Java collections
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This document discusses the main interfaces in the Java Collections Framework - List, Set, Map, Collection, Iterator and Queue. It describes the key characteristics of each interface and provides examples of common implementing classes. It also covers important utility methods from the Collections class for working with collections and choosing the appropriate collection implementation based on needs.
Java collections
- 2. Main Interfaces in Java Collection Framework • List • Set • Map • Collection • Iterator • Queue
- 3. List Interface • Ordered or sequenced collection of objects. • You can access each element in the list via an index. • Examples of classes that implement the List interface – ArrayList, LinkedList. • You can have duplicate entries or null values.
- 4. Set Interface • Cannot have duplicate elements. • No guarantee on the order of elements. • Can have atmost 1 null value. • Classes that implement this interface are HashSet, TreeSet and LinkedHashSet.
- 5. Map Interface • Map has a collection of keys and values and each key maps to a specific value. • No duplicate keys. • Each key can map to at most 1 value. • Examples of classes that implement the Map interface are HashMap, TreeMap and LinkedHashMap.
- 7. Class: java.util.Collections • Contains only static methods that operate on or return collections. • Polymorphic algorithms that operate on collections. • Wrapper methods, which return a new collection backed by a specified collection. • Throw a NullPointerException if a null object is passed as a parameter.
- 8. Important Methods of Collections Class • Wrapper Classes for Collections • Thread Safe Collection: Collections.synchronizedList(someListObject); • Immutable Collection: Collections.unmodifiableList(someListObject); • Type Safe Collection: Collections.checkedList(List<E> list, Class<E> type)
- 9. Other important methods of Collections class • addAll • binarySearch • min, max • Sort • reverseOrder • Empty{List,Set,Map} methods. • replaceAll
- 10. Difference between ConcurrentHashMap and Collections.synchronizedMap • ConcurrentHashMap: allows concurrent modification of the Map from several threads without the need to block them. Use this option if performance is critical, and each thread only inserts data to the map, with reads happening less frequently. • Collections.synchronizedMap(map) creates a blocking Map which will degrade performance, albeit ensure consistency (if used properly). Use this option if you need to ensure data consistency, and each thread needs to have an up-to- date view of the map. Use this when lot of reads and very few writes.
- 11. Iterator interface • Iterate over any of the collection classes and access the elements in an iterative order. • List.iterator() will provide an iterator. • iterator.hasNext() – check if another element is available. • iterator.next() – retrieves the next element. • Java Collection classes are fail-fast which means that if the Collection is changed while some thread is traversing over it using an iterator, the iterator.next() will throw a ConcurrentModificationException.
Editor's Notes
- #5: No nulls are allowed in TreeSet.