Working with printf() in java

package sample.com;
    
    import java.lang.reflect.Array;
    import java.util.*;
    import java.util.ArrayList;
    public class Calculation {
        Random ran = new Random();
        ArrayList<Character> randomCharactersArray = new ArrayList<>();
        ArrayList<Character> charNotUsedArray = new ArrayList<>();
        ArrayList<Double> randomNumberArray = new ArrayList<>();
        char[] allowableCharacter = {'A', 'B', 'C', 'D', 'E', 'F', 'a', 'b', 'c', 'd', 'e', 'f'};
    
        //method to generate 10 characters.
        public void randomCharacters(int num) {
            for (int i = 0; i < num; i++) {
                char randomizedLowerCharacters = (char) (ran.nextInt(num) + 'a');
                char randomizedUpperCharacters = (char) (ran.nextInt(num) + 'A');
                randomCharactersArray.add(randomizedLowerCharacters);
                randomCharactersArray.add(randomizedUpperCharacters);
            }
    
            System.out.println(" Random Values are " + Arrays.toString(randomCharactersArray.toArray()));
        }
    
        public double randomNumbers(){
            double number = 0;
            double max = 3.0;
            double min = 0.5;
    
            for(int i =0; i < randomCharactersArray.size(); i++){
    //           number = ((Math.random() * 3.0 - 0.5) + 0.5);
                number = ran.nextDouble(max -min) + min;
                randomNumberArray.add(number);
    //            System.out.printf("%.1f the number is \n",number);
            }
            return number;
        };
    
    
        public void displayMessage(){
    //        randomNumberArray.forEach(number->{
    //            System.out.printf("%.1f %.1d The number is " +  Arrays.toString(randomNumberArray.toArray()));
    //        });
            for(int i = 0; i < randomNumberArray.size(); i++){
                if(randomCharactersArray.get(i) == randomCharactersArray.get(randomCharactersArray.size()-1)){
    //                System.out.printf("'%s'",randomCharactersArray.get(i));
                    System.out.print(randomCharactersArray.get(i));
                }else{
                    System.out.print(randomCharactersArray.get(i) + " ,");
    //                System.out.printf("%s,",randomCharactersArray.get(i));
                }
    
            }
    //        System.out.print("%.1f %.1d The number is " +  Arrays.toString(randomNumberArray.toArray()));
    
        }
    
        public void charactersNotUsed(){
            char charNotUsed = 0;
            for(int i = 0; i < randomCharactersArray.size(); i++){
                if(!randomCharactersArray.contains(allowableCharacter[i])){
                    charNotUsed = allowableCharacter[i];
                    charNotUsedArray.add(charNotUsed);
                    System.out.println("\nchar Not Used " + charNotUsed);
                }
            }
            System.out.printf("\n%s,Letters not used:",  Arrays.toString(charNotUsedArray.toArray()));
    //        System.out.println(" Characters not used" + Arrays.toString(charNotUsedArray.toArray()));
        }
    
    }
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    //        for(int i=0; i < randomCharactersArray.size(); i++){
    //            Double myDouble = Double.valueOf(randomCharactersArray.get(i));
    //            doubleRandomArr.add(myDouble);
    ////            System.out.printf("%.1f\n",RANDOM_VALUE);
    //        }