Arrays, OOP, Recursion & Algorithms
Master the transition from simple programs to handling multiple data values, advanced Object-Oriented concepts, and standard algorithmic problem-solving.
Class 3, Part A: 1D Arrays & Problem Solving ▼
An array is a container object that holds a fixed number of values of a single type. It allows you to store and manipulate multiple data values efficiently.
Declaration, Creation & Initialization
// Declaration & Initialization int[] marks = {75, 82, 65, 90, 55}; // Key Properties int firstMark = marks[0]; // Index starts at 0 int totalStudents = marks.length; // Length is 5 marks[2] = 70; // Updating a value
Traversing Arrays
Standard For Loop
Best when you need the index to update values or track positions.
for(int i=0; i<marks.length; i++)
Enhanced For-Each Loop
Cleaner syntax for reading values sequentially without modifying the array.
for(int mark : marks)
Array Problem Solving Toolkit
Sum & Average
Use an accumulator variable in a loop. Divide sum by .length for average.
Max & Min
Assume the first element is max/min, then update if a larger/smaller value is found.
Count Even/Odd
Use num % 2 == 0 inside a loop with a counter variable.
Reverse Array
Swap elements from the outside in (index 0 with length-1, etc.).
Searching & Sorting Algorithms ▼
Linear Search
Checking every element one by one until the target is found.
for (int i = 0; i < numbers.length; i++) { if (numbers[i] == target) { System.out.println("Found at index: " + i); break; } }
Basic Sorting Logic
Sorting arranges data in a specific order (ascending/descending).
↓ (Compare & Swap adjacent elements)
Sorted: [ 1, 2, 3, 5, 8 ]
Bubble Sort
Repeatedly steps through the list, compares adjacent elements, and swaps them if they are in the wrong order.
Selection Sort
Finds the minimum element from the unsorted part and puts it at the beginning of the sorted part.
2D Arrays & Arrays of Objects ▼
2D Arrays (Matrices)
An array of arrays, organized in rows and columns. Requires nested loops to traverse.
int[][] matrix = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} }; // Nested loops for Matrix Input/Output/Addition for(int i=0; i<matrix.length; i++) { // Rows for(int j=0; j<matrix[i].length; j++) { // Columns System.out.print(matrix[i][j] + " "); } }
Arrays of Objects
This is the crucial bridge connecting Arrays with Object-Oriented Programming (OOP).
Student[] students = new Student[5]; students[0] = new Student("Kamal", 20); students[1] = new Student("Nimal", 21); // Now you can loop through objects! for(Student s : students) { if(s != null) s.display(); }
Part B: Recursion (A Method Calling Itself) ▼
Recursion is a technique where a method solves a problem by calling itself with a smaller version of the problem. It requires a Base Case (to stop) and a Recursive Case (to continue).
Factorial Example
static int factorial(int n) { // Base Case if (n == 0 || n == 1) { return 1; } // Recursive Case return n * factorial(n - 1); }
Visualizing the Stack
↓
5 × factorial(4)
↓
5 × 4 × factorial(3)
↓
5 × 4 × 3 × 2 × factorial(1) → returns 1
↓
Result: 120
StackOverflowError.Practice Problems: Fibonacci sequence, Sum of 1 to n, Reversing a String, Power calculation (x^n).
Class 4, Part A: Inheritance & Polymorphism ▼
Inheritance (The "Is-A" Relationship)
Allows a new class (Child/Subclass) to inherit fields and methods from an existing class (Parent/Superclass) using the extends keyword.
class Person { String name; void displayName() { System.out.println(name); } } class Student extends Person { int marks; // Inherits 'name' and 'displayName()' from Person }
super keyword (calls parent constructor/methods), Access Modifiers (private is hidden, protected is inherited, public is everywhere).Polymorphism (Runtime / Method Overriding)
"Many forms." A parent class reference variable can hold a child class object, and the correct overridden method is called at runtime.
class Animal { void sound() { System.out.println("Animal sound"); } } class Dog extends Animal { @Override void sound() { System.out.println("Bark"); } } class Cat extends Animal { @Override void sound() { System.out.println("Meow"); } } // Runtime Polymorphism Animal a; a = new Dog(); a.sound(); // Output: Bark a = new Cat(); a.sound(); // Output: Meow
Part B: Standard Algorithms (Syllabus Concepts) ▼
The syllabus focuses on the problem-solving concepts behind these classic computer science algorithms.
1. String Matching
Finding a substring within a text. Uses character comparison, .equals(), and .contains().
2. Counting Coins (Greedy)
Making change (e.g., 37 = 25 + 10 + 2). The Greedy Approach always picks the largest possible coin first.
3. Knapsack Problem
Maximizing value without exceeding weight capacity. 0/1 Knapsack (take all or leave) vs Fractional (take parts).
4. Huffman Coding
Data compression. Builds a binary tree based on character frequency, assigning shorter binary codes (0/1) to common characters.
5. Activity Selection
Given tasks with Start/Finish times, find the maximum number of compatible (non-overlapping) activities. (Greedy by finish time).
6. Scheduling
Allocating tasks to resources based on Time, Priority, and Constraints to optimize throughput or minimize wait time.
Final Integrated Project: Student Management System ▼
Combine Arrays, Inheritance, Polymorphism, and Algorithms into one real-world application.
System Architecture
↑ (extends)
class Student (studentId, marks[], methods)
↓ (stored in)
Student[] students (Array of Objects)
Required Methods in Student Class
calculateTotal()
Iterates through the marks[] array to find the sum.
calculateAverage()
Divides total by the length of the marks array.
findGrade()
Uses if/else if logic based on the average.
displayStudent()
Polymorphic method overriding a parent display method.
Main Menu Operations (Algorithmic Logic)
- Add Students: Instantiate objects and store in the array.
- Search Student: Implement Linear Search by ID or Name.
- Sort Students: Use Bubble Sort to arrange the array by highest marks.
- Display Pass/Fail: Loop through the array and apply conditional logic.
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