Java Fundamentals

JVM and Conditional Statements

Lecture 4

Java Virtual Machine, compilation, execution, and control flow

Summary of the last lecture

We began to see how to use Eclipse
We programmed our first Java classes

How Java handles the produced code execution?

Java environment: the JVM

What is a Virtual Machine?
A virtual machine is a software engine, that can handle the execution of instructions, like an operating system, to ensure the program global execution.
It is like a (virtual) independent operating system, running on the real operating system

Java environment: the JVM (2)

Pro and cons of a Virtual Machine system

Pro : Code portability the main advantage of using a virtual machine system, is to be portable. When you write your code, you don't have to worry about OS specific operations: the JVM makes the abstraction, and you can focus on your business code.
Con : Performance issues the major disadvantage is that the JVM adds a layer to perform the translation of the java code in the OS native instructions. You can have some performance problems in some specific cases. ▸ less sensible since Java 1.6

Java environment: the JVM (3)

From design to compilation

From compiled classes to execution

Summary

Java files .java are compiled into .class
.class files are interpreted by the JVM into native OS operations
The JDK allows us to develop and compile java code. The JDK furnishes a lot of integrated tools to help the Java developer
The JRE can only run already compiled java code: .class files
The JDK contains the JRE, which itself contains the JVM

Compilation and Execution in IntelliJ IDEA (1)

IntelliJ IDEA handles compilation automatically:

IntelliJ builds your project automatically as you type
Compilation output is located in the out/ folder
You can manually trigger a build with Ctrl+F9 (Cmd+F9 on Mac)
Behind the scenes, IntelliJ uses javac (Java compiler):
```
javac Identity.java
```

Compilation and Execution in IntelliJ IDEA (2)

IntelliJ project configuration:

File > Project Structure to configure SDK and dependencies
Project SDK: Choose your Java version (Java 17+ recommended)
Language Level: Select Java language features to use
Libraries: Add external JAR files if needed

IntelliJ intelligently manages your classpath and build configuration

Compilation and Execution in IntelliJ IDEA (3)

Running and debugging your code:

Run: Click the green play button next to your main method, or press Shift+F10
Debug: Click the debug icon (bug symbol) or press Shift+F9
IntelliJ automatically detects classes with main methods
Behind the scenes, it executes the java command:
```
java Identity
```

Your First Executable Program

In order to be executable, your class must have a static main method

Type main and press Tab in IntelliJ - it will auto-complete!

package fr.tbr.exercises;

public class JavaSyntaxDemo {
     private String demoVersion = "1.0_DEV";

     //Constructor
     public JavaSyntaxDemo() {
     }

     public String getDemoVersion(){
        return this.demoVersion;
     }

     public static void main(String[] args){
    	 System.out.println("=> Begin execution");
    	 JavaSyntaxDemo demo = new JavaSyntaxDemo();
    	 System.out.println("Working with version : " + demo.getDemoVersion());
    	 System.out.println("<= Exit");
     }
}

Console applications : `in` and `out`

Getting input from the user :

Scanner scanner = new Scanner(System.in);
try { //this is a try-catch block, we will discuss it further
    int num1 = scanner.nextInt();
    System.out.println("Input 1 accepted");
    int num2 = scanner.nextInt();
    System.out.println("Input 2 accepted");
} catch (InputMismatchException e) {
    System.out.println("Invalid Entry");
}

Printing in the console


System.out.println("Say something in the console");

Exercise: Bank Account System

Initialize programmatically a customer, an account, each of them with concrete values.
Perform a withdrawal, and a interests calculation,
then display the customer and account values (name and balance in the standard output)

Conditional statements in Java : Tests

To test something in Java, you have to use the if(...){ }else{ } statement
The expression in the if parenthesis, must be a boolean

Example :

// A boolean to do the demo
final boolean tester = true;
final boolean anotherTester = false;

/* The if instruction is followed by a parenthesis block, allowing to
 * put the condition. If the condition is matched, then the curly
 * bracket is executed. Else the "else" block is performed */

if (tester) {
    // do something
} else {
    // do something else
}

Conditional statements in Java : Switches

Branching logic : switch - case statement

Example :


int toBeTested = 2;
// Switches can be based on integer values, chars, or enums. Since java
// 7, it can even be based on Strings
switch (toBeTested) {
case 1:
	// treat case one;
	break; // put this break so the case stops
case 2:
	// in this example, this the case we pass into
	break;

default:
	// this is the instruction to perform if no cases were satisfied
	break;
}

Java Fundamentals

JVM and Conditional Statements

Summary of the last lecture

Java environment: the JVM

Java environment: the JVM (2)

Java environment: the JVM (3)

From design to compilation

From compiled classes to execution

Summary

Compilation and Execution in IntelliJ IDEA (1)

Compilation and Execution in IntelliJ IDEA (2)

Compilation and Execution in IntelliJ IDEA (3)

Your First Executable Program

Console applications : `in` and `out`

Exercise: Bank Account System

Exercise 2: Bank Account System, user input

Conditional statements in Java : Tests

Conditional statements in Java : Switches

Exercise 3: Calculate the perimeter and the area of different figures

Slide Overview

Java Fundamentals

JVM and Conditional Statements

Summary of the last lecture

Java environment: the JVM

Java environment: the JVM (2)

Java environment: the JVM (3)

From design to compilation

From compiled classes to execution

Summary

Compilation and Execution in IntelliJ IDEA (1)

Compilation and Execution in IntelliJ IDEA (2)

Compilation and Execution in IntelliJ IDEA (3)

Your First Executable Program

Console applications : in and out

Exercise: Bank Account System

Exercise 2: Bank Account System, user input

Conditional statements in Java : Tests

Conditional statements in Java : Switches

Exercise 3: Calculate the perimeter and the area of different figures

Slide Overview

Console applications : `in` and `out`