Java Course 4: Exceptions & Collections


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Lecture 4 from the IAG0040 Java course in TTÜ.
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Java Course 4: Exceptions & Collections

  1. 1. Java course - IAG0040 Exceptions, CollectionsAnton Keks 2011
  2. 2. java.lang.Object ● All objects in Java extend java.lang.Object ● It provides the following methods: – toString() - returns a String representation of an object, by default it returns getClass().getName() + “@” + hashCode(); – equals(Object o) – checks for equality with another Object, by default just compares references: return this == o; – hashCode() - returns a (possibly) unique and uniformly distributed int value for the object internal state. Used in hash tables. – getClass() - returns the Class object, representing its runtime class. – wait() and notify() - used for synchronization of threads – clone() - can be overridden to allow cloning (copying) of objects ● equals, hashCode, toString, and clone are overridden quite oftenJava course – IAG0040 Lecture 4Anton Keks Slide 2
  3. 3. Exceptions● Exceptions exist to separate real code from error checking● Exceptions are special classes, instances of which can be thrown: – throw new Exception(“Hello!”);● Thrown exceptions can be caught: – try { } catch (Exception e) { } finally { }● Hierarchy: Exception RuntimeException (regular errors) (unchecked exceptions) Throwable (base class) Error (system or fatal errors)Java course – IAG0040 Lecture 4Anton Keks Slide 3
  4. 4. Exceptions (cont) ● Exceptions automatically collect stack trace on creation ● A method must declare all checked Exceptions it throws: – public void hello() throws IOException {...} – then compiler forces you to either declare throws too or catch the declared exception – forced error checking ● Unchecked exceptions (extending RuntimeException) can be thrown without declaration, like NullPointerException ● Errors are never thrown from the code manually, they are fatal like OutOfMemoryError, NoClassDefFoundError ● Any Throwable can contain a nested Throwable, which caused it – can be obtained using the getCause() methodJava course – IAG0040 Lecture 4Anton Keks Slide 4
  5. 5. System properties ● Provide a mean of configuration ● Handled by java.util.Properties class ● Each property is a dot-separated name-value pair: – ● Can be read using System.getProperties() and similar methods ● Additional properties can be specified on command-line: – java ● Can be stored in files with .properties extension – load() and store() methods provided – files are always in ISO-8859-1 (other encodings allowed in 1.6)Java course – IAG0040 Lecture 4Anton Keks Slide 5
  6. 6. Introduction to collections ● A Collection is a container of Objects, it groups many Objects into a single one ● Arrays are too static (but can also be considered collections) ● Arrays have very few built-in features ● Initially, Java contained a few collection classes, like Vector, Hashtable (and Properties), Stack, etc ● Java 1.2 introduced the Collections Framework ● Another example of a collections framework is the STL (Standard Template Library) in C++Java course – IAG0040 Lecture 4Anton Keks Slide 6
  7. 7. What is a Collections Framework The Java Collections Framework consists of: ● Interfaces – abstract data types representing various collections. Allow collections to be manipulated independently of their implementations. ● Implementations – these are the concrete implementations of the interfaces. They are reusable data structures. ● Algorithms – these are able to perform useful computations, like searching and sorting, on the implementations of the interfaces. So, the algorithms are polymorphic and therefore are reusable functionality.Java course – IAG0040 Lecture 4Anton Keks Slide 7
  8. 8. Benefits of Collections ● Reduce programming effort ● Increase program speed and quality ● Allow interoperability among unrelated APIs ● Reduce effort to learn and use new APIs ● Reduce effort to design new APIs ● Help to reuse the codeJava course – IAG0040 Lecture 4Anton Keks Slide 8
  9. 9. Interfaces Here are the core Collections interfaces: Collection Set List Queue Map SortedSet SortedMap Note: Collection is at the root, Map is separate All Collections interfaces and implementation classes reside in the java.util package.Java course – IAG0040 Lecture 4Anton Keks Slide 9
