Unit 1: Java fundamentals and control structures

• API: thousands of ready classes organised in packages — java.lang, java.util, java.io, java.awt, java.net.

java// Documentation section: program to greet a student
package college;                 // optional package statement
import java.util.Scanner;        // import statements

public class Hello {             // class definition; file must be Hello.java
    public static void main(String[] args) {   // entry point
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter your name: ");
        String name = sc.nextLine();
        System.out.println("Welcome to SBS, " + name);
    }
}

• main: public (accessible to the JVM), static (called without an object), void (returns nothing), String[] args (command-line arguments).

javafinal double PI = 3.14159;       // constant
int age = 19;                    // variable
long population = 1_400_000_000L;
float cgpa = 8.6f;
char grade = 'A';
boolean passed = true;
double avg = (double) 245 / 3;   // explicit casting
int x = 'A' + 1;                 // implicit widening: 66
var city = "Ludhiana";           // local type inference (Java 10+)

• Variable kinds: local (inside methods), instance (per object), class or static (shared by all objects). Type conversion: widening is automatic (int → long → float → double); narrowing needs a cast.

javaString s = "SBS";
String t = s + " Bathinda";           // concatenation creates a new object
System.out.println(t.length());        // 12
System.out.println(t.toUpperCase());   // SBS BATHINDA
System.out.println(s.equals("sbs"));   // false (use equalsIgnoreCase)
System.out.println(t.substring(4));    // Bathinda

• Strings are immutable; literals are pooled. Use equals() not == to compare content.

• Precedence: unary > multiplicative > additive > shift > relational > equality > bitwise > logical > conditional > assignment. Integer division truncates (7 / 2 = 3); % gives the remainder.

javaint marks = 72;
if (marks >= 75) System.out.println("Distinction");
else if (marks >= 60) System.out.println("First division");
else System.out.println("Pass");

int day = 3;
switch (day) {
    case 1: System.out.println("Monday"); break;
    case 3: System.out.println("Wednesday"); break;
    default: System.out.println("Other day");
}
String type = switch (day) {               // switch expression (Java 14+)
    case 6, 7 -> "Weekend";
    default -> "Weekday";
};

javafor (int i = 1; i <= 10; i++) System.out.print(i * i + " ");   // squares

int n = 12345, sum = 0;
while (n > 0) { sum += n % 10; n /= 10; }                      // digit sum = 15

int k = 1;
do { System.out.println(k); k++; } while (k <= 3);             // runs at least once

int[] marks = {78, 85, 90};
for (int m : marks) System.out.println(m);                     // enhanced for

outer:
for (int i = 1; i <= 3; i++)
    for (int j = 1; j <= 3; j++) {
        if (i * j == 4) break outer;                            // labelled break
        if (j == 2) continue;
        System.out.println(i + "," + j);
    }

javaouter:
for (int i = 1; i <= 3; i++)
    for (int j = 1; j <= 3; j++) {
        if (j == 2) continue;
        if (i == 3) break outer;
        System.out.println(i + "," + j);   // 1,1 1,3 2,1 2,3
    }