The document outlines a Java program for an address book, which includes various functionalities such as adding, deleting, updating, and searching for entries. It highlights a critical error involving a NullPointerException related to the 'action' variable being null. Additionally, the document details the associated classes and methods used to manage the address book entries.

1. How to fix this error:
Exception in thread "main" q. Exit
java.lang.NullPointerException: Cannot invoke "String.hashCode()" because "action" is null
at AddressBook.main(AddressBook.java:23)
import java.util.Scanner;
public class AddressBook {
public static void main(String[] args) {
Tablea addressBook = new Tablea();
Scanner scanner = new Scanner(System.in);
String name, address;
String action = null;
boolean moreEntries = true;
while (moreEntries) {
System.out.println("/nMenu");
System.out.println("n.Add Entry");
System.out.println("d. Delete Entry");
System.out.println("u. Update Entry");
System.out.println("l. Search");
System.out.println("a. View All");
System.out.println("q. Exit");
// Insert a new entry into the address book
switch (action) {
case "n":
System.out.print("Enter a name: ");
name = scanner.nextLine();
System.out.print("Enter an address: ");
address = scanner.nextLine();
addressBook.insert(name, address);
System.out.println("New Contact has been added,");
System.out.print("Add another entry? (y/n): ");
String answer = scanner.nextLine().toLowerCase();
moreEntries = answer.equals("y") || answer.equals("yes");

2. break;
// Delete an entry from the address book
case "d":
System.out.println("Enter a name to delete: ");
name = scanner.nextLine();
boolean deleted = addressBook.delete(name);
if (deleted) {
System.out.println("Address deleted");
} else {
System.out.println("Name not found");
}
break;
// Update an entry in the address book
case "u":
System.out.print("Enter a name to update: ");
name = scanner.nextLine();
System.out.print("Enter a new address: ");
address = scanner.nextLine();
boolean updated = addressBook.update(name, address);
if (updated) {
System.out.println("Address updated");
} else {
System.out.println("Name not found");
}
break;
// Lookup an entry in the address book
case "l":
System.out.print("Enter a name to look up: ");
name = scanner.nextLine();
String result = addressBook.lookUp(name);
if (result != null) {
System.out.println("Address: " + result);
} else {

3. System.out.println("Name not found");
}
break;
// Display all entries in the address book
case "a":
System.out.println("All entries:");
addressBook.displayAll();
break;
// Quit the program
case"q":
System.out.println("Quitting...");
}
}
}
}
OTHER FILES THAT GO WITH THIS
public class Node {
private String name;
private String address;
private Node next;
Node() {
// add here ..
}
Node(String name, String address) {
// add here ..
this.name = name;
this.address = address;
this.next = null;
}
public String getKey() {
// add here ..
return this.name;
}

4. public void setKey(String name) {
// add here ..
this.name = name;
}
public String getValue() {
// add here ..
return this.address;
}
public void setValue(String address) {
// add here ..
this.address = address;
}
public Node getNext() {
// add here ..
return this.next;
}
public void setNext(Node next) {
// add here ..
this.next = next;
}
}
import java.util.Scanner;
public class Tablea {
private Node mark;
public Node getMark() {
return this.mark;
}
public void setMark(Node mark) {
this.mark = mark;
}
public boolean insert(String name, String address) {
Node newNode= new Node(name,address);
if (this.mark== null) {
this.mark = newNode;
}else {

5. newNode.setNext(this.mark.getNext());
this.mark.setNext(newNode);
}
return true;
}
public String lookUp(String name) {
Node current = this.mark;
while(current!=null) {
if (current.getKey().equals(name)) {
return current.getValue();
}
current = current.getNext();
}
return null;
}
public boolean delete(String name) {
Node current = this.mark;
Node prev = null;
while (current!= null) {
if (current.getKey().equals(name)) {
if (prev == null) {
this.mark = current.getNext();
}else {
prev.setNext(current.getNext());
}
return true;
}
prev = current;
current = current.getNext();
}
return false;
}
public boolean update(String name, String newValue) {
Node current = this.mark;
while (current != null) {
if (current.getKey().equals(name)) {
current.setValue(newValue);
return true;
}
current = current.getNext();
}
return false;
}

6. public boolean markToStart() {
if (this.mark== null) {
return false;
}
this.mark = null;
return true;
}
public boolean advanceMark() {
if (this.mark== null) {
return false;
}
this.mark = this.mark.getNext();
return true;
}
public String keyAtMark() {
if (this.mark== null) {
return null;
}
return this.mark.getKey();
}
public String valueAtMark() {
if (this.mark== null) {
return null;
}
return this.mark.getValue();
}
public int displayAll() {
Node current = this.mark;
int count = 0;
while (current != null) {
System.out.println(current.getKey()+":"+ current);
}
return count;
}
}