1. Temperature Conversion

import java.util.*;

public class TemperatureConversion {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("1. Celsius to Fahrenheit");
        System.out.println("2. Fahrenheit to Celsius");
        System.out.println("3. Celsius to Kelvin");
        System.out.println("4. Kelvin to Celsius");
        int ch = sc.nextInt();
        double t = sc.nextDouble();
        switch (ch) {
            case 1: System.out.println((t * 9/5) + 32); break;
            case 2: System.out.println((t - 32) * 5/9); break;
            case 3: System.out.println(t + 273.15); break;
            case 4: System.out.println(t - 273.15); break;
            default: System.out.println("Invalid Choice");
        }
    }
}

2. Bubble Sort

import java.util.*;

public class BubbleSort {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int arr[] = new int[n];
        for(int i=0;i<n;i++) arr[i]=sc.nextInt();
        for(int i=0;i<n-1;i++) {
            for(int j=0;j<n-i-1;j++) {
                if(arr[j]>arr[j+1]) {
                    int temp=arr[j];
                    arr[j]=arr[j+1];
                    arr[j+1]=temp;
                }
            }
        }
        for(int x:arr) System.out.print(x+" ");
    }
}

3. Stack Operations

import java.util.*;

public class StackExample {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        Stack<Integer> st = new Stack<>();
        while(true) {
            int ch = sc.nextInt();
            if(ch==1) st.push(sc.nextInt());
            else if(ch==2) { if(!st.isEmpty()) System.out.println(st.pop()); }
            else if(ch==3) System.out.println(st);
            else break;
        }
    }
}

4. String Manipulation

import java.util.*;

public class StringManipulation {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s = sc.nextLine();
        System.out.println(new StringBuilder(s).reverse().toString());
        System.out.println(s.toUpperCase());
        System.out.println(s.toLowerCase());
        System.out.println(s.length());
    }
}