Java Fundamentals

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;
  }
  
  
  

Enums: Fixed Sets of Constants

An enum (enumeration) defines a fixed set of named constants:

public enum Day {
    MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
}

public enum AccountType {
    SAVINGS, CHECKING, INVESTMENT
}

// Usage
Day today = Day.MONDAY;
AccountType type = AccountType.SAVINGS;

• Better than constants: Type-safe, auto-complete support
• Better than Strings: Can't misspell, compiler catches errors

Enums with Switch Statements

Enums work perfectly with switch statements:

public enum TrafficLight {
    RED, YELLOW, GREEN
}

public void handleLight(TrafficLight light) {
    switch (light) {
        case RED:
            System.out.println("Stop!");
            break;
        case YELLOW:
            System.out.println("Caution - prepare to stop");
            break;
        case GREEN:
            System.out.println("Go!");
            break;
    }
}

Enums with Values

Enums can have fields, constructors, and methods:

public enum AccountType {
    SAVINGS(0.02),    // 2% interest
    CHECKING(0.0),    // 0% interest
    INVESTMENT(0.05); // 5% interest

    private final double interestRate;

    AccountType(double rate) {
        this.interestRate = rate;
    }

    public double getInterestRate() {
        return interestRate;
    }
}

// Usage
AccountType type = AccountType.SAVINGS;
double rate = type.getInterestRate();  // 0.02

Debugging: Finding and Fixing Bugs

Debugging is the process of finding and fixing errors in your code.

• Print debugging: Adding System.out.println() statements
  • Quick but messy - must remove after
• IDE Debugger: Step through code line by line
  • Professional approach - inspect variables, call stack

Setting Breakpoints

A breakpoint tells the debugger to pause execution at a specific line:

• To set: Click in the left margin next to a line number (red dot appears)
• To remove: Click the red dot again
• To run in debug mode: Press Shift+F9 (not F10!)

public void calculateInterest(double balance) {
    double rate = 0.05;
    double interest = balance * rate;  // <-- Set breakpoint here
    System.out.println("Interest: " + interest);
}

When execution reaches the breakpoint, the program pauses and you can inspect values.

Debugger Controls

Once paused at a breakpoint, use these controls:

Variables panel: Shows all variable values at current point

Exercise: Debug This Code

The following code has a bug. Use the debugger to find it:

public class BuggyCalculator {
    public static double calculateAverage(int[] numbers) {
        int sum = 0;
        for (int i = 0; i <= numbers.length; i++) {  // Bug here!
            sum += numbers[i];
        }
        return sum / numbers.length;
    }

    public static void main(String[] args) {
        int[] grades = {85, 90, 78, 92, 88};
        double avg = calculateAverage(grades);
        System.out.println("Average: " + avg);
    }
}

1. Set a breakpoint inside the for loop
2. Run in debug mode (Shift+F9)
3. Step through and watch the value of i
4. What happens when i reaches 5?

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

• A triangle
• A circle
• A square