U1 Data Types Lesson Notes • 28 min read

Java is used around the world to create applications and is one of the most popular coding languages. The reason Java is so popular is because of it’s security and versatility provided by it’s Object Oriented nature.

1.1 Basics

public class Main {
  int x = 5;

  public static void main(String[] args) {
    Main myObj = new Main();
    System.out.println(myObj.x);
  }
}

Main.main(null);

5

1.2 Variables and Data Types

Variables

A Variable is a name given to a memory location that is holding the specified value. Here are some naming practices:

• Use camel case. likeThis.
• Don’t start with a number.
• Spaces are not allowed.
• No reserved characters, like $, @, and &

Java is a strongly typed language so you always need to declare the type of the variable. Variables can also be declared on their own or in the same line as when they are given a value:

int primitive5;
primitive4 = 1;

//Or...
int primitive6 = 1;

What are the greatest values integers and doubles can store?

Primitive Data

Primitive data determines the size and type of information. Primitive types are the most simple type of variable. They are simply store a short amount of raw data, and are not associated with another class.

Here are the different primitive types:

• byte: An 8-bit signed two’s complement integer.
• short: A 16-bit signed two’s complement integer.
• int: A 32-bit signed two’s complement integer.
• long: A 64-bit signed two’s complement integer.
• float: A single-precision 32-bit IEEE 754 floating point.
• double: A double-precision 64-bit IEEE 754 floating point.
• boolean: Stores either true or false.
• char: Stores a single character.

For this class you need to know:

boolean primitive3 = true; //Stores a true of false binary value
int primitive1 = 0; //Whole number
double primitive2 = 1.1; //Decimal values. Floating point numbers.
char primitive4 = 'a'; //Single character

Data Type	Size (bits)
boolean	8
int	32
double	64
char	16

public class GreatestValue {
    public static void main(String[] args) {
        System.out.println(Integer.MAX_VALUE);
        System.out.println(Integer.MIN_VALUE);
        System.out.println(Double.MAX_VALUE);
        System.out.println(Double.MIN_VALUE);
    }
}
GreatestValue.main(null);

2147483647
-2147483648
1.7976931348623157E308
4.9E-324

Reference Types

Some data types, like String, start with a capital letter. This is because they are not primiative, but are refrence types. They are called this because they refrence an object.

“A reference type is a code object that is not stored directly where it is created, but that acts as a kind of pointer to a value stored elsewhere.”

int integer = 7120; //"int" starts with a lowercase
String string = "abc"; //"String" starts with an uppercase, because it is an object and not a primitive type

All Reference Types Reference Objects: String Example

String is the most common reference type. Here is an example of how a String type is really just referencing an object.

public class WorseString {
    private char[] charArray;

    public WorseString(String inputString) {
        this.charArray = inputString.toCharArray();
    }

    public String getChars() {
        return new String(this.charArray);
    }

    @Override
    public String toString() {
        return getChars();
    }
}

WorseString string = new WorseString("Hello, World!");
System.out.println(string);

Hello, World!

Therefore, these two things are the same:

String string = "abc";
String string = new String("abc");

Literal vs String literal

• Literal: Source code representation of a fixed value — 3
• String Literal: In double quotes, a String — “3”

1.3 Expressions and Assignment Statements

Calculations and evaluating arithmetic statements is important when coding to create algorithms and other code. Make sure you are doing arithmetic statements with int or double values and not String literals

Operators

Operator	Example Equation	Output	Use
+	5 + 3	8	Add numbers together.
-	5 - 3	2	Subtract one number from another.
*	5 * 3	15	Multiply numbers together.
/	5 / 3	1	Divide one number by another.
/ (with double)	5 / 3.0	1.67	Divide one number by another.
%	5 % 3	2	Find the remainder of a division operation.

Tip: In the AP subset, you only have to worry about operations with int values. However, it’s good to know how to use arithmetic statements with doubles and other types.

If you do an operation with two ints or doubles, it returns the respective type. If you mix types, Java returns the one with more bits, a double in this case.

