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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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