ArrayList, Hashtables, Exceptions, and Files

Course Evaluations in Miverva

ArrayList

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

Using java.util.ArrayList

"Any type": Anything that is not a primitive type

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

Using java.util.ArrayList

"Any type": Anything that is not a primitive type

Examples:

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();

/* 
     * 2 - 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_2 = myArray.get(1); // You can cast an Integer into an int.

/* 
     * 4 - 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.

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).

Hashtable

Think of a Hashtable as a dictionary

We use a Hastable to associate a key to a value

Hashtable

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

Using java.util.Hashtable

"Any type": Anything that is not a primitive type

not a primitive type: Objects, fixed-size arrays, Integer, Character, Double, Long, String

Examples:

Using a Hashtable

Hashtable methods: size, put, get and remove

// 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();

/* 
     * 2 - Adding a new entry to the Hashtable ( a Key,Value pair) :
     *     The .put( K someKey, V someValue ) method of the Hashtable class.
     */
    long oldNumber = 4387563321L;
    phonebook.put("Marcel", oldNumber); // This adds a new entry
    phonebook.put("Ante", 4388871414L); // 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 = 4387563321L;
    phonebook.put("Marcel", newNumber); // This replaces the old value

/* 
     * 4 - 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 4388871414

String key2 = "Etaoin Shrdlu";
    Long number2 = phonebook.get(key2); // 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

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).

Exceptions

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

Trying to call a method of a null reference:

Trying to get an integer number, from a string that does not represent an integer

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

NullPointerException

NumberFormatException

Programmatically handling run-time errors

You can handle the exception by using the `try` and `catch` statements

Your program will first try to execute the code, and catch an Exception if an error happens.

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.

A user of this method may pass you an array with null references. Accessing a null element will throw a NullPointerException

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

You can defer error handling by making your code throw an Exception

Defering error handling to the user of the method

You can also throw an Exception with your own custom message

You can defer error handling by making your code throw an Exception

File Input/Output

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.

This code will throw an exception if the file cannot be opened, or if it does not exist

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.

This code will throw an exception if the file cannot be opened, or if it does not exist

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.

This code will throw an exception if the file cannot be opened, or if it does not exist

Resources

Using an ArrayList:
http://www.homeandlearn.co.uk/java/array_lists.html
Using a Hashtable:
http://www.tutorialspoint.com/java/java_hashtable_class.htm
Exceptions:
https://docs.oracle.com/javase/tutorial/essential/exceptions/
Reading and writing files:
http://www.homeandlearn.co.uk/java/read_a_textfile_in_java.html
How to think like a Computer Scientist in Java (ArrayList, Exceptions, Files):
Chapter 16 and Appendix B

ArrayList, Hashtables, Exceptions, and Files

Course Evaluations in Miverva

ArrayList

java.util.ArrayList

This is one of the Java implementations of a LinkedList. I.e. an ArrayList does not have a fixed size.

Using java.util.ArrayList

Using java.util.ArrayList

Using an ArrayList

ArrayList methods: size, add, get, and remove

Using an ArrayList

Other ArrayList methods ( listed in the Java Documentation ):

Hashtable

Hashtable

Think of a Hashtable as a dictionary

We use a Hastable to associate a key to a value

Hashtable

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

Using java.util.Hashtable

Using a Hashtable

Hashtable methods: size, put, get and remove

Using a Hashtable

Other Hashtable methods ( listed in the Java Documentation ):

Exceptions

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

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

Programmatically handling run-time errors

You can handle the exception by using the try and catch statements

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.

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

Defering error handling to the user of the method

You can also throw an Exception with your own custom message

File Input/Output

Reading a text file

You need the FileReader and BufferedReader classes

This code will throw an exception if the file cannot be opened, or if it does not exist

Writing a text file

You need the FileWriter and BufferedWriter classes

This code will throw an exception if the file cannot be opened, or if it does not exist

Writing a text file

You need the FileWriter and BufferedWriter classes

This code will throw an exception if the file cannot be opened, or if it does not exist

Resources

You can handle the exception by using the `try` and `catch` statements

You need the `FileReader` and `BufferedReader` classes

You need the `FileWriter` and `BufferedWriter` classes

You need the `FileWriter` and `BufferedWriter` classes