Thread: why could this happen?

I just completed my homework(see it in attachment) but when I run it, it always appears like that

run:
===================================
Menu
Exception in thread "main" java.lang.NullPointerException
===================================
A) String reversal
B) Vowel capitalization
C) <Tag> removal
D) String complement
E) Compute Boolean expression
F) Quit
===================================
	at TextProcessor.getChoice(TextProcessor.java:75)
	at TextProcessor.main(TextProcessor.java:184)
Choices:Java Result: 1
BUILD SUCCESSFUL (total time: 0 seconds)

my code is below

public class TextProcessor {
    // field declaration
    private Scanner keyboard;
 
 
 
    // constructor
    // print the program heading
    // set up the keyboard Scanner object
    public void constructor(){
        System.out.print("Text Processor");
        keyboard = new Scanner(System.in);
    }
 
 
 
    // show menu method: given, DO NOT MODIFY
    // print the menu lines
    public void showMenu(){
        System.out.println("===================================");
        System.out.println("Menu");
        System.out.println("===================================");
        System.out.println("A) String reversal");
        System.out.println("B) Vowel capitalization");
        System.out.println("C) <Tag> removal");
        System.out.println("D) String complement");
        System.out.println("E) Compute Boolean expression");
        System.out.println("F) Quit");
        System.out.println("===================================");
    }
 
 
 
    // get choice from user
    // repeat until getting a valid choice
    // requirement: case-insensitive choice
    // i.e. accepts both UPPER and lower-CASE choice
    // assumption: takes the first character of the input line
    // return a char, indicating the user's choice
    public char getChoice(){
        //TO DO
        String str = "initial";
        char result = 'z';
        while (str.charAt(0) != 'a'&&str.charAt(0) != 'A'&&
                str.charAt(0) != 'b'&&str.charAt(0) != 'B'&&
                str.charAt(0) != 'c'&&str.charAt(0) != 'C'&&
                str.charAt(0) != 'd'&&str.charAt(0) != 'D'&&
                str.charAt(0) != 'e'&&str.charAt(0) != 'E'&&
                str.charAt(0) != 'f'&&str.charAt(0) != 'F'){
            System.out.print("Choices:");
            str = keyboard.next();
 
            if (str.charAt(0) == 'a' || str.charAt(0) == 'A')
                result = 'A';
            if (str.charAt(0) == 'b' || str.charAt(0) == 'B')
                result = 'B';
            if (str.charAt(0) == 'c' || str.charAt(0) == 'C')
                result = 'C';
            if (str.charAt(0) == 'd' || str.charAt(0) == 'D')
                result = 'D';
            if (str.charAt(0) == 'e' || str.charAt(0) == 'E')
                result = 'E';
            if (str.charAt(0) == 'f' || str.charAt(0) == 'F')
                result = 'F';
        }
        return result;
    }
 
    // string reversal method
    // ...
    public void  stringReversal(){
        //TO DO
        System.out.print("Input a line:");
        String input = keyboard.nextLine();
 
        System.out.print("Reverse-line:");
 
        int length = input.length();
        for(int i = length - 1; i >= 0; i--){
            System.out.print(input.charAt(i));
        }
 
    }
 
    // vowel capitalization
    public void vowelCapitalization(){
         //TO DO
         System.out.print("Input a line:");
         String input = keyboard.nextLine();
 
         input = input.replace('a', 'A');
         input = input.replace('e', 'E');
         input = input.replace('i', 'I');
         input = input.replace('o', 'O');
         input = input.replace('u', 'U');
 
         System.out.print("VOWELED-line:");
         System.out.print(input);
 
    }
 
 
    // <tag> removal
    // assumption: arrowed <> tags must appear in pair
    // assumption: no nested <> tags
    public void tagRemoval(){
         //TO DO
         System.out.print("Input a line:");
         String input = keyboard.nextLine();
 
         int length = input.length();
         for (int i = 0; i < length; i++ ){
             if (input.charAt(i) != '<')
                 System.out.print(input.charAt(i));
             else {
                 i++;
                 while (input.charAt(i) != '>')
                     i++;
             }
 
         }
 
    }
 
    //string complement
    public void stringComplement(){
        //To do
        System.out.print("Input a line:");
        String input = keyboard.nextLine();
 
        int length = input.length();
        for (int i = 0; i < length; i++){
            if ((input.charAt(i) > 'A' && input.charAt(i) <= 'Z')
                    ||(input.charAt(i) > 'a' && input.charAt(i) <= 'z'))
                System.out.print(input.charAt(i-1));
            else {
                if (input.charAt(i) == 'A' || input.charAt(i) == 'a')
                    System.out.print(input.charAt(i+25));
                else {
                       System.out.print(input.charAt(i));
                }
            }
        }
 
    }
 
 
     // main program
    public static void main(String[] args)
    {
        // create the  TextProcessor object
        TextProcessor processorObject = new TextProcessor();   
 
        while (true)
        {
            // ask the  processorObject to show menu
            processorObject.showMenu();
 
            // use the processorObject  to ask for user choice
            char userChoice = processorObject.getChoice();
 
            // process the user choice and send relevant messages
            if(userChoice == 'A'){
                processorObject.stringReversal();  //uncomment this line if you implemented this function
                continue;
            }
            else if(userChoice == 'B'){
                processorObject.vowelCapitalization(); //uncomment this line if you implemented this function
                continue;
            }
            else if(userChoice == 'C'){
                processorObject.tagRemoval(); //uncomment this line if you implemented this function
                continue;
            }
            else if(userChoice == 'D'){
                processorObject.stringComplement(); //uncomment this line if you implemented this function
                continue;
             }
            else if(userChoice == 'E'){
                processorObject.expressionEvaluation(); //uncomment this line if you implemented the bonus part
                continue;
             }
            else break;   // end the menu loop 
        }
        System.out.println("Bye bye!");  
    }

I would appreciate it if somebody can help~

Originally Posted by copeg

That exception is quite descriptive: go to the line that is noted (line 75, which is in the getChoice() method). Use println's to see what could be null (hint: when is keyboard ever instantiated? - where you think it is, add some println's to verify this is the case - you will hopefully see it is not). Another hint: see the following: Providing Constructors for Your Classes (The Java™ Tutorials > Learning the Java Language > Classes and Objects)

Thank u so much! I have found what I did wrong.