We Keep Coding

Searching for a String in an ArrayList of Objects

I have been trying to figure this out for hours and I have had no luck doing so,
I'm trying to iterate over my Arraylist<Booking> which utilizes my Booking class file and trying to understand how I'm able to search it for the matching, case-insensitive term.
this is my current method:
private void searchBookings() {
if (bookings.size() <= 0) {
JOptionPane.showMessageDialog(null, "There are no bookings.", "Search Bookings", 3);
} else {
String searchTerm = JOptionPane.showInputDialog(null, "Please input search term: ", "Search Bookings", 3);
for (int i = 0; i < bookings.size(); i++) {
while (!bookings.get(i).getStudent().getName().equalsIgnoreCase(searchTerm)) {
i++;
if (bookings.get(i).getStudent().getName().equalsIgnoreCase(searchTerm)) {
String output = String.format("%-30s%-18s%-18b$%-11.2f\n", bookings.get(i).getStudent(), bookings.get(i).getLessons(), bookings.get(i).isPurchaseGuitar(), bookings.get(i).calculateCharge());
this.taDisplay.setText(heading + "\n" + output + "\n");
}
}
}
}
JOptionPane.showMessageDialog(null, "There is no booking with that name.", "Search Bookings", 3);
}
I know it's messy but, just trying to make do.
I am trying to retrieve the name of the booking as I am searching by name as well as provide an error message if that names does not exist, to do that I must
use bookings.getStudent().getName() I have had some luck as I can return the value but now I am not able to provide my error message if I do not find it. Any help is appreciated.
package com.mycompany.mavenproject1;
public class Booking {
private Student student;
private int lessons;
private boolean purchaseGuitar;
// CONSTANTS
final int firstDiscountStep = 6;
final int secondDiscountStep = 10;
final int tenPercentDiscount = 10;
final int twentyPercentDiscount = 5;
final double LESSON_COST = 29.95;
final double GUITAR_COST = 199.00;
double LESSON_CHARGE = 0;
final int MINIUMUM_LESSONS = 1;
public Booking() {
}
public Booking(Student student, int lessons, boolean purchaseGuitar) {
this.student = new Student(student.getName(), student.getPhoneNumber(), student.getStudentID());
this.lessons = lessons;
this.purchaseGuitar = purchaseGuitar;
}
public Student getStudent() {
return student;
}
public void setStudent(Student student) {
this.student = student;
}
public int getLessons() {
return lessons;
}
public void setLessons(int lessons) {
this.lessons = lessons;
}
public boolean isPurchaseGuitar() {
return purchaseGuitar;
}
public void setPurchaseGuitar(boolean purchaseGuitar) {
this.purchaseGuitar = purchaseGuitar;
}
public double calculateCharge() {
double tempCharge;
if (lessons < firstDiscountStep) {
LESSON_CHARGE = (lessons * LESSON_COST );
} else if (lessons < secondDiscountStep) {
tempCharge = (lessons * LESSON_COST) / tenPercentDiscount;
LESSON_CHARGE = (lessons * LESSON_COST) - tempCharge;
} else {
tempCharge = (lessons * LESSON_COST) / twentyPercentDiscount;
LESSON_CHARGE = (lessons * LESSON_COST) - tempCharge;
}
if (isPurchaseGuitar()) {
LESSON_CHARGE += GUITAR_COST;
}
return LESSON_CHARGE;
}
#Override
public String toString() {
return student + ","+ lessons + "," + purchaseGuitar +"," + LESSON_COST;
}
}

If I understood you correctly, you are searching for a given student name in your collection of bookings. And if it is present, set a formatted text.
First of all, use a for-each loop, because you don't use the index.
Secondly, return from the for-each loop, when you found your student.
private void searchBookings() {
if (bookings.size() <= 0) {
JOptionPane.showMessageDialog(null, "There are no bookings.", "Search Bookings", 3);
} else {
String searchTerm = JOptionPane.showInputDialog(null, "Please input search term: ", "Search Bookings", 3);
for (final Booking booking : bookings) // for-each
{
if (booking.getStudent().getName().equalsIgnoreCase(searchTerm))
{
String output = booking.getFormattedOutput();
this.taDisplay.setText(heading + "\n" + output + "\n");
return; // break out of the loop and method and don't display dialog message
}
}
}
JOptionPane.showMessageDialog(null, "There is no booking with that name.", "Search Bookings", 3);
}
Then there are multiple other things, which you could improve.
Don't get all the data from a booking just to format it externally. Let the Booking class handle the formatting and return you the string you desire. (move the formatting in a function inside the Booking class)
Instead of recreating a Student you receive in your Booking constructor, make the Student class immutable, and then you can just reuse the object provided.
Try also making the Booking class immutable. You provided some setters, but do you really want to change the student in a booking? Or would you rather create a new booking for the other student?
The calculteCharge method could be stateless. Just get the LESSON_CHARGE value and hold it in a local variable. Your method would also get threading-proof.
Make your constants final and better yet make them members of the class (by adding the static modifier) instead of every member.
Lastly, representing a money amount with a floating (double is better but not good either) number, you will run into funny situations. Try this calculation: 0.1+0.1+0.1+0.1+0.1+0.1+0.1+0.1+0.1+0.1 for example.
One way would be to create a Money class which holds the value in cents as an integer. And when you want to display the amount you can divide it by 100 and format it accordingly. That way, you can also restrict it become negative.
PS: Sometimes we desperately try to find a solution that we don't give ourselves some rest. After a little break, you might recognize the problem. Oh and try debugging with breakpoints. Or this, if you use IntelliJ IDEA (which I would highly recommend, the community edition is free).

You're re-incrementing your counter variable, which is really not going to help. Try the following:
private void searchBookings() {
if (bookings.size() <= 0) {
JOptionPane.showMessageDialog(null, "There are no bookings.", "Search Bookings", 3);
} else {
String searchTerm = JOptionPane.showInputDialog(null, "Please input search term: ", "Search Bookings", 3);
boolean studentFound = false;
for (int i = 0; i < bookings.size(); i++) {
if (bookings.get(i).getStudent().getName().equalsIgnoreCase(searchTerm)) {
String output = String.format("%-30s%-18s%-18b$%-11.2f\n", bookings.get(i).getStudent(),
bookings.get(i).getLessons(), bookings.get(i).isPurchaseGuitar(),
bookings.get(i).calculateCharge());
this.taDisplay.setText(heading + "\n" + output + "\n");
studentFound = true;
break;
}
}
}
if (!studentFound) {
JOptionPane.showMessageDialog(null, "There is no booking with that name.", "Search Bookings", 3);
}
}

Java: Why is there a NullPointerException when there is no null? [duplicate]

This question already has answers here:
What is a NullPointerException, and how do I fix it?
(12 answers)
Closed 5 years ago.
This is my first time using this site so hopefully this makes sense. I have my code below and when I execute it its giving me a NullPointerException on line 38 in my Library class,
(the line says if (items[i].getTitle() != null && items[i].getTitle().equals(title)) {)
and another NullPointerException on like 29 in my MainMethod class,
(line is if (loaned == true) {).
To check the contents of items[i], after I added an item I had printed in the console to see if it was null and it successfully printed so I am confused as to why the NullPointerExpcetion is present. Any and all help will be greatly appreciated.
public class MediaItem {
