Example on how to use primitive data types. This particular example takes in Strings, booleans, ints, and doubles to convert certain units from metric to imperial.

import java.util.Scanner;

public class Primitives {

    public static void main(String[] args) {
        Scanner input;
        double firstUnit, secondUnit;
        boolean userConfirm;
        int userOption;

        // Takes in String input
        input = new Scanner(System.in);
        System.out.println("Enter your name: ");
        String name = input.nextLine();
        input.close();

        System.out.println("Hello " + name + "!");
        System.out.println("Welcome to the unit convertor!");

        // Loop of conversions
        while (true) {

            // Takes in boolean input
            System.out.println("Would you like to convert something (true/false): ");
            input = new Scanner(System.in);
            userConfirm = input.nextBoolean();
            input.close();

            if (userConfirm) {

                //Takes in int input
                System.out.println("Here are a list of options below:");
                System.out.println("\t1. Pounds (lbs) to grams (g)");
                System.out.println("\t2. Grams (g) to pounds (lbs)");
                System.out.println("\t3. Feet (ft) to meters (m)");
                System.out.println("\t4. Meters (m) to feet (ft)");
                System.out.println("What would you like to convert (enter int): ");
                input = new Scanner(System.in);
                userOption = input.nextInt();
                input.close();

                switch (userOption) {

                    // Lbs to grams
                    case 1:
                        System.out.println("Enter the amount in lbs (double): ");
                        input = new Scanner(System.in);
                        firstUnit = input.nextDouble();
                        secondUnit = firstUnit * 453.592;
                        System.out.println(firstUnit + " lbs is " + secondUnit + " g.");
                        input.close();
                        break;
                    
                    // Grams to lbs
                    case 2:
                        input = new Scanner(System.in);
                        System.out.println("Enter the amount in g (double): ");
                        firstUnit = input.nextDouble();
                        secondUnit = firstUnit * 0.00220462;
                        System.out.println(firstUnit + " g is " + secondUnit + " lbs.");
                        input.close();
                        break;

                    // Ft to m
                    case 3:
                        input = new Scanner(System.in);
                        System.out.println("Enter the amount in ft (double): ");
                        firstUnit = input.nextDouble();
                        secondUnit = firstUnit * 0.3048;
                        System.out.println(firstUnit + " ft is " + secondUnit + " m.");
                        input.close();
                        break;

                    // M to ft
                    case 4:
                        input = new Scanner(System.in);
                        System.out.println("Enter the amount in m (double): ");
                        firstUnit = input.nextDouble();
                        secondUnit = firstUnit * 3.28084;
                        System.out.println(firstUnit + " m is " + secondUnit + " ft.");
                        input.close();
                        break;
                }

            } else {
                break;
            }
            


            
        }
        
    }
}

Primitives.main(null);
Enter your name: 
Hello USER!
Welcome to the unit convertor!
Would you like to convert something (true/false): 
Here are a list of options below:
	1. Pounds (lbs) to grams (g)
	2. Grams (g) to pounds (lbs)
	3. Feet (ft) to meters (m)
	4. Meters (m) to feet (ft)
What would you like to convert (enter int): 
Enter the amount in g (double): 
54.0 g is 0.11904948 lbs.
Would you like to convert something (true/false):