Modulus

Modulus gets the remainder if you were to divide two numbers. One common use is to find odd/even numbers.

• 5 % 2 = 1
• 100 % 10 = 0

You try:

• 8 % 3 = 2
• 4 % 1 = 0

Modulus joins multiplication and division in the order of operations

Assignment Operator

= is called the assignment operator because it is used to assign a value to a variable. It is the last in the order of operations.

Casting

Casting is converting one type of variable to another ex: double to int

public class Casting {
    public static void main(String[] args) {
        double castTest = 3.2;
        System.out.println((int) castTest); // this is casting syntax
        castTest = 3.7;
        System.out.println((int) castTest);
        System.out.println((int) (castTest+0.5));

        int castTest2 = 3;
        System.out.println(castTest2/2);
        System.out.println(castTest2/2.0);
    }
}
Casting.main(null);

3
3
4
1
1.5

What will this output?

int castTest2 = 7;
System.out.println(castTest2/3);
System.out.println((int) (castTest2+0.5));

2
7

Wrapper Classes

For many operations in Java, you need to have a class. Some examples are:

• ArrayLists or HashMaps
• If you require nullability (meaning the value could be null)
• Generics
• Methods that require objects as input

To accomplish this, we use a wrapper class. A wrapper class is essentially a class which ‘wraps’ the primitive type and makes it into an object rather than a primitive.

What is a downside of using wrapper classes? Why not always use them?

Wrapper classes take more memory to use, but this is extremely negligable.

Increased memory usage

//This code fails
ArrayList ArrayList = new ArrayList<int>();

//This code works
ArrayList ArrayList = new ArrayList<Integer>();

public class Wrappers {
    Integer ageWrapper = 17;
    int age = Integer.parseInt("17");
    String gpaString = "3.9";
    double gpaDouble = Double.parseDouble(gpaString);

    public static void main(String[] args) {
        Wrappers wrapper = new Wrappers();
        System.out.println(wrapper.ageWrapper);
        System.out.println(wrapper.age);
        System.out.println(wrapper.gpaDouble);
    }
}
Wrappers.main(null);

17
17
3.9

How do you complete this output so that it outputs an integer?

String grade = "95";
System.out.println(Integer.parseInt(grade));

95

How do you complete this output so that it outputs a double?

String grade = "95.5";
System.out.println(Double.parseDouble(grade));

95.5

Enums

What are they?

Enums are a type of data, which allows a variable to be a predetermined set of values

Uses

• Examples: days of the week

Things you can do with Enums

• ordinal
• switch
• for loops

public class EnumTest { 
    enum Units {
    PRIMITVE_DATA_TYPES,
    CLASSES,
    BOOLEAN,
    ITERATION,
    WRITING_CLASSES,
    ARRAY,
    ARRAY_LIST,
    TWO_DIMENSIONAL_ARRAY,
    INHERITANCE,
    RECURSION;
}
public static void main(String[] args) { 
  System.out.println("What is the third unit in AP CSA? Answer: " + Units.BOOLEAN);
  Units classUnit = Units.CLASSES;
  System.out.println("What is the unit is Classes in AP CSA? Answer: " + (classUnit.ordinal() + 1));
  Units selectedUnit = Units.ARRAY_LIST;

  switch(selectedUnit) {
    case PRIMITVE_DATA_TYPES:
      System.out.println("The selected unit is: primitive data types");
      break;
    case BOOLEAN:
       System.out.println("The selected unit is: boolean");
      break;
    case CLASSES:
      System.out.println("The selected unit is: classes");
      break;
    case ITERATION:
      System.out.println("The selected unit is: iteration");
      break;
    case WRITING_CLASSES:
      System.out.println("The selected unit is: writing classes");
      break;
    case ARRAY:
      System.out.println("The selected unit is: array");
      break;
    case ARRAY_LIST:
      System.out.println("The selected unit is: array list");
      break;
    case TWO_DIMENSIONAL_ARRAY:
      System.out.println("The selected unit is: 2d array");
      break;
    case INHERITANCE:
      System.out.println("The selected unit is: inheritance");
      break;
    case RECURSION:
      System.out.println("The selected unit is: recursion");
      break;
  }
  for (Units allUnits: Units.values()) {
    System.out.println(allUnits);
  }
} 
}
EnumTest.main(null);

