Java Fundamental period - Lecture 4

Summary of the last lecture

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

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 : 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 Eclipse (1)

• Eclipse builds the project automatically and incrementally
• Eclipse compilation output is hidden in the package explorer
• Those operations can manually be done by a call to the javac command line tool
• Example :

  javac Identity.java

Compilation and Execution in Eclipse (3)

• Running your project : Right Click (Project) > Run As > Java Application
• Debugging your project : Right Click (Project) > Debug As > Java Application
• Those operations can manually be done by a call to the java command line tool
• Example :

  java Identity.java

Compilation and Execution in Eclipse (4)

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)

Exercise 2: Bank Account System, user input

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;
  }
  
  
  

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

• A triangle
• A circle
• A square