbersList);

System.debug('List is present inside the set -> ' + numbers.containsAll(numbersList));

retainAll():
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Set<Integer> numbers = new Set<Integer>{30,30,30,20,20,10,40,40};
System.debug('Initial Set -> ' + numbers);

List<Integer> numbersToRetain = new List<Integer>{20,30,100};
System.debug('Numbers to retain set -> ' + numbersToRetain);

numbers.retainAll(numbersToRetain);
System.debug('Final Set -> ' + numbers);