What is the third unit in AP CSA? Answer: BOOLEAN
What is the unit is Classes in AP CSA? Answer: 2
The selected unit is: array list
PRIMITVE_DATA_TYPES
CLASSES
BOOLEAN
ITERATION
WRITING_CLASSES
ARRAY
ARRAY_LIST
TWO_DIMENSIONAL_ARRAY
INHERITANCE
RECURSION

Homework

All of your homework is on this form. (Link is https://forms.gle/M6FgxZwX1AnWdZmL9)

Long Answer Questions

Question 1

Create a program using enums and a switch statement. The values in the enum are the days of the week. If it is Tuesday, then print, “Today is Tuesday!”.

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

    public static void tuesdayChecker(Day day) {
        switch (day) {
            case TUESDAY:
                System.out.println("Today is Tuesday!");
                break;
            default:
                System.out.println("Today is not Tuesday!");
                break;
        }
    }

    public static void main(String[] args) {
        // test for if it's a tuesday
        tuesdayChecker(Day.TUESDAY);

        // test for if it's not a tuesday
        tuesdayChecker(Day.WEDNESDAY);
    }
}

tuesdayAnalysis.main(null)

Today is Tuesday!
Today is not Tuesday!

Question 2

Create a calculator program that uses both int and double types depending on user input. Include at least 3 operations.

public class SpecialCalculator {
    public static Number operationSelection(double num1, double num2, int operation, int intOrDouble) {
        if (intOrDouble == 1) { // int operations with int variables
            switch (operation) {
                case 1:
                    Number intSum = (int) num1 + (int) num2;
                    return intSum;
                case 2:
                    Number intDifference = (int) num1 - (int) num2;
                    return intDifference;
                case 3:
                    Number intProduct = (int) num1 * (int) num2;
                    return intProduct;
                case 4:
                    Number intQuotient = (int) num1 / (int) num2;
                    return intQuotient;
                default:
                    System.out.println("Invalid input.");
                    return 0;
            }
        } else { // double operations with double variables
            switch (operation) {
                case 1:
                    Number doubleSum = num1 + num2;
                    return doubleSum;
                case 2:
                    Number doubleDifference = num1 - num2;
                    return doubleDifference;
                case 3:
                    Number doubleProduct = num1 * num2;
                    return doubleProduct;
                case 4:
                    Number doubleQuotient = num1 / num2;
                    return doubleQuotient;
                default:
                    System.out.println("Invalid input.");
                    return 0.0;
            }
        }
    }

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

        System.out.println("Would you like to perform integer operations or double operations?");
        System.out.println("1. Integer");
        System.out.println("2. Double");
        int typeChoice = scanner.nextInt();

        System.out.println("\nChoose an operation:");
        System.out.println("1. Addition");
        System.out.println("2. Subtraction");
        System.out.println("3. Multiplication");
        System.out.println("4. Division");
        int operationChoice = scanner.nextInt();

        System.out.println("\nEnter the first number:");
        double num1 = scanner.nextDouble(); // start as doubles, but converted if
        System.out.println("Enter the second number:");
        double num2 = scanner.nextDouble();

        Number result = operationSelection(num1, num2, operationChoice, typeChoice);
        System.out.println("\nResult: " + result);

        scanner.close();
    }
}

SpecialCalculator.main(null);

Would you like to perform integer operations or double operations?
1. Integer
2. Double



Choose an operation:
1. Addition
2. Subtraction
3. Multiplication
4. Division

Enter the first number:
Enter the second number:

Result: 2

Additions From Class

Here’s the overflow demonstration with int:

int max = Integer.MAX_VALUE;
int overflow = max + 1;
System.out.println(overflow);

-2147483648