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CS101

Course Content and Resources for CS101, sponsored by Heights Philadelphia, taught through Arcadia University.

Arcadia Course Canvas Link

Topic 13: String Methods

Associated Lab: lab15_StringComparison

Always use .equals() to compare Strings in Java — never ==. == compares memory references, not text content.

Common String methods:

 s.equals("hi")           // true if same text
 s.equalsIgnoreCase("Hi") // ignores case
 s.length()               // number of characters
 s.isEmpty()              // true if ""
 s.contains("fox")        // true if found
 s.startsWith("The")      // true if starts with
 s.endsWith("dog")        // true if ends with
 s.toUpperCase()          // "HELLO"
 s.toLowerCase()          // "hello"
 s.replace("a", "b")     // replace all "a" with "b"
 s.trim()                 // remove leading/trailing spaces
 String[] parts = s.split(","); // split into array

Note: String methods return new Strings — they do not change the original.

Topic 12: ArrayList, File Scanner

Associated Labs: lab13_ArrayList and lab14_FileScanner

An ArrayList is like an array, but it can grow and shrink in size.

 import java.util.ArrayList;
 ArrayList<String> names = new ArrayList<String>();

Note: ArrayLists use object types (String, Integer, Double), not primitives (int, double).

Common methods:

 names.add("Alice");       // add to the end
 names.get(0);             // access by index
 names.set(0, "Bob");      // change an element
 names.remove("Alice");    // remove by value
 names.size();             // number of elements
 names.contains("Bob");    // check if element exists

A Scanner can read from a file just like it reads from System.in:

 import java.io.File;
 File file = new File("data.txt");
 Scanner fileScanner = new Scanner(file);
 while (fileScanner.hasNextLine()) {
    String line = fileScanner.nextLine();
 }
 fileScanner.close();

Topic 11: Arrays

Associated Labs: lab11_Arrays and lab12_Arrays2d

An array is a variable that holds multiple values of the same type.

To create an array with values already known:

 int[] numbers = {10, 20, 30, 40, 50};

To create an empty array of a specific size:

 int[] numbers = new int[5];

Access or modify elements using their index (starting at 0):

 System.out.println(numbers[0]); // prints the first element
 numbers[1] = 99;                // changes the second element

Use a for-each loop to iterate over elements:

 for (int num : numbers) {
    System.out.println(num);
 }

Multidimensional Arrays

A multidimensional array is an array of arrays — it lets you store data in a grid (rows and columns).

To create a 2D array:

 int[][] grid = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
 };

To create an empty 2D array (3 rows, 4 columns):

 int[][] grid = new int[3][4];

Access elements using two indices — [row][column]:

 System.out.println(grid[0][2]); // row 0, column 2 → prints 3
 grid[1][0] = 99;               // changes row 1, column 0

Topic 10: Methods

Associated Lab: lab10Methods

Methods are a named, reusable block of code that performs a specific task.

Java methods use the syntax:

 public static <returnType> <methodName>(<parameters>) {
   // code that runs when the method is called
 }
  • returnType is the data type the method sends back (e.g. int, String, boolean). Use void if the method returns nothing.
  • parameters are inputs the method receives, each with a type and name, separated by commas. A method does not need to have parameters.

For example, a method named "add" that that takes two integers, a and b, as parameters and returns their sum as an int:

 public static int add(int a, int b) {
    return a + b;
 }

You call a method by placing parenthesis to the right of the method. Inside the parenthesis, add comma separated arguments matching the type(s) of the method's parameters.

 int result = add(3, 5); // Passing in the arguments 3 and 5

In this example, the return value from the method is assigned to result

Topic 9: Scope

Topic 8: While Loops

Associated Lab: Lab8: While Loops

Java while loops use the syntax:

 while (<condition>) {
   // code runs repeatedly as long as <condition> is true
 }
  • The condition is checked before each iteration. If it is true, the block of code will run once and then check condition again, continuing to run as long as condition is true. If condition is false from the start, the loop body never runs.

For example, to print the numbers from 1 to 5:

 int i = 1;
 while (i <= 5) {
    System.out.println(i);
    i++;
 }

A do-while loop runs the body at least once, then checks the condition:

 do {
   // code runs at least once
 } while (<condition>);

Topic 7: For Loops

Associated Lab: Lab7: For Loops

Java for-loops use the syntax:

 for (<start>; <stop>; <how>) {
   // code is run from <start> to <stop>, incrementing by <how>
 }
  • For the start, initialize a variable to some integer
  • For the stop, create a condition which defines when the for loop stops running
  • For the how, define how the variable that you initialized in the start is going to be updated each loop

For example, to print the numbers from 3 to 12, incrementing by 3 each time, you could use:

 for (int n = 3; n <= 12; n+=3) {
    System.out.println(n);
 }

Topic 6: Compiled vs Interpreted

Associated Lab: Lab6-Compiled-Interpreted-Handout

Topic 5: Booleans, Conditionals

Associated Lab: lab5_booleans_conditionals

Topic 4: User CLI Input and Type Casting/Conversion/Parsing

Associated Lab: lab4_input_and_casting

Topic 3: Java Variables & Arithmetic

Associated Lab: lab3_variables

Topic 2: Java Print Statements

Associated Lab: lab2_java_print_statements

The basic boilerplate for a java class is:

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

The name of the file must match the name of the class. In this case it must be helloWorld.java.

Topic 1: Command Line (CLI)

Associated Lab: lab1_shell_script

Tips:

  • Be very careful with the rm command. There is no way to undo! It is easy to accidently delete important files forever.
  • If you get stuck in a weird interface, try:
    • Hit Enter a few times
    • Hit CTRL - c or CTRL - d
    • Hit q
    • (last resort) Close window and start over

Other Resources

Touch Typing

Integrated Development Environments (IDEs) for Java

  • VS Code with Java Extensions - Free lightweight editor with plugins for (almost) everything
  • Harvard CS50 Codespaces - Online version of VS Code accessible via browser (connects to remote machine)
    • Purposefully does NOT have tab-completion
    • Includes CS50s Duck Debugger, a free ChatGPT powered AI that will guide your learning, but will not write code for you.
  • Intellij IDEA - Professional level IDE; No additional setup required for Java development
    • Requires significant RAM and may be too slow on old machines
  • Eclipse IDE - Similar to Intellij, with slightly smaller RAM footprint and older-looking interface

Windows Intellij IDEA Setup

Windows Tips for Reducing Memory load

  • Uninstall ubuntu
    • Use windows key, search "Ubuntu", select "Uninstall"
    • Open the app Powershell as an Administrator
    • Run commands
      • wsl --unregister Ubuntu
      • wsl --uninstall
  • Uninstall the follow apps
    • Microsoft 365 Copilot

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