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Managing resizable arrays with ArrayList

From: Java Essential Training

Video: Managing resizable arrays with ArrayList

There are many situations in applications where you need to manage ordered collections of data. That means that once you add data to the collection, it stays in the same order and you can reference the items in the data collection by their index position. This simple array in Java is useful but it has a downside, it's not resizable at runtime. And so Java provides a special class for resizable arrays, it's named array list. You declare an array list as an instance of a class, so it's a complex object.

Managing resizable arrays with ArrayList

There are many situations in applications where you need to manage ordered collections of data. That means that once you add data to the collection, it stays in the same order and you can reference the items in the data collection by their index position. This simple array in Java is useful but it has a downside, it's not resizable at runtime. And so Java provides a special class for resizable arrays, it's named array list. You declare an array list as an instance of a class, so it's a complex object.

I'll start in the empty main method of this project ArrayList. And I'll begin by typing the name of the class ArrayList. Remembering that Java is case- sensitive, ArrayList is spelled with an uppercase A and an uppercase L. The ArrayList class is not a member of the java.lang package. So you must provide an import for it at the top of your code. After typing in the name of the class, press Ctrl+ Spacebar and Eclipse will add the import for you. Eclipse also adds a little bit of additional notation after the class name.

The less than and greater than characters are sometimes known as the diamond operator. And in Java the diamond operator is used to declare something called a generic. A generic is a special data type, and in this context it means this array list will contain items only of a particular data type. It is possible to create an array list of mixed data, but most of the time we don't use it for that purpose. Most of the time array lists and other collections contain items all of the same data type. You declare which data type by simply setting the data type between the less than and greater than characters, so this array list could contain integers or can contain shorts or, as I'm going to do here, it could contain strings or any other complex object.

So that's the complete data type including the generic declaration. This will be an array list of strings. Next as with all declarations I'll provide the variable name I'll call it list, and then I'll initialize it using the constructor method. I'll once again type in the name of the class, this will be a constructor method call, and then I'll press control space again and I'll choose the version of the array list constructor method that doesn't receive any arguments. Eclipse looks at the original declaration, and says you said this was going to be an array list of strings, so when I call the constructor method, I'm going to use that same string generic declaration and that's correct, I'll complete the declaration and initialization with the semicolon.

Now I can add items to the list. I said it was going to contain strings, so anything I pass in must either be a string or be translatable to a string. To add items to the array list call the add method. There are two versions of the add method, you can either pass in a value and it will be appended to the end of the list or you can pass in an integer, an index, and the value and then the item will be added in the position that you determine. I am just going to use the simple add version that accepts the string and I am going to add three items to the list, California, Oregon and Washington.

I'm going to output the list to the console. I'll use System.out.printline, and I am just going to add the list object. The list object will be serialized into a string for me because its two string method handles that operation automatically. I'll save and run the application, and I see my items displayed in a comma delimited list wrapped in square brackets and that's how an array list is translated automatically.

Now I'm going to add another item. I'll call list.add and I'll just keep on moving north, I'll add Alaska, and then I'll copy and paste my print line command and run the application again. And I see that I've successfully added an item at runtime, something I wouldn't have been able to do if this were a simple array. You can add items to an array list at runtime and you can also remove items. In order to remove items, you can provide either the index position or an object reference that matches.

In the case of strings, you can remove items simply by providing a matching string. I am going to remove the first item in the list when I type .remove and press Ctrl+ Spacebar, I see that there are these two versions of the remove method. It's overloaded. And I'll use the first version. I'll provide an index value of zero meaning remove the first item. I once again copy and paste my print line command and I'll output the list. And I'll see that after removing the first item California I'm left with a list of three items starting with Oregon.

Next I'm going to get an item in a particular position in the list. I'll declare a string variable named state. In order to get an item from the list, you use the method list.get. You provide an integer value. So I'm going to provide a value of one remembering that an array list just like a simple array starts its numbering at Zero, this would mean get the second item in the list. I once again paste in my print line command and I'll change it. I'll start with the literal "The second state is" and then I'll append state.

I'll save and run, and I get the second state as Washington and that's correct. You can also find an item in the list. That is, get its index position by passing an object reference or in the case of strings by passing in the string value. So I am going to declare an integer variable named pos, and I'll get its value by list.indexOf an I'll pass in Alaska. And then I'll once again use a print line command and l'll output Alaska is at position and I'll append pos.

I'll run the application and I see Alaska is at position 2, which taking into consideration the zero-based numbering is at position 2, Oregon position zero, Washington position one, Alaska position 2. Take a look at the documentation of the array list for more information about how you can use this powerful class. I'll double click the name of the class in my code and go to Dynamic Help and click the link for its javadocs, expand the help view and then go to the method list.

