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Using two-dimensional arrays

From: Java Essential Training

Video: Using two-dimensional arrays

Java supports the use of multidimensional arrays. Just as with simple arrays a multidimensional array contains items all of the same data type. And once the sizes of the dimensions of the array have been set you can't change them at runtime. I'll show you how to use a multidimensional array in this empty project 2DArrays. I'll start by declaring the array. This is going to be a multidimensional array of strings, so I'll start with the data type followed by brackets, but because this is going to be a two-dimensional array, I add another pair of brackets.

Using two-dimensional arrays

Java supports the use of multidimensional arrays. Just as with simple arrays a multidimensional array contains items all of the same data type. And once the sizes of the dimensions of the array have been set you can't change them at runtime. I'll show you how to use a multidimensional array in this empty project 2DArrays. I'll start by declaring the array. This is going to be a multidimensional array of strings, so I'll start with the data type followed by brackets, but because this is going to be a two-dimensional array, I add another pair of brackets.

Next, I assign the array name, I'll call it states. Then after the equals assignment operator, you put in the new keyword, once again you put in the data type. And now just as with simple arrays you declare the array size, but now you're declaring both the top-level dimension and the child dimension. I'll set the size of my top-level dimension at three and my child dimension at 2. Now my goal is to create a primary array of three items each one representing a state. And then within the secondary array or child array there'll be two items, one for the name of the state and one for its capitol.

Now I'll set the values. Use similar syntax to set values. I'll start with states, bracket, 0, bracket, 0. The first item in the first primary array. And I'll set that to a value of California. Next I'll create the second item in the first primary array using the syntax states, bracket, 0, bracket, 1, and I'll set that to the name of the capitol of California, Sacramento. Now I'm going to copy and paste those two lines of code a couple of times.

I'll create two new copies. And for the second pair, I'll change the primary array item to one. And for the third pair I'll set the primary array item to two. Now I'll set the values, the second state will be Oregon, and its capitol is Salem. And for the third pair I'll use Washington and Olympia. Now to output these values, I am going to use a loop inside a loop.

I'll use two for loops and I'll manage my output by using a StringBuilder object to build strings for each combination of state name and capitol. I'll start by typing the word for and pressing control space and I'll choose for, iterate over array. In the primary for loop the integer counter variable will be named i. And I'll be looping through the primary array, this syntax is the same as for a simple array. I'm iterating through the primary dimension, the one that has three items.

Then I'll click into the primary for loop and I'll add a secondary for loop. I'll once again type for and choose iterate over array. And this time, Eclipse knows that I've already used the variable i, so it uses the variable j for the secondary counter. Now to make sure that I'm iterating over the secondary array, I'll place the cursor after name of the array, states, and I'll add bracket, i, bracket. So now I have an outer loop and an inner loop.

Now my goal is each time through the loop, I'm going to output the state name and its capitol all in a single line. To put it together I'll declare a StringBuilder. I'll put the declaration and initialization of the StringBuilder in the outer loop, so then I can manipulate and append to it in the inner loop. So I'll set the data type to StringBuilder and its variable name to sb and I'll initialize it. Within the for loop, I'm simply going to add the values to the StringBuilder using sb.append and I'll output states, open bracket, i, open bracket, j.

And then after the for loop I'll use a println command to output the StringBuilder to the console System.out.println. And I'll just refer directly to the StringBuilder object. I'm not quite done yet but let's see the results so far. I'll save and run the application and I'm seeing that I am successfully getting both the state name and its capitol output to the same line. So now I need to add a little bit of conditional logic so I can separate those values.

I'll go to the inner loop and I'll add some conditional code. I'll type if and press control space and choose the if statement and I'll set the condition to if J has a value of zero. So if I'm working on the state name then j will have a value of zero. And before I append the state name, I want to append a little bit of text. So I'll say sb.append, I'm appending a string to the existing StringBuilder, and so now the text should say the capitol of and then the name of the state. Let's check that.

The capitol of California, so now, all I have to do is separate those two values and I'll use an 'else' clause for that. If I'm just about to append the capitol name to the string builder, I'll put in a space the word is and another space and let's see how I'm doing. And now I have sensible output, the capitol California is Sacramento, the capitol of Oregon is Salem and so on.

So I have successfully organized my data into a multidimensional array. Now the strength of this approach is that if you know exactly how much data you're going to be working with, it's a pretty straightforward process to store it in memory and then access it at runtime. If on the other hand you have to be able to expand the array or shrink it at runtime, because you don't know how much data you're going to be dealing with and you can't just declare the array, and then deal with a fixed size, then you should start looking at the array list class instead.

And I'll talk about that in another video.

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This video is part of

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Java Essential Training

71 video lessons · 75603 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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