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Java Code Structures

Every Java application projects are just collection of Java files ( aka Class files ). Understanding the structure of Java file is crucial for understanding Java programming.

In this module, we will cover following topics:

  • Anatomy of Java File
  • Class Declaration
  • Method
  • Executable Code



› Anatomy of Java File

A file with .java extension is a code file where we write our Java code (instructions to computer ).   You will create this code file in the src/main/java folder on your project.  The java code in this file is human-readable and will be compiled into machine code when we turn our project into a program.

Java source file contains following items:

Components Purpose
Class Declaration Defines region of code that belongs to the Java file
Data (Field) List of data available in the class
Methods Contains executable code that achieves specific function

Let's look at the components of the Demo.java file:

These components are present in almost all Java files. So, let's understand them with more details and explanations.



› Class Declaration

Every Java file must contain class declaration. It begins with keyword class followed by the java file name and code block { } that indicates the start and end of the code region. 

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public class Demo {  

}
The code block indicates any code between { and } belongs to the Java file.   Therefore, if you write any java code outside the class code blocks, these code will be treated as invalid, resulting in a programming error.

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public class Demo {
    // code goes here
}
int age = 20;               <-- invalid code
System.out.println(age);    <-- invalid code

Again, it is important to remember that the Java file and class declaration names must be identical.   Otherwise, it will also result in a programming error.

✔️ Java file name and Class declaration name are identical.

❌ Java file name and Class declaration name are different.



› Method

A method is a container for a group of useful executable code.   As an analogy, the button on your TV remote controller is similar to a method.   It groups and hides complicated engineering details as a mere button.

Using just a method name, we can execute often-repeated many lines of code. Some fantastic benefits of the method are:

  • assigning a descriptive name to a group of organized code
  • reduction on repeated lines of code
  • reduction in development complexity
  • update on code happen on a single location
  • we can re-use the method on another programming project

From our example Demo.java file, we can observe the following two methods: 

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public static void main(String[] args) {
    System.out.println(message);
    sleep(1);
    System.out.println(audience);
}
  • the method name is the main
  • it groups and hides the highlighted executable code

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public static void sleep(int second) {
    try {
        Thread.sleep(second * 1000);
    } catch (InterruptedException ignore) {
        // Ignore
    }
}
  • the method name is sleep
  • it groups and hides the highlighted executable code
  • highlighted code instructs CPU to pause for given seconds

Remember that you can create as many non-identical methods in a given Java file. So if you find yourself using the same lines of executable code here and there in your project, it's time to group these lines of code into a method and store it in some Java file (class) for reuse.



› Executable Code

In Java, a single line of code directly executable to the CPU is called statements.   Whether a single statement or multiple statements, it achieves a meaningful task when executed.

Single Statement

When executed, this single statement instructs CPU to print a message "HELLO TESTER!" to the terminal. 

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System.out.print("HELLO TESTER!");

Multiple Statement

When executed, this multiple statement instructs CPU to take a screenshot from a browser. 

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TakesScreenshot tkDriver = null;
tkDriver = (TakesScreenshot) driver;
tkDriver.getScreenshotAs(OutputType.BASE64);

As we went over from previous section, its a good idea to group multiple statements into a method. So that we can use these code by a single method name and reuse it elsewhere in the project. 

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public static void takeScreenshot() {
    TakesScreenshot tkDriver = null;
    tkDriver = (TakesScreenshot) driver;
    tkDriver.getScreenshotAs(OutputType.BASE64);
}

As you can see, whether single or multiple, each statement must end with a semicolon ; to indicate the end of the instruction to the CPU.