The ArrayList Class and File IO

1. The ArrayList Class and File IO CS0007: Introduction to Computer Programming

2. The ArrayList Class • Array data structure: • Pro: Can access any element with its index • Con: Cannot add or remove elements (statically sized) • Linked list data structure • A linked list is a data structure in which an element of the list is a node that references the next element in the list. • A node in a linked list is made up of some data and a reference to another node. • Pro: Can add and remove elements (dynamically sized) • Con: Have to traverse list from beginning to find an element • Arrays are weak where linked lists are strong, and vice versa. Is there any data structure to take advantage of this. • Answer: Yes • The Java API provides a class called ArrayList. • An ArrayList is a data structure that is a combination of an array and a linked list.

3. The ArrayList Class • ArrayListscontain elements that can be accessed by their index, but also have advantages over arrays: • An ArrayList automatically expands as items are added to it. • An ArrayList automatically shrinks as items are removed from it. • To use an ArrayList, you must import it from the java.util package: importjava.util.ArrayList; • The declaration and creation of an ArrayList follows this form: ArrayList<dataType> identifier= newArrayList<dataType>(); • So If I wanted to create an array list whose elements are of type String: ArrayList<String> myList= newArrayList<String>(); • Important Note: An ArrayList can only hold elements that are objects. • That means an ArrayList cannot hold elements of a primitive type.

4. The ArrayList Class • You can add items to the end of an ArrayList by using the add method myList.add("Eric"); myList.add("Bob"); myList.add("Steve"); • This will make "Eric" the first element, "Bob" the second element, and "Steve" the third element. • Like in an array, these elements have indices starting at 0. • To get the number of items stored in the ArrayList, use the size method. myList.size(); • To get an element by its index, use the get method myList.get(1); • Example: ArrayListExample.java

5. Enhanced for Loop and ArrayLists • As stated previously, the enhanced for loop iterates through the elements of any collection. • We’ve seen this with arrays before: String[] names = {"Eric", "Thomas", "Steve"}; for(String name : names) { System.out.println(name); } • We can also do this with ArrayLists. • Example: EnhancedForArrayList.java

6. The toString Method and ArrayLists • Every class in Java inherits methods from a base class called Object. • If you were to create a class, it would automatically inherit these methods. • These methods are placeholders and do nothing meaningful when you inherit them. • The point of these methods is to override them. • When you override an inherited method, you replace a previous implementation of that method with another. • One of these methods is called toString. • toString is a method whose purpose is to return a String representation of an object. • ArrayList has its own implementation of this. • Example: ArrayListToString.java

7. Inserting, Removing and Replacing Elements • To remove an element from a list you use the remove method with the index of the element you want to remove: myList.remove(1); • Note: This will remove the second element and all elements behind that element will move up in the list. • For example: If an element has index 5 before we remove the element at index 1, after the removal, it will have index 4. • To insert an element into a specific index you use the add method (similar to adding an element to the end of the list): myList.add(3, "Chris"); • This will add "Chris" to the list at index 3 and all elements with indices greater than 2 will move back one. • For example: If an element has index 5 before we add the element at index 3, after the addition, it will have index 6. • Finally, you can replace an element by using the set method: myList.set(2, "Joe"); • This will replace the value of the element at index 2 with the value "Joe". • Example: ArrayListInsertRemoveReplace.java

8. Java File Input and Output • So far we have written programs, data could not be saved in between runs of the program. • This is because the data we’ve used in our programs have been stored in RAM. • Once our program is done running the data stored in RAM disappears. • In order for us to save data for use outside our programs or for different runs of our programs, we must save our data outside of RAM. • The most simple way of doing this is writing to a text file on the computer’s hard drive. • In a text file all data is encoded as text, such as Unicode. • That way, when our program is finished executing, we will still have a file with our data. • The steps taken when reading or writing to a file: • Open the file. • This opens a connection between the file and a program (often called a stream) • Data is written or read from a file. • When the program is finished with the file, the file must be closed. • Two kinds of files in input and output: • Input File – File in which data is read from. • Output File – File in which data is written to.

9. Writing to a File • The Java API provides a class for writing to a file called PrintWriter. • You must import the PrintWriter class. importjava.io.PrintWriter; • The general form to create a PrintWriter object: PrintWriteridentifier = newPrintWriter(fileName); • What is fileName? • It is the file you want to write to. • Text files typically have a .txt suffix • The file will be created in the directory in which the program was compiled • Note: This may be different places depending on the IDE you use. • If the file already exists, it will be erased and overwritten! • So to create a FileWriter object that opens a file called “myFile.txt”: PrintWriteroutputFile = newPrintWriter("myFile.txt");

10. Writing to a File • Important Note: Creating a PrintWriter object throws an exception that must be handled. • We will not discuss exception handling in this course at length, so just perform the following steps to allow your program to compile: • Add the words “throws FileNotFoundException” to the end of the main method’s header: publicstaticvoidmain(String[] args)throwsFileNotFoundException • Import java.io.FileNotFoundException importjava.io.FileNotFoundException; • There are two methods that can be used to write to a file: • println– prints its argument to the file and ends with a new line character. • print – prints its argument to the file • When you are finished writing to the file, you must close it using the close method of the PrintWriter object. • Example: FileOutput.java

11. Reading from a File • To read from a file we need to use both the Scanner class and another class in the Java API called File. • The File class is Java’s way of representing a file in code. • For this we need to import both Scanner and File classes: importjava.io.File; importjava.util.Scanner; • Also we have to perform the steps to handle the FileNotFound exception, like when we wrote to a file. • Next, we need to create both a File and Scanner objects: File theFile= new File("myFile.txt"); Scanner inputFile = newScanner(theFile); • This creates a File object that is associated with the file “myFile.txt”. • For now make sure this file is in the same directory that you compile your program from. • Again, this could be different depending on which IDE you use. • If the file does not exist, it will cause an error! • To read a single line from a file we can use the nextLine method on the Scanner object. • Finally, to close the file we use the close method on the Scanner object. • Example: FileInput.java

12. Detecting the End of a File • It is often the case we want to read all of the contents of the file, or simply keep reading from the file until the end. • There is a method in the Scanner class that detects if there is anything left to read in a file called hasNext. • hasNext returns true if there is more to read in a file, or false if there is not. • So this can be used in a loop to read all of the lines in a file… • Example: HasNext.java