Week 2 - Variables and Operators

Variables and Operators

Declaring Variables

• Allocate Storage Location of that Data-Type

• Type informs java of possible Values.

  String aString;
  int anInt;
  double a64BitFloat;
  float a32BitFloat;
  char aCharacter;
  

• Note the capital letter in String

  • A string is Special Object in Java
  • OK, so why don’t we have to create strings like we create other objects i.e. String aString = new String("aString");?
    • Java founders determined Strings were so common, Strings needed to be treated differently (i.e. reference the same literal string even if assigned to 2 different variables.
    • Consider the following blocks of code:

      String a = "Next iteration";
      String b = "Next iteration";
      System.out.println("What will this be?" +  ((a == b) ? "True" : "False") ); // ? are they the same ?
      
      for (int i = 0; i < 10; i++) {
        System.out.println("Next iteration"); // If the literal String weren't a reference, this would instantiate the string every single loop.
      }
      

    • Thus, Strings are interned to:
      • Lower memory requirements.
      • Make comparisons faster.
    • BUT WAIT, if strings are shared/cached references, wouldn’t changing the string for variable a also change it for b?
      • Yes, but that’s why the designers also made literal strings immutable. String methods that modify a string will return new string, not the original literal.
      • This is also why you should not compare strings with == but a.equals(b) because == compares the reference while the other compares the string.

• Declare variables of the same type on 1 line!

  int a, b, c = 5; // This is OK!
  

• Variables names must be unique (within its scope), and not a reserved keyword like static, private, public, final, class, etc.

public class Example {
    public static void main(String[] args) {
        double pi = 3.14;
        double pi = 3.141592653;  // Nope, Nope, Nope!

        System.out.println("The value of pi is: " + pi);
    }
}

• Variables are case sensitive

int firstName = 1;
int FirstName = 2;

• Camel Case is Java’s preferred naming convention.

int firstName;

Other Naming Conventions

• Variable names cannot contain certain special symbols, such as exclamation marks (!).
• Spaces are not allowed.
• Numbers can be used within a variable name as long as the name does not begin with a number.
• A variable’s name cannot already be in use.

Excercise

• What is wrong with these variable names?

last day of month = 20
1day = 1952
beware! = 1910
1920 = 1

Assignment - Initialize a Variable

• Stores/Updates variable value, but you must assign the right data-type.

String a = "123";
String a = 123; // Error: wrong type.

• You only have to define the variable type when declaring it, any assignment after must leave off the type.
  • The variable type persists.

    int value = 10;
    value = 4;
    

• Variables of different type(s) can be assinged as long as there is no loss in precision.

  double fpNum = 42.5;
  fpNum = 42;
  

State - Forget diagrams (like in the book) - Use IntelliJ debug

• Why waste your time, use the state diagrams that already exist in your IDE ;-)
• Breakpoints & variable analysis is the modern way to debug variables.

Arithmetic Operators

• Symbols for doing Math
• See Arithmetic Operators
• Type aware, int division is int only result! (i.e. 59/60 == 0)

Operations with Floating Point

• Fractional numbers, with fractional precision, BUT NOT PERFECT!, which leads to rounding errors.
  • Irrational numbers can’t be represented.
  • Nor can repeating fractions.
  • Rounding Errors Explained
• 1 or more double numbers will cause operators to use floating point calculations.
• If you’re equation depends on fractional precision use doubles to avoid 59/60 error, or convert to whole numbers like $$$ libraries do.
• Java will implicitly convert integer assignment to a double, however.

Operators for Strings

• + concatenates
  • Many variables can be joined to a string using the + operator:

String text = "contains text";
int wholeNumber = 123;
double floatingPoint = 3.141592653;
boolean trueOrFalse = true;

System.out.println("Text variable: " + text);
System.out.println("Integer variable: " + wholeNumber);
System.out.println("Floating-point variable: " + floatingPoint);
System.out.println("Boolean: " + trueOrFalse);

Text variable: contains text
Integer variable: 123
Floating-point variable: 3.141592653
Booolean: true

• .equals("compares"); compares!

Errors

• Compile Errors - Syntax errors, Keywords as variable names, etc. causes Parser to barf.
• Runtime Errors - Errors that cause exceptions only when the program is run i.e. FileNotFoundException
• Logic Error - Compiles and runs without errors, but the output is invalid. double percent = 59 / 60;

Show how to Read Strings, Integers, Doubles, and More

import java.util.Scanner;

public class Program {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.println("Write text and press enter ");
        String text = scanner.nextLine();
        System.out.println("You wrote " + text);
    }
}

import java.util.Scanner;

public class Program {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.println("Write a value ");
        int value = Integer.parseInt(scanner.nextLine().trim());
        System.out.println("You wrote " + value);
    }
}

import java.util.Scanner;

public class Program {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.println("Write a value ");
        double value = Double.parseDouble(scanner.nextLine().trim());
        System.out.println("You wrote " + value);
    }
}

Exercise

1. Write a program that asks the user for an integer value. The program should then add 1 to the value provided by the user, and print that new value.
2. Write a program that asks the user for a floating-point number using the variable type Double. The program then prints the user’s input value.
3. Make the following code ouput:

Summary:
    Chicken: 4
    Bacon: 5.600000
    Tractor: John Deer!

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        int chicken = 3;
        double bacon = 5.5;
        String tractor = "None!";
        String summary = """

                Summary:
                    Chicken: %d
                    Bacon: %f
                    Tractor: %s
                """;
        Scanner s = new Scanner(System.in);
        try {
            System.out.printf("Chicken: %d%n", chicken);
            String input = s.nextLine().trim();
            chicken = (input.isEmpty()) ? chicken : Integer.parseInt(input);
            System.out.printf("Bacon: %f%n", bacon);
            input = s.nextLine().trim();
            bacon = (input.isEmpty()) ? bacon : Double.parseDouble(input);
            System.out.printf("Tractor: %s%n", tractor);
            input = s.nextLine().trim();
            tractor = (input.isEmpty()) ? tractor : input;
            System.out.printf(summary, chicken, bacon, tractor);
        } catch (NumberFormatException nfe) {
            System.out.println("Error parsing number.");
        }
    }
}

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