You'll see that there are methods in they are not just for adding removing and searching, but also for getting the size of the array list and managing how it uses its memory. Use array list to manage ordered collections of data, or you need to resize the collection at runtime, and where you need to be able to add remove and search for items, it's an incredibly powerful class that does much more than the simple array.

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This video is part of

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Java Essential Training

71 video lessons · 68734 viewers

David Gassner
Author

 
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  1. 10m 8s
    1. Welcome
      1m 3s
    2. Is this course for you?
      5m 35s
    3. Using the exercise files
      3m 30s
  2. 31m 24s
    1. The history of Java
      5m 19s
    2. Java compilation and syntax
      8m 54s
    3. Understanding the principles of Java
      8m 28s
    4. Choosing a development environment
      8m 43s
  3. 19m 5s
    1. Installing Java on Windows
      6m 42s
    2. Installing Eclipse on Windows
      3m 19s
    3. Exploring Java on Mac OS X Leopard and Snow Leopard
      2m 27s
    4. Installing Java on Mac OS X Lion
      3m 27s
    5. Installing Eclipse on Mac OS X
      3m 10s
  4. 46m 10s
    1. Creating a Hello World application
      11m 7s
    2. Exploring the Eclipse IDE
      8m 55s
    3. Compiling and running from the command line
      8m 2s
    4. Passing arguments to the application
      8m 17s
    5. Using the Java API documentation
      4m 5s
    6. Memory management and garbage collection
      5m 44s
  5. 58m 57s
    1. Everything is an object
      5m 59s
    2. Declaring and initializing variables
      9m 15s
    3. Working with numbers
      8m 32s
    4. Converting numeric values
      6m 40s
    5. Understanding operators
      7m 58s
    6. Working with character values
      5m 14s
    7. Working with boolean values
      5m 13s
    8. Outputting primitive values as strings
      5m 33s
    9. Creating a simple calculator application
      4m 33s
  6. 53m 40s
    1. Writing conditional code
      5m 35s
    2. Using the switch statement
      8m 50s
    3. Repeating code blocks with loops
      7m 35s
    4. Creating reusable code with methods
      6m 31s
    5. Declaring methods with arguments
      5m 41s
    6. Overloading method names with different signatures
      5m 53s
    7. Passing arguments by reference or by value
      5m 35s
    8. Creating a more complex calculator application
      8m 0s
  7. 20m 30s
    1. Using the String class
      5m 44s
    2. Building strings with StringBuilder
      3m 34s
    3. Parsing string values
      3m 19s
    4. Working with date values
      7m 53s
  8. 20m 44s
    1. Understanding compile-time vs. runtime errors
      4m 5s
    2. Handling exceptions with try/catch
      4m 55s
    3. Throwing exceptions in methods
      2m 50s
    4. Using the debugger
      8m 54s
  9. 32m 22s
    1. Using simple arrays
      4m 47s
    2. Using two-dimensional arrays
      6m 17s
    3. Managing resizable arrays with ArrayList
      7m 14s
    4. Managing unordered data with HashMap
      6m 5s
    5. Looping through collections with iterators
      7m 59s
  10. 52m 2s
    1. Understanding encapsulation
      5m 59s
    2. Creating and instantiating custom classes
      8m 8s
    3. Organizing classes with packages
      6m 47s
    4. Creating and using instance methods
      6m 52s
    5. Storing data in instance variables
      6m 56s
    6. Using constructor methods
      5m 40s
    7. Managing instance data with getter and setter methods
      8m 26s
    8. Using class variables and Enum classes
      3m 14s
  11. 41m 15s
    1. Understanding inheritance and polymorphism
      9m 12s
    2. Extending custom classes
      9m 1s
    3. Overriding superclass methods
      3m 8s
    4. Casting subclass objects
      5m 3s
    5. Understanding interfaces and implementing classes
      4m 2s
    6. Creating your own interfaces
      4m 14s
    7. Using abstract classes and methods
      6m 35s
  12. 32m 17s
    1. Managing files with the core class library
      7m 46s
    2. Managing files with Apache Commons FileUtils
      7m 32s
    3. Reading a text file from a networked resource
      7m 52s
    4. Parsing an XML file with DOM
      9m 7s
  13. 17m 39s
    1. Creating your own JAR files
      4m 54s
    2. Understanding the classpath
      5m 2s
    3. Documenting code with Javadoc
      7m 43s
  14. 47s
    1. Goodbye
      47s

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