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20 Cards in this Set
- Front
- Back
What is the difference between an ArrayList and a Vector?
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Both implement the List interface, but ArrayList is not thread safe whereas Vector is.
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Whats the difference between a HashTable and a HashMap?
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Both implement the Map interface but HashTable is thread safe whereas HashMap is not.
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Describe a Collection that sorts the elements for you.
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TreeSet or TreeMap. A TreeSet's Iterator is guaranteed to traverse the set in ascending element order while a TreeMap has the keys stored in natural ascending order.
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What is an iterator?
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Iterator is an interface, implementations of which allow the user to enumerate all of the elements in a collection. Each collection implements its own variety of Iterator depending on the type of collection, order of its elements, etc.
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What are some top level interfaces in Java's Collection classes?
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List, Set, Queue, Map and BeanContext.
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What is the difference between a List and a Set?
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Set does not allow duplicate elements while a List does.
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If you override the hashCode() method of a class, what other things should you consider doing to it?
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Override the equals() method so that it is consistent with hashCode so that it may be looked up properly in a collection.
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How would you implement your own ArrayList?
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Have an array of a given type and make sure you have enough room when adding an element. Otherwise, create a bigger array, and copy existing element references to the new array and manipulate.
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Suppose you are given a sorted list. How do you perform a binary search of the list?
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The class java.util.Collections has methods to perform a binary search.
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What is the Collections class in Java?
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java.util.Collections is a utility class that defines a number of static methods that operate on Collections.
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What are some examples of useful methods in the Collections class?
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binarySearch, sort, shuffle, min, max are all examples.
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How do you enumerate all the elements in a HashMap?
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You can get the entry set and enumerate each element and key in the set or you can get the key set and use that to retrieve the values.
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What will happen if I get the KeySet from a HashMap and delete an element from the KeySet?
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The corresponding element will be deleted from the HashMap.
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What will happen if I try to add an element to the KeySet I obtain from a HashMap?
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UnsupportedOperationException. You can not add an element to a HashMap's KeySet although you can remove elements from it.
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What are generics in Java?
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Generics create a mechanism by which a class or an interface can take one or more types as parameters and make that class or interface tied to that type or types. For example ArrayList <Integer> aList; // Creates a list that will accept only Integers.
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Explain the for-each feature of Java.
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It is a special for loop that allows one to iterate over a Collection without using indexes. It's less error prone and cleaner code.
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Give the syntax of a for-each loop.
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List<Integer> myList = new ArrayList<Ineger>();
for ( Integer I : myList) { //do something to or with the current element which is I. } |
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What are synchronized wrappers?
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These are wrappers that add thread safety to an arbitrary Collection. Each of the 6 core Collection sub-interfaces has a static factory method that returns such a wrapper backed up by the specified Collection.
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What is type erasure in Java?
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The Java compiler actually erases types parameters from a generic after enforcement to ensure backward compatibility. This means the type is not available at run time.
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What are wild cards in generics? How are they useful?
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So Class A extends B and B implements interface C:
List<?> Any individual type. List <? extends B> could be A OR it could be B. List <? super C > could only be A or Object since it must be a super class of A. As they pertain to Collections, it means that Collections can now be made type safe so that if you put something into a collection you will guaranteed to get the same type back out. |