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Writing conditional code

From: Java Essential Training

Video: Writing conditional code

Java supports a number of constructs that allow you to easily control the flow of your application. Conditional Blocks, Switch Statements, Loops, and so on. I'm going to start in this chapter by showing you how to build a basic conditional statement using the keyword if. I am working in a project named Conditional that's part of the Chapter 05 Exercise Files, but it just has an empty class with the basic Main method. I'll place the cursor inside the Main method and I am going to start by declaring an integer variable.

Writing conditional code

Java supports a number of constructs that allow you to easily control the flow of your application. Conditional Blocks, Switch Statements, Loops, and so on. I'm going to start in this chapter by showing you how to build a basic conditional statement using the keyword if. I am working in a project named Conditional that's part of the Chapter 05 Exercise Files, but it just has an empty class with the basic Main method. I'll place the cursor inside the Main method and I am going to start by declaring an integer variable.

I'll give it a data type of int and a name of monthNumber and I'll set it to a value initially of 3. Now I am going to evaluate that numeric value and ask the question, does this month number fall in the range of 1 to 3. In Java the conditional blocks starts with the keyword if. You then place a Boolean condition that is a condition that evaluates to true or false inside of set of parenthesis, and then within the pair of braces, you put in the code you want to execute if the condition is true.

Now rather than hand coding all that, Eclipse will the model the code for you. After typing the word if, without any spaces after it press Ctrl+Spacebar and you will get a list of available code templates. Choose the first item in the list, if statement, an Eclipse fills out the rest of the conditional block, the required parenthesis around the Boolean condition and the braces in which you will place the executable code. Now the question I want to ask is does the month number fall between 1 and 3. So I am going to put in a complex condition here.

I'll start off with a Boolean expression of monthNumber>=1, and then I'll use the double ampersand which is the conditional end and I'll complete the expression with monthNumber <=3 Notice that as I type there is a little rectangle that's wrapping the condition. When I am done typing in the condition I can press the tab key and the cursor will drop between the braces and I can start writing the executable code. Within the if clause, I'll use System.out.println.

and I'll out put the string "You're in Quarter 1." I'll save the changes and Run the application and I should see the output "You're in Quarter 1", so it's working. Now once you've used an if clause, you can also use one or more else-ifs. The structure of else-if is the same as an if, but it can only follow in an initial if. So I am going to make a copy of this code block and I'll paste it in to place, and I'll prefix the next one with else. I'll change the numbers that I am evaluating to 4 and 6 and I'll change the Quarter output to Quarter 2.

I'll Run the code again and I am in still Quarter 1, but then I'll change the month number at the top of the code to month number 5 and Run it again, and I am told you are in Quarter 2. Now again, you can have as many else- ifs as you want, and then optionally you can have an else, which is the catcher, the code that will execute if none of the previous conditions are true. The else looks like this, it doesn't take any parenthesis or conditions, but it does take its own code block, and here I'll output of string saying, "You are not in the first half of the year!" Now that code is getting a little wide, so let me maximize my code editor and we can see all three conditions.

One for Quarter 1, one for Quarter 2, and finally, one for all of the other possible numeric values. I'll Save and Run the application and I am still in Quarter 2, but now I'll change the monthNumber variable to say number 8 for August and Run again and I get the output "You're not in the first half of the year!" So that's basically the structure of an if statement. Now when you are comparing values and you are working with primitive data types, you can use these mathematically quality operators >=, <= and so on.

But as I previously described, if you are working with strings, you have to use methods of the String class to do comparisons. Let's create another class. I'll go to my package explorer to the default package, I'll right click and create a New class, and I'll name this CompareStrings. It's going to be its own little application, so I'll check the main method and click finish. I am going to get ride of all these extra comments that Eclipse puts in, and now within the Main method I'll declare a String variable named month and I'll give it an initial value of "February".

Now I am going to compare that string value to another string. So, once again I'll put in an if clause typing the word if and pressing control space and pressing Enter or Return and then I'll look for the string February like this. if month.equals("February") then I say, System.out.println ("It's the second month!"). I'll Save and Run this application and I get the output. The mechanism of comparing values will shift from one data type to another.

For primitive values, you can use the mathematical equality operators. For Strings you have to use methods of the String class like equals or contains, but the structure of the if, else-if, and else statements will stay the same. Regardless of the data type that you are comparing, you can use the if, else-if, and else clauses to determine whether you would want to execute particular code, all depending on the current condition.

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This video is part of

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Java Essential Training

71 video lessons · 68928 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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