  10. 10. Collection interface ● Is the root and the most generic one, no direct implementations provided ● A Collection contains elements, nothing else is defined ● Operations on a Collection: – add(...) - adds an element – contains(...) - checks if the specified element exists – remove(...) - removes an element – clear() - removes all elements – size() / isEmpty() - for checking the number of elements – toArray() - converts the Collection to an array – Some of the methods also operate on other Collections rather than on single elements, like addAll(...), removeAll(...), etcJava course – IAG0040 Lecture 4Anton Keks Slide 10
  11. 11. Iterator and Iterable interfaces● Collections can be iterated using Iterators.● Collection interface extends Iterable, therefore any Collection can be used in for each loops● Collection provides the iterator() method, which returns the specific Iterator implementation.● Iterators methods: – boolean hasNext() - returns true if there are more elements available – Object next() - returns the next available element – void remove() - removes the current element (optional)● Iterators are fail-fast, they may throw ConcurrentModificationExceptionJava course – IAG0040 Lecture 4Anton Keks Slide 11
  12. 12. Set interface ● Is a mathematical set ● Contains no duplicate elements ● Some implementations may accept null element ● Set doesnt add any new methods to the Collection ● equals(...) checks for contents, implementation independent ● contains(...) is the most common use case of Sets ● SortedSet provides methods: first(), last(), headSet(), tailSet() and subSet()Java course – IAG0040 Lecture 4Anton Keks Slide 12
  13. 13. Set implementations ● HashSet – the fastest implementation based on a hash table. Iteration order is not guaranteed. Addition of many new elements may be expensive due to resizing. ● TreeSet – a SortedSet, based on a red-black tree. Iteration returns elements in ascending order. Elements must be Comparable or a separate Comparator must be provided. ● LinkedHashSet – same as HashSet, but backed with a linked list and guarantees the order of iteration (defined by the insertion). ● EnumSet – specific Set for enums, implemented using bit masks, very fast and memory-efficient.Java course – IAG0040 Lecture 4Anton Keks Slide 13
  14. 14. Set task ● Write a program, which removes all duplicate elements from an array of Strings – Name your class DuplicateRemoverImpl and put into your own package. – Implement the – Pay close attention to the javadoc – Write a main() method, which demonstrates that the program works ● Which Set implementation will you use?Java course – IAG0040 Lecture 4Anton Keks Slide 14
  15. 15. List interface ● List is an ordered and indexed sequence of elements – Positional access: get(...), set(...) and others – Search: indexOf(...) and lastIndexOf(...) – Iteration: ListIterator, which can iterate in both directions, return indexes and replace objects. – Range-view: subList(...) returns a view of a portion of the list as another List, doesnt copy. All operations on a sublist are reflected in the parent list – add(...) appends to the end, remove(...) removes the first occurence, equals(...) checks for contents and order ● List may contain duplicate elementsJava course – IAG0040 Lecture 4Anton Keks Slide 15
  16. 16. List implementations ● ArrayList – a List, backed by an array – Insertions and deletions can be ineffective due to array resizing or copying of elements. – Index based access is very effective ● LinkedList – a classic linked list with the List interface – Effective insertions, deletions and iteration – Ineffective index based access – Additional Queue, Stack or Deque functionality: addFirst(), getFirst(), removeFirst() and the same for the last elementJava course – IAG0040 Lecture 4Anton Keks Slide 16
  17. 17. Queue interface ● A collection for holding elements prior to processing ● Typically, a FIFO queue (but can be LIFO as well) ● Implementations specify the ordering properties ● New methods: – offer() - adds the element if possible (returns false otherwise) – poll() - retrieves and removes the element from head – peek() - retrieves the element from head without removing it ● Java 1.6 added Deque – double ended queueJava course – IAG0040 Lecture 4Anton Keks Slide 17
  18. 18. Queue implementations ● There are many in java.util.concurrent package ● LinkedList – also implements the Queue interface – nulls are allowed – offer() inserts at the end (tail) – poll() and peek() operate with the first element ● PriorityQueue – a queue with prioritized elements – Only permits Comparable elements or a specific Comparator – Head is the least element according to the comparison – Backed by an array, nulls are not permitted ● ArrayDeque – array-backed Deque and QueueJava course – IAG0040 Lecture 4Anton Keks Slide 18
