Unit 1 - Hello World and Primitive Data Types

Unit 1 - Hello World and Primitive Data Types#

The assumption is that you have your development environment set up by now.

Hello World#

Your first task is to create a file Main.java in VScode and run the following code :

public class Main{
    public static void main(String[] args) {
    System.out.println("Hello World");
    }
}

Play around with the code and explore the outputs. Some things to try:

  • Write multiple print statements

  • Compare the difference between the commands System.out.print and System.out.println

  • Try printing numbers and/or characters

  • Try ommiting the quotations

  • Try single quotes ' instead of double quotes "

Errors#

If you tried running the following code block, you probably saw an error message in your development environment.

public class Main{
    public static void main(String[] args) {
    System.out.println('Hello World');
    }
}

Errors are nothing to be afraid of in coding and are common occurences. There are three types of errors you will encounter:

  • Syntax

  • Runtime

  • Logic

Syntax Errors#

  • Bad grammar in Java

    • Misspelled words

    • Incorrect case

    • Missing semicolons

  • Java does not understand.

E.g.:

  1. Svstem.out.pruntln("");

  2. system.out.println("");

  3. public static main(String[] arqs) {

✅ Error message
❌ Program does not run

Runtime Errors#

  • Program syntax is correct, but program asks computer to do something impossible or not allowed.

  • Program runs but ends with an error.

E.g.:

int n = reader.nextInt();
int x = 100 / n;
// Error when n == 0

✅ Error message after stopping
❌ Program stops

Logical Errors (bugs)#

  • Program does not do what was intended.

  • Can be hard to find.

E.g.:

public static boolean isEven(int n) {
    return n % 2 == 1;
}

❌ No Error message
✅ Program runs

Avoiding Errors#

  • IDE (don’t count on it!)

  • Comments (good for spotting logic errors)

  • Psuedocode (can ensure your logic is correct and help avoid runtime errors)

  • Readable code (can prevent syntax errors)

  • Practice (even experience programmers still get errors)

Comments#

  • Single line comments:

    // this is a single-line comment

  • Multi-line (or C-style) comments:

    /* this is a 
multi-line comment */

Variables and Data Types#

  • A variable is a named memory location that holds a data value.

  • Variables have a data type (e.g., int, double, boolean).

  • To declare a variable, specify the type, a name, and end with a semicolon (;).

  • Example: int number;

  • You can change the value of a variable throughout the program.

  • Declaring a variable as final makes its value unchangeable.

Data Types#

  • Primitive (This chapter)

    • Boolean

      • boolean

    • Numeric

      • Character

        • char

      • Numerical

        • Integer

          • byte

          • short

          • int

          • long

        • Floating-point

          • float

          • double

  • Non-Primitive (Future Chapters)

    • String

    • Array

    • etc.

Expressions and Assignment Statements#

When assigning values to variables in Java, we use the = sign. The right side of the = is the value or expression, and the left side is the variable that will store the value.

Example:#

int num = 200;

  • Data type: int (integer)

  • Literal: 200

A literal is a fixed value directly represented in the code without any computation.

Notes:#

  • Literals can be assigned to any primitive type variable.

  • Literals can be in different formats: base 10, base 16 (hexadecimal), base 8 (octal), and string.

Arithmetic Operators#

Java supports various arithmetic operations that can be performed on numerical values.

The arithmetic operators consist of:#

  • + (Addition)

  • - (Subtraction)

  • * (Multiplication)

  • / (Division)

  • % (Remainder, also known as modulo)

You can use these operators when assigning values to variables.

Example:#

int sum = 5 + 3;
int difference = 10 - 4;
int product = 2 * 3;
int quotient = 10 / 2;
int remainder = 10 % 3;

Compound Assignment Operators#

Java provides compound assignment operators that combine an arithmetic operation with assignment.

Common compound operators:#

  • +=, -=, *=, /=, %=

Increment and Decrement Operators:#

  • ++ (Increment): Adds 1 to the stored value.

  • -- (Decrement): Subtracts 1 from the stored value.

These operators can be used on variables to modify their values directly.

Example:#

int num = 5;
num += 3; // num is now 8
num++;    // num is now 9
num -= 2; // num is now 7

Casting and Ranges of Variables#

Type casting is when you assign a value of one primitive data type to another type.

Widening Casting (Automatic):#

  • Converting a smaller type to a larger size type automatically.

  • Order: byte -> short -> char -> int -> long -> float -> double

Example:#

int myInt = 9;
double myDouble = myInt; // Automatic casting: int to double

Narrowing Casting (Manual):#

  • Converting a larger type to a smaller size type.

  • Order: double -> float -> long -> int -> char -> short -> byte

Example:#

double myDouble = 9.78;
int myInt = (int) myDouble; // Manual casting: double to int

Escape Sequences#

Escape sequences are combinations of the backslash (\) and another character, which send special instructions to the compiler.

Common escape sequences:#

  • \n – New line

  • \" – Quotation mark

  • \t – Tab

  • \\ – Backslash

Example:#

System.out.println("Hello\nWorld"); // Prints "Hello" on one line and "World" on the next

Exercises#

All questions should be done in the same program file. Please try and complete the lab without help from online resources!

For more questions and a unit summary please visit CSAwesome

  1. Multiline Comment
    Using a multiline comment (/* to start the comment and */ to end the comment), write the name of the file, your name, and the date on separate lines.
    On ALL of the questions in this homework, use single-line comments (//) to state which question you are answering.

  2. Hello World Program
    Create a “Hello World” program as done in the beginning of this chapter.
    Print a new line with the words “My name is “, then using a new print statement, print your name on the same line (total of 3 print statements).

    The output should look like this:

    Hello World
My name is Your Name

  3. Variable Declaration
    Declare four variables, using logical naming for all variables:

    • One of type String, assign your name to this variable.

    • One of type int, assign your age to this variable.

    • One of type double, assign to this variable a number between 5 and 6.

    • One of type boolean, its value should be true if this homework is hard and false if this homework is easy. (Assign the value based on what you think of the homework now, we will change this value throughout this homework).

  4. Arithmetic Operations
    Using the variable of type double from the previous question, use arithmetic operations and assignment operators to change the value of the variable to the number 42.2.
    Try using the variables you already made to do these operations. For example:

    int a = 56;
double b = 5.5;
b += a;
b -= 20;
b = b + 0.7; // if you print b it will be equal to 42.2

  5. Repeat Arithmetic Operations
    Repeat question 4 two more times with your original numbers. Get creative! (Can you do it in only 1 line of code? Can you do it with the help of the % operator? Can you use all operators and assignment operators?)

  6. Casting and Rounding
    Use casting methods to assign the value of your variable of type double to a variable of type int. Do this twice:

    • The first time, round the value to 42.

    • The second time, round it to 43.

    For example:

    double a = 27.2;
int b, c;
b = (int) (a); // b will equal 27
c = (int) (a + 1); // c will equal 28

  7. Casting with Arithmetic
    Use casting methods and arithmetic operators to create a new variable of type double for your age using the variable of type int for your age. (What happens when we give the two variables the same name?)

    For example:

    int iddoAge = 23;
double iddosAge = (double) (iddoAge) + 0.75; // I am born in October so I am ¾ way to my 24th
// iddosAge should now be 23.75

  8. Boolean Update
    Was this homework difficult? Change the value of the boolean variable we initialized earlier to reflect your feelings.

  9. BONUS (will be part of Unit 2 Homework Lab)
    In your program, output the following with the help of escape sequences:

    Pikachu welcomes you to the world of Pokemon!
(\__/)
(o^.^)
z(_(“)(“)

    (Question taken from TEALS program)

Contents