ArrayList, Hashtables, Exceptions, and Files

Transcription

1 ArrayList, Hashtables, Exceptions, and Files

2 Course Evaluations in Miverva

3 ArrayList

4 java.util.arraylist This is one of the Java implementations of a LinkedList. I.e. an ArrayList does not have a fixed size. An ArrayList is a Generic data Type Meaning we can build ArrayList in which the element have "any type" I.e. ArrayList of integer numbers, of floating point numbers, of objects, etc. "Any type": Anything that is not a primitive type

5 Using java.util.arraylist "Any type": Anything that is not a primitive type 1 3 // This creates an empty ArrayList for elements of type T ArrayList<T> myarray = new ArrayList<T>(); Java provides non-primitive type versions of primitive types, that can be used with an ArrayList int Integer long Long double Double char Character You can assign a null value to the previous non-primitive types. You can assign a primitive value/variable to its non-primitive counterpart, only if it is not null

6 Using java.util.arraylist "Any type": Anything that is not a primitive type Examples: // This creates an empty ArrayList of int numbers ArrayList<Integer> myarray1 = new ArrayList<Integer>(); // This creates an empty ArrayList of char ArrayList<Character> myarray = new ArrayList<Character>(); // This creates an empty ArrayList of String ArrayList<String> myarray3 = new ArrayList<String>(); // This creates an empty ArrayList of Person objects ArrayList<Person> myarray = new ArrayList<Person>();

7 Using an ArrayList ArrayList methods: size, add, get, and remove // This creates an empty ArrayList of int numbers ArrayList<Integer> myarray = new ArrayList<Integer>(); /* * 1 - Getting the number of elements inside an ArrayList: * The.size() method of the ArrayList class. */ int n = myarray.size(); /* * - Inserting an element at the end of the ArrayList: * The.add( T newelement ) method of the ArrayList class. */ int a = 5; myarray.add(a); // You can cast an int into an Integer myarray.add(11); // You can cast an int into an Integer /* * 3 - Getting an element at the i-th position of the ArrayList: * The.get( int i ) method of the ArrayList class. */ Integer ith_element = myarray.get(0); int ith_element_ = myarray.get(1); // You can cast an Integer into an int. /* * - Removing an element at the i-th position of the ArrayList: * The.remove( int i ) method of the ArrayList class. */ myarray.remove(); // You can cast an Integer into an int.

8 Using an ArrayList Other ArrayList methods ( listed in the Java Documentation ):.add( int index, T newelement) inserts newelement at position given by index.remove( T target ) removes the first occurrence of the target.indexof( T target ) Searches for the target and returns the index of the first occurrence..lastindexof( T target ) Returns the index of the last occurrence of the target..clear() Remove all elements form an array..tostring() A string representation of the contents (Useful for debugging).

9 Hashtable

10 Hashtable Think of a Hashtable as a dictionary We use a Hastable to associate a key to a value

11 Hashtable Keys, as well as values, can be of "any type"

12 Using java.util.hashtable "Any type": Anything that is not a primitive type 1 3 // This creates an empty Hashtable with keys of type K, and values of type V Hashtable<K, V> myarray = new Hashtable<K, V>(); not a primitive type: Objects, fixed-size arrays, Integer, Character, Double, Long, String Examples: // This creates an empty Hashtable with keys of type String, and values of type String Hashtable<String, String> dictionary = new Hashtable<String, String>(); // This creates an empty Hashtable with keys of type String, and values of type Integer Hashtable<String, Integer> phonebook = new Hashtable<String, Integer>(); // This creates an empty Hashtable with keys of type Long, and values of type Student Hashtable<Long, Student> comp0students = new Hashtable<Long, Student>();

13 Using a Hashtable Hashtable methods: size, put, get and remove

14 // This creates an empty Hashtable with keys of type String, and values of type Long Hashtable<String, Long> phonebook = new Hashtable<String, Long<(); /* * 1 - Getting the number of keys inside a Hashtable: * The.size() method of the Hashtable class. */ int n = phonebook.size(); /* * - Adding a new entry to the Hashtable ( a Key,Value pair) : * The.put( K somekey, V somevalue ) method of the Hashtable class. */ long oldnumber = L; phonebook.put("marcel", oldnumber); // This adds a new entry phonebook.put("ante", L); // This add a new entry /* * 3 - Replacing the value associated to a given key: * The.put( K somekey, V somevalue ) method of the Hashtable class. */ long newnumber = L; phonebook.put("marcel", newnumber); // This replaces the old value /* * - Getting the value associated to a particular key: * The get( K somekey ) method of the Hashtable class. */ String key1 = "Ante"; long number1 = phobebook.get(key1); // This returns String key = "Etaoin Shrdlu"; Long number = phonebook.get(key); // This returns null /* * 5 - Removing an entry fron the Hashtable: * The remove( K targetkey ) method of the Hashtable class. */ phonebook.remove("marcel"); // This removes the "Marcel" key, and its associated value

15 Using a Hashtable Other Hashtable methods ( listed in the Java Documentation ):.containskey( K target ) checks if the Hashtable contains an entry with the target key.containsvalue( V target ) checks if the Hashtable contains an entry with the target value.clear() Remove all entries from the Hashtable..toString() A string representation of the contents (Useful for debugging).

