DSA Day 1: Getting Started and Learning the Basics of Java

Welcome to Day 1 of my Data Structures and Algorithms (DSA) journey! Today, I am laying down the ultimate foundation. I will cover all the core basics of Java that you need to be familiar with before solving complex algorithms.

This guide maps directly to the standard topics for Step 1.1: Things to know in Java (similar to C++ basics).

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1. Input & Output in Java

To interact with console inputs and outputs, Java offers standard utilities.

Standard Output

System.out.print() outputs text, while System.out.println() outputs text and moves the cursor to a new line.

Standard Input (Scanner Class)

The Scanner class in java.util package is commonly used to read different types of inputs.

import java.util.Scanner;

public class BasicIO {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        
        System.out.print("Enter an integer: ");
        int x = sc.nextInt();
        
        System.out.print("Enter a string (single word): ");
        String str = sc.next();
        
        System.out.println("Output: " + x + " and " + str);
        sc.close();
    }
}

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2. Java Basics & Data Types

Java is a statically typed language. Every variable must be declared with a data type.

Type	Size	Range / Description
int	4 bytes	$-2^{31}$ to $2^{31}-1$
long	8 bytes	$-2^{63}$ to $2^{63}-1$ (used for large values)
float	4 bytes	Single-precision fractional numbers
double	8 bytes	Double-precision fractional numbers (preferred for precision)
char	2 bytes	Single character (e.g., 'a', '5')
boolean	1 bit	true or false

Note: In competitive programming, be mindful of overflows. If the answer can exceed $2 \times 10^9$, use long instead of int.

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3. If-Else / Conditional Statements

Conditional statements are used to make decisions in your code.

int marks = 85;

if (marks >= 90) {
    System.out.println("Grade A");
} else if (marks >= 80) {
    System.out.println("Grade B");
} else {
    System.out.println("Grade C");
}

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4. Switch Case

The switch statement executes one statement from multiple conditions. It is cleaner than nested if-else when checking a single variable against specific constants.

int day = 3;
switch (day) {
    case 1:
        System.out.println("Monday");
        break;
    case 2:
        System.out.println("Tuesday");
        break;
    case 3:
        System.out.println("Wednesday"); // Output
        break;
    default:
        System.out.println("Weekend");
}

Note: Always remember the break statement to prevent fall-through behavior.

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5. What are Arrays and Strings?

Arrays

An array is a linear data structure containing a fixed number of elements of the same data type.

// Declaration & Initialization
int[] arr = new int[5];
arr[0] = 10;
arr[1] = 20;

// Directly initializing
int[] primeNumbers = {2, 3, 5, 7, 11};
System.out.println("First prime: " + primeNumbers[0]);

Strings

A String in Java is an object that represents a sequence of characters. Strings in Java are immutable (cannot be modified after creation).

String s = "Hello";
String joined = s + " World"; // Creates a new String object
System.out.println("Length: " + joined.length());
System.out.println("Char at index 1: " + joined.charAt(1)); // 'e'

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6. Loops

For Loops

Use a for loop when you know the number of iterations in advance.

for (int i = 0; i < 5; i++) {
    System.out.println("Iteration: " + i);
}

While Loops

Use a while loop when the number of iterations is determined by a condition.

int count = 5;
while (count > 0) {
    System.out.println("Count: " + count);
    count--;
}

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7. Functions (Pass by Value vs Pass by Reference)

This is a critical topic in Java and is frequently tested in interviews.

Rule: Java is strictly Pass-by-Value.

However, what is passed depends on the data type:

A. Primitives (Passed by Value)

When you pass a primitive variable to a method, a copy of the actual value is passed. Changes inside the method do not affect the original variable.

public class PrimitiveExample {
    public static void modifyValue(int num) {
        num = 100; // Only changes local copy
    }
    
    public static void main(String[] args) {
        int x = 10;
        modifyValue(x);
        System.out.println("x is still: " + x); // Output: 10
    }
}

B. Objects & Arrays (References Passed by Value)

When you pass an object or an array, Java passes a copy of the reference (memory address). Both the original variable and the parameter point to the same object in memory.

• Modifying the object’s properties or array indices will affect the original.
• Reassigning the parameter to a new object inside the method will not affect the original reference.

import java.util.Arrays;

public class ReferenceExample {
    public static void modifyArray(int[] arr) {
        arr[0] = 999; // Modifies the actual object heap memory
    }
    
    public static void reassignArray(int[] arr) {
        arr = new int[]{1, 2, 3}; // Points parameter to a new address; original is untouched
    }
    
    public static void main(String[] args) {
        int[] myArr = {10, 20, 30};
        
        modifyArray(myArr);
        System.out.println(Arrays.toString(myArr)); // Output: [999, 20, 30]
        
        reassignArray(myArr);
        System.out.println(Arrays.toString(myArr)); // Output: [999, 20, 30]
    }
}

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🏁 Day 1 Action Items

To practice these concepts, here are the problems I’m coding:

1. Find Factorial: Write a function using a loop.
2. Reverse a String: Construct a new string by reading character by character backward.
3. Swap Two Variables: Write a helper function (and think about how reference types allow you to modify array elements to simulate swapping).

Stay tuned for Day 2, where I’ll explore Time Complexity Analysis (Big O notation) and dive deep into Arrays!