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Managing unordered data with HashMap

From: Java Essential Training

Video: Managing unordered data with HashMap

Most programming languages have the ability to store unordered collections of data where you can randomly access items in the collection by their keys. In some languages these are known as associative arrays, in others structures. But they always serve the same purpose, an unordered collection of data where you can get to one item at a time. In Java, we do this with a class called the HashMap, just like the array list it's a class. And when you declare it you have to import it. I'll start in this empty main method in the project HashMap and I'll declare an instance of the HashMap class.

Managing unordered data with HashMap

Most programming languages have the ability to store unordered collections of data where you can randomly access items in the collection by their keys. In some languages these are known as associative arrays, in others structures. But they always serve the same purpose, an unordered collection of data where you can get to one item at a time. In Java, we do this with a class called the HashMap, just like the array list it's a class. And when you declare it you have to import it. I'll start in this empty main method in the project HashMap and I'll declare an instance of the HashMap class.

I'll type in the name of the class and press control space. Eclipse recognizes that this class must be imported and it adds the import at the top of the code. Just as with the array list the HashMap is designed to store data all of the same type but where the array list means one data type, the HashMap needs two. Each item in a HashMap will have a key and a value. Notice that these are represented in the Diamond Operator that is where the generic is declared using the letters K and V.

In some languages the key always has to be a string, but in Java you can really use any object. For simplicity though in this exercise I'll declare both my keys and my values to be strings. So I'll type in String, press tab and type String again and then I'll declare the variable name, map. Next, I'll instantiate the class. I'll type in the code = new then once again I'll type the name of the class and press control space. There are four different versions of the constructor method for HashMap.

There is one with no arguments, there is one with an initial capacity, and there are a couple that let you initialize the object in other ways. I'm going to choose the simplest the one with no arguments. Eclipse looks at my original declaration and matches my constructor method call. Now the codes get in little wide so I'll maximize my editor. I'll add the semicolon at the end of the line and now I've created my first HashMap. In order to add items to the HashMap use the method named put.

I'll call map.put and I'll see that there's only one version of the put method. It requires two values, in this case two strings. And once again using the names of states, I am going to use the name of the state as the key and its capital as the value. So I'll type in California then tab over and type in Sacramento. I'll add another item to the map this will have a state name of Oregon and a capital of Salem and finally I'll add Washington and Olympia.

Now I'd like to output the contents of the map to the command line. Just as with the array list, the HashMap has a two string method that will serialize its contents so it's easy to read. So I'll use System.out.println and just pass in the map object. I'll save and run my application and there is the results. Each item contains a key and a value separated by the equals operator and the entire list is wrapped in curly braces.

Just as with array lists, HashMaps are resizable at runtime. So I'll add in yet another State Alaska and its capital Juneau and I'll copy and paste my printline command. I'll organize my codes, so that it's all grouped together correctly, and I'll run the application and there you see that I am successfully adding an item. You can find an item in a map by referring to the key.

So for example, let's say that I wanted to find the capital of Oregon. I'll declare a string variable, I'll call it cap and I'll call map.get and I will pass in the key Oregon. And then I'll use of printline command and I'll say the capitol of Oregon is and I'll output the value of cap. And I'll run the code and I get the capitol of Oregon is Salem, that's correct.

Now once again as I mentioned this second item in the HashMap does not have to be a string, it could be any kind of complex object. And as you get into creating your own custom classes that represent data, you'll find that HashMaps are very useful for storing large collections of data in memory. Next, I am going to remove an item. With the array list you removed an item by its index position, but remember a HashMap isn't in any particular order. So if you want to remove an item, you refer to its key.

So I'll call map.remove and I'll pass in California and I'll print out the map, and I've removed California and I'm left with the three remaining states. Take a look at the documentation for the HashMap class to learn more about it. I'll select the name of the class and go to Dynamic Help, click for the javadoc and maximize and go to the method list. You'll see that there are methods to determine whether a particular item is available in the list. Those methods are name contains key and contains value.

There is a method for determining whether the HashMap is empty, it's called isEmpty and returns a Boolean. There's put methods, a putAll method, size methods and values methods. I'll show you how to use a couple of these other methods in a later video when I show you how to loop through the HashMap using something called an iterator object. But this should give you enough information to get started with HashMaps storing unordered data collections in your Java applications.

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This video is part of

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Java Essential Training

71 video lessons · 76567 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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