  19. 19. Map interface ● Map maps keys to values (aka associative array) ● Doesnt extend Collection, but provides similar methods – put(), get(), remove() operate with single key-value pairs – containsKey(), containsValue() check for existense – Collection views: keySet(), values(), entrySet() ● Map.Entry interface is for elements of a Map (key and value container) ● SortedMap is a Map with sorted keys, has analogous methods as SortedSetJava course – IAG0040 Lecture 4Anton Keks Slide 19
  20. 20. Map implementations ● HashMap – the fastest implementation based on a hash table. ● TreeMap – a SortedMap, based on a red-black tree. Keys are in the ascending order. ● LinkedHashMap – a HashMap with guaranteed key iteration order. ● EnumMap – specific Map for enum keys, implemented as arrays, very fast and efficient. ● IdentityHashMap – same as HashMap, but uses == for equality tests instead of the equals() method, slightly faster ● WeakHashMap – very specific, holds references to keys as weak references, allowing garbage collector to destroy these objects while in the Map (prevents memory leaks)Java course – IAG0040 Lecture 4Anton Keks Slide 20
  21. 21. Map task ● Write a program that calculates word frequency table in text – Text is represented by a String ● Use the s.split(“s”) method for parsing – Program should output words in alphabetical order – Name your class WordFrequencyCalculator and put into your own package – Write a main() method, which demonstrates that the program works ● Which Map implementation will you use?Java course – IAG0040 Lecture 4Anton Keks Slide 21
  22. 22. Legacy collections ● Vector – now implements List, substituted by ArrayList ● Enumeration – substituted by Iterator, which has shorter methods ● Stack – now implements List, substituted by LinkedList ● Hashtable – now implements Map, same as HashMap ● BitSet – doesnt implement Set, a bit vector implementation, no direct substitutes in the Collections framework, but sometimes EnumSet will do the job betterJava course – IAG0040 Lecture 4Anton Keks Slide 22
  23. 23. More on implementations ● Implementation classes have been discussed ● There are many abstract implementations, like AbstractCollection, AbstractSet, AbstractList, AbstractSequentialList, etc, provided for writing new custom Collections ● Special helper classes Arrays and Collections provide additional functionality and algorithms (static methods)Java course – IAG0040 Lecture 4Anton Keks Slide 23
  24. 24. Arrays helper class ● Arrays class provides operations on arrays – asList() - provides a view of an array as a List – binarySearch() - searches for an element from a sorted array – equals() - checks two arrays for equality – fill() - fills an array with the specified element – sort() - sorts an array (using a tuned QuickSort algorithm) – toString() - can be used for displaying of arrays – deepToString() - the same for multidimensional arraysJava course – IAG0040 Lecture 4Anton Keks Slide 24
  25. 25. Collections helper class ● Provides constants and operations on Collections – EMPTY_XXX or emptyXXX() - immutable empty collection – sort(), binarySearch(), fill(), copy(), min(), max(), shuffle(), replaceAll(), rotate(), swap() – singletonXXX() - immutable collection with one element – enumeration() - for support of legacy classes ● Wrappers – checkedXXX() - a dynamically typesafe view – unmodifiableXXX() - an unmodifiable view – synchronizedXXX() - a synchronized viewJava course – IAG0040 Lecture 4Anton Keks Slide 25
  26. 26. Tips ● Program to interfaces – List list = new ArrayList(); ● Copy (or conversion) constructors – Set set = new TreeSet(map.values()); ● Checking if the Collection is empty – collection.isEmpty() – collection.size() == 0 may be very expensive ● Remove all nulls (or other elements): – collection.removeAll(Collections.singleton(null)) ● Convert to arrays – String[] s = c.toArray(new String[c.size()]);Java course – IAG0040 Lecture 4Anton Keks Slide 26
  27. 27. Tips (cont) ● Iterate Maps with Map.Entry if you need both keys and values – for(Map.Entry e : map.entrySet()) {} ● Initial capacity in case of HashSet, HashMap, and ArrayList – new ArrayList(512) ● Operations on sublists are reflected in the main lists – list.subList(15, 16).remove(object); ● All collections implement toString() – useful for displaying the contents quicklyJava course – IAG0040 Lecture 4Anton Keks Slide 27