2. Learning the Basics
Overview
In this chapter, we will be executing programs that do not have the typical linear flow that we have seen so far. You will first learn to use if, else, else if, and switch-case statements to control the flow of your programs. You will practice running for, while, and do-while loops in order to perform repetitive tasks in Java, and how to pass command-line arguments to modify how programs run. By the end of this chapter, you will be able to implement immutable, static (global) variables, alongside Java's variable type inference mechanism.
Introduction
Business applications have lots of special-case conditions. Such conditions may include finding changes in allocation rules starting at a particular year, or handling different types of employees differently based on their designation. To code for such special cases, you will require conditional logic. You basically tell the computer to perform a set of actions when a particular condition is met.
Before we delve into advanced Java topics, you need to know the basics of Java syntax. While some of this material might seem simple, you'll find you need to use the techniques and syntax shown in this chapter repeatedly in your applications.
As you've seen in Chapter 1, Getting Started, Java's syntax borrows heavily from C and C++. That's true for conditional statements that control the flow of your programs as well. Java, like most computer languages, allows you to do this. This chapter covers the basic syntax of the Java language, especially ways in...
Controlling the Flow of Your Programs
Imagine paying a bill from your e-wallet. You will only be able to make the payment if the credit balance in your e-wallet is greater than or equal to the bill amount. The following flowchart shows a simple logic that can be implemented:
Figure 2.1: A representative flow chart for an if-else statement
Here, the credit amount dictates the course of action of the program. To facilitate such scenarios, Java uses the if statement.
With the if statement, your application will execute a block of code if (and only if) a particular condition is true. In the following code, if the happy variable is true, then the block of code immediately following the if statement will execute. If the happy variable is not true, then the block of code immediately following the if statement will not execute.
boolean happy = true;// initialize a Boolean variable as true if (happy) //Checks if happy is true System.out...
Looping and Performing Repetitive Tasks
In this chapter, we cover using loops to perform repetitive tasks. The main types of loop are as follows:
forloopswhileloopsdo-whileloops
for loops repeat a block a set number of times. Use a for loop when you are sure how many iterations you want. A newer form of the for loop iterates over each item in a collection.
while loops execute a block while a given condition is true. When the condition becomes false, the while loop stops. Similarly, do-while loops execute a block and then check a condition. If true, the do-while loop runs the next iteration.
Use while loops if you are unsure how many iterations are required. For example, when searching through data to find a particular element, you normally want to stop when you find it.
Use a do-while loop if you always want to execute the block and only then check if another iteration is needed.
Looping with the for Loop
A for loop executes the same block...
Handling Command-Line Arguments
Command-line arguments are parameters passed to the main() method of your Java program. In each example so far, you've seen that the main() method takes in an array of String values. These are the command-line arguments to the program.
Command-line arguments prove their usefulness by giving you one way of providing inputs to your program. These inputs are part of the command line launching the program, when run from a Terminal shell window.
Exercise 15: Testing Command-Line Arguments
This exercise shows how to pass command-line arguments to a Java program, and also shows how to access those arguments from within your programs:
- Using the techniques from the previous exercises, create a new class named
Exercise15. - Enter the following code:
public class Exercise15 { public static void main(String[] args) { for (int i = 0; i < args.length; i++) { ...
Summary
This chapter covered a lot of Java syntax—things you need to learn to be able to tackle the more advanced topics. You'll find yourself using these techniques in just about every Java application you write.
We started out by controlling the flow of the program using conditional statements such as if, else if, else, and switch statements. We then moved on to the different loops that can be used to perform repetitive tasks. After this, we looked at how to provide values during runtime using command-line arguments. This is one way to pass inputs to your Java applications. Every example in this chapter created a class, but we have not yet done much with these classes.
In the next chapter, you'll learn about classes, methods, and object-oriented programming, and how you can do a lot more with classes.
