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Repeating code blocks with loops

From: Java Essential Training

Video: Repeating code blocks with loops

The Java language includes a number of syntax styles that allow you to loop through the contents of arrays and other types of data collections. I'll demonstrate four approaches to looping in this project named Loops. The main class in this project has a set of code that's commented out currently, it declares a static variable, an array of strings called months. I'm going to loop through this data and output these strings a number of times using a different syntax style each time. I'll start by uncommenting this code.

Repeating code blocks with loops

The Java language includes a number of syntax styles that allow you to loop through the contents of arrays and other types of data collections. I'll demonstrate four approaches to looping in this project named Loops. The main class in this project has a set of code that's commented out currently, it declares a static variable, an array of strings called months. I'm going to loop through this data and output these strings a number of times using a different syntax style each time. I'll start by uncommenting this code.

I'll select the commented code and then press Ctrl+/ on Windows or Command+/ on Mac and that removes the double slash comments at the beginnings of the lines. Then I'll place the cursor inside the main method. The first type of loop I'll show you uses a numeric counter variable. It's a conventional for loop and the syntax looks exactly the same as it would in JavaScript and other similar languages. I'll start by typing in the word for, and then pressing Ctrl+Space. Eclipse gives me a number of choices and I'll choose the first one, for iterate over array.

This sort of loop uses a temporary variable, sometimes called a counter variable. When you generate the code using Eclipse, it will name the variable i for integer and it will datatype it as an int. Now if you're doing with a fairly small amount of data, you could change the data type to a short or even a byte, but typically ints are used in this case. When you're looping through an array, you should set the initial value of the counter variable i as 0. This is because in Java arrays are 0-based.

The first item is item 0. The second is item 1 and so on. Then in order to make sure that you hit every item in the array, you add a comparison. You look at the counter variable I, and you compare it to the length of the array, as long as i is less than the length, you'll execute any code within the for-loop. But when i matches or exceeds the length, you'll stop, and again, that's because of the 0-based array. Say you have an array of 10 items, if the length is 10 and i starts at 0, then you can keep executing code as long as i is less than 10.

When it actually is 10, it's exceeded the available items in the array. The third part of the for construct is the increment. Most commonly, you'll see this, i++, meaning, increment the value by 1, but you can put in any operation you want here. You could use i--, you could say i+ = 5 and then you would be stepping by 5, and so on. So now I'll place the cursor inside the for-loop, and put in System.out.println and I'm ready to output something.

Now when Eclipse generated the code, it thought I might want to output the values from args, but instead I'm going to output the values for months and that's the array that I declared at the top. And in the print line command I'll output months, open bracket, i, closed bracket, and now I'm going to be outputting the string at that position of the array. I'll Save and Run the code and there is the result. Now to make this a little bit easier to see, I'm going to detach the console right-click and choose Detached, move it up onto the screen and expand it, and now I have a lot more room to see the output from my application.

So that's a for loop with the counter variable. Here's another approach to using a for loop. This is called a for-each, but unlike some C Style languages, you don't actually use the word each. Here is how it actually looks. Once again I'll type in the word for, and press Ctrl+Space and I'll for-each from the list of choices and here's how Eclipse models it. Within the for-loop it declares a variable by default of type string, and it gives it a name. I'm going to change the name from string to month and then it asks what array or collection you want to loop through.

I'm going to be looping through months. So in English this would be read as, for-each month in the month's array, but you declared in Java using a data type variable before the colon and the array or collection you are looping through after the colon. Now I can refer directly to the variable month. So I am going to go make a copy of this code, System.out.println, I'll paste it in, but now I don't need refer to the collection anymore, I'm already doing the iteration, I'll just refer to the variable that's created temporarily each time through the loop.

I'll go up to my original for loop and comment it out and I'll Save and Run the application, and once again there is my output. In most cases with the arrays, you can use either of these two approaches. The only real advantage to using the counter variable, as opposed to the for-each syntax is that you're creating new counter variables, rather than complex objects. Whereas, with the for-each, particularly with the strings and other complex objects, you are creating a new object on each time through the loop. In most environments it really won't matter, and you should use the syntax that you're most comfortable with.

I'll comment this version out, and now we'll move to the while loop. There are two forms of the while loop, the while loop where you put the condition at the front and the while loop where you put the condition at the back. Before you create a while loop, you should declare a variable and give it an initial value, unlike the for-construct, the while construct does not do the variable tracking for you. So I'll create a variable called counter, and set it to a value of 0. Now I'll type in the word of while and I'll press Ctrl+Space and I'll choose while loop with condition.

The while loop with condition takes a look at a Boolean expression and asks should I keep on looping? I'm going to set my condition as counter cursor into the while loop, I'll paste in that code, and I'll change the name of the variable I'm looking at from I, to counter, because that's what I declared it as earlier, but again, the while loop isn't going to increment that value for me, I have to do it myself. So I'll place the cursor after the print line command and increment the value with counter++; I'll save and run the code.

And once again it does exactly the same thing as the for-loops, but with a slightly different syntax. Finally, there is the do-while loop. For the do-while loop you place the condition at the end of the loop. I'm going to make a copy of this code and I'll paste it down at the bottom. I'll comment out the original and now let's create a do-loop. I'll take this while command and I'll move it after the code block. Then I'll the place the cursor at the beginning of the code block and put in the word do, and after the while command, I'll finish with a semicolon.

So now I'm saying start by outputting the value and then increment the value and then do the evaluation rather than doing the evaluation at the beginning. Take a look at what happens. It still works exactly the same. So these are the four possible syntax styles that you can use for looping through arrays and other collections. You can use the counter for loops, you can use for-each syntax, you can use a simple while-loop or you can use a do-while loop. In many cases it's just a matter of program or preference, which syntax you like, or are most comfortable with.

You will find that there are differences in what problems you can solve between say a while and do-while, but for the most part it's your choice.

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This video is part of

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Java Essential Training

71 video lessons · 69638 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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