You can give whole code(from displaying menu) inside a while loop and give condition as true so that after using a case it will automatically repeat(as you are using 6 to EXIT). And if any invalid input is given for eg:10 the case will go to default section and will execute the code there
import java.util.Scanner;
public class basicCalc {
public static void main(String[] args) {
// TODO Auto-generated method stub
Scanner input = new Scanner(System.in);
boolean mainLoop = true;
int choice;
while(true){
System.out.println("Calculator Main Menu\n");
System.out.print("1.) Addition \n");
System.out.print("2.) Subtraction.\n");
System.out.print("3.) Multiplication.\n");
System.out.print("4.) Division.\n");
System.out.print("5.) Generate Random Number.\n");
System.out.print("6.) Exit\n");
System.out.print("\nEnter Your Menu Choice: ");
choice = input.nextInt();
switch(choice){
case 1:
//Definitions
int adNumf, adNuml, sum;
System.out.print("Please Enter The First Number: ");
adNumf = input.nextInt();
System.out.print("\nPlease Enter The Second Number: ");
adNuml = input.nextInt();