Notes

Unit 1-10 Notes

Unit 1: Primitive Types

Key Concepts:

• Variables and data types (int, double, boolean, char)
• Arithmetic operations
• Casting (widening/narrowing)

Example:

int x = 5;
double y = 2.0;
double result = x / y; // result = 2.5

Type casting:

int a = 7;
int b = 2;
double c = (double) a / b; // 3.5

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Unit 2: Using Objects

Key Concepts:

• Object creation using new
• String methods
• Wrapper classes (e.g., Integer, Double)
• Autoboxing/unboxing

Example:

String word = "APCSA";
System.out.println(word.length());       // 5
System.out.println(word.substring(2));   // "CSA"
System.out.println(word.indexOf("C"));   // 2

Wrapper Example:

Integer n = 10;         // autoboxing
int x = n.intValue();   // unboxing

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Unit 3: Boolean Expressions and if Statements

Key Concepts:

• Relational operators: ==, !=, >, <, >=, <=
• Logical operators: &&, ||, !
• Control flow with if, else if, and else

Example:

int age = 16;
if (age >= 18) {
    System.out.println("Adult");
} else {
    System.out.println("Minor");
}

Boolean expression:

boolean valid = !(x < 10 || x > 100);

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Unit 4: Iteration

Key Concepts:

• for, while, and do-while loops
• Loop control: break, continue
• Nested loops

Examples:

for (int i = 0; i < 5; i++) {
    System.out.print(i + " "); // 0 1 2 3 4
}

int i = 0;
while (i < 5) {
    System.out.print(i + " ");
    i++;
}

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Unit 5: Writing Classes

Key Concepts:

• Defining a class and constructor
• Instance variables
• this keyword

Example:

public class Dog {
    private String name;

    public Dog(String name) {
        this.name = name;
    }

    public void bark() {
        System.out.println(name + " says woof!");
    }
}

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Unit 6: Array

Key Concepts:

• Array declaration, instantiation, and indexing
• Iterating through arrays
• Common algorithms (sum, max, etc.)

Example:

int[] scores = {90, 80, 70};
for (int score : scores) {
    System.out.println(score);
}

Find max value:

int max = scores[0];
for (int i = 1; i < scores.length; i++) {
    if (scores[i] > max) max = scores[i];
}

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Unit 7: ArrayList

Key Concepts:

• Importing and using ArrayList
• add(), get(), set(), remove(), size()
• For-each loop limitations

Example:

ArrayList<String> names = new ArrayList<>();
names.add("Alice");
names.add("Bob");
names.set(0, "Ann");
System.out.println(names); // [Ann, Bob]

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Unit 8: 2D Arrays

Key Concepts:

• Declaration and nested loops
• Row-major traversal
• Applications in grid problems

Example:

int[][] grid = {
    {1, 2, 3},
    {4, 5, 6}
};
System.out.println(grid[1][2]); // 6

for (int row = 0; row < grid.length; row++) {
    for (int col = 0; col < grid[row].length; col++) {
        System.out.print(grid[row][col] + " ");
    }
}

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Unit 9: Inheritance

Key Concepts:

• extends keyword
• Method overriding
• super keyword

Example:

public class Animal {
    public void speak() {
        System.out.println("Animal sound");
    }
}

public class Dog extends Animal {
    public void speak() {
        System.out.println("Woof");
    }
}

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Unit 10: Recursion

Key Concepts:

• Base case and recursive call
• Stack behavior
• Tracing recursive calls

Example:

public static int factorial(int n) {
    if (n == 0) return 1;
    return n * factorial(n - 1);
}

String reversal:

public static String reverse(String s) {
    if (s.length() <= 1) return s;
    return reverse(s.substring(1)) + s.charAt(0);
}