16 Exceptions

17 Programmatically handling run-time errors Exceptions: An exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions Trying to get data from an array, or ArrayList, at an index that is greater that its number of elements 1 int[] myarray = {60, 6, 6, 65, 67, 69, 71}; // this array has 7 elements 3 int c5 = myarray[8]; // This will make the computer scream at you with red lines 5 ArrayList<Integer> mylist = new ArrayList<Integer>(); 6 7 mylist.get(7); // This will make the computer scream at you with red lines 8 Trying to call a method of a null reference: 1 City city = null; 3 city.explore(); // This will make the computer scream at you with red lines 5 6 City[] cities = new City[35]; 7 8 City somecity = cities[10]; // This is less than 3, so it is ok 9 10 somecity.setneighbours; // This will make the computer scream at you with red lines 11

18 Trying to get an integer number, from a string that does not represent an integer 1 String notanumber = "This is not a number."; 3 int anumber = Integer.parseInt(notAnumber); // This will make the computer scream at you

19 Programmatically handling run-time errors Exceptions: An exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions The previous examples throw standard Java Exceptions. ( Read the red messages printed by the computer ) IndexOutOfBoundsException 1 ArrayList<Integer> mylist = new ArrayList<Integer>(); 3 mylist.get(7); // This will make Java scream at you with red lines NullPointerException 1 City[] cities = new City[35]; 3 City somecity = cities[10]; // This is less than 3, so it is ok 5 somecity.setneighbours(); // This will make Java scream at you with red lines 6 NumberFormatException 1 String notanumber = "This is not a number."; 3 int anumber = Integer.parseInt(notAnumber); // This will make the computer scream at you

20 Programmatically handling run-time errors You can handle the exception by using the try and catch statements try{ // Some code that may cause a run time error } catch ( Exception e){ // the actions that you want to take when the error happens } Your program will first try to execute the code, and catch an Exception if an error happens.

21 Programmatically handling run-time errors This is useful for when you cannot predict if the data that your code receives is correct; i.e. when you deal with user input public static int find( String[] a, String target){ try{ // try searching the array } catch ( Exception e ){ // take some action like telling the user what is wrong } } A user of this method may pass you an array with null references. Accessing a null element will throw a NullPointerException

22 Defering error handling to the user of the method You can decide not to handle the error yourself, and let the user of your method do it public static int find( String[] a, String target) throws NullPointerException{ // try searching the array } // if a NullPointerException happens, then the user of your code will have to catch it public static void main(string[] args){ // the user of the method will have to do the error handling try{ int n = find( somearray, sometarget ); } } catch (NullPointerException){ // take some action like finding out what is wrong } You can defer error handling by making your code throw an Exception

23 Defering error handling to the user of the method You can also throw an Exception with your own custom message public static int find( String[] a, String target) throws Exception{ try{ // try searching the array } } catch ( Exception e ){ } Exception myexception = new Exception("What the hell are you doing?"); // throw the exception throw(myexception); public static void main(string[] args){ // the user of the method will have to do the error handling try{ int n = find( somearray, sometarget ); } } catch (Exception e){ // the user will get our message here } You can defer error handling by making your code throw an Exception

24 File Input/Output

25 Reading a text file You need the FileReader and BufferedReader classes The FileReader handles the opening and reading of text files, character by character The BufferedReader allows you to read the file line by line instead. // Open a file for reading FileReader fr = new FileReader("someFile.txt"); // Create a BufferedReader so you can read it line by line BufferedReader br = new BufferedReader(fr); // Initialize a variable that will hold the latest line that you read. Initialize it String currentline = br.readline(); // Read lines until there are no more line ( br.readline()returns null in taht case) while ((currentline!= null){ // print the line System.out.println(currentLine); } // Read the next line currentline = br.readline(); // VERY IMPORTANT: this closes the file, so other programs can read or write to it br.close(); fr.close(); This code will throw an exception if the file cannot be opened, or if it does not exist

26 Writing a text file You need the FileWriter and BufferedWriter classes The FileWriter handles the opening and writing of text files, character by character The BufferedWriter allows you to write multiple characters at once. // Open a file for writing, character by character // This erases any previous file with the same name FileWriter fw = new FileWriter("someOtherFile.txt"); // Create a BufferedWriter, so you can write multiple characters at once BufferedWriter bw = new BufferedWriter(fw); bw.write("is this real life?"); // VERY IMPORTANT: this closes the file, so other programs can read or write to it. // AND MORE IMPORTANTLY: it writes the file to your harddrive bw.close(); fw.close(); This code will throw an exception if the file cannot be opened, or if it does not exist

27 Writing a text file You need the FileWriter and BufferedWriter classes The FileWriter handles the opening and writing of text files, character by character The BufferedWriter allows you to write multiple characters at once. // Open a file for writing, character by character // If the file exists, the new text will be appended at the end boolean append = true; FileWriter fw = new FileWriter("someOtherFile.txt", append); // Create a BufferedWriter, so you can write multiple characters at once BufferedWriter bw = new BufferedWriter(fw); bw.newline(); bw.write("is this just fantasy?"); // VERY IMPORTANT: this closes the file, so other programs can read or write to it. // AND MORE IMPORTANTLY: it writes the file to your harddrive bw.close(); fw.close(); This code will throw an exception if the file cannot be opened, or if it does not exist

28 Resources Using an ArrayList: Using a Hashtable: Exceptions: Reading and writing files: How to think like a Computer Scientist in Java (ArrayList, Exceptions, Files): Chapter 16 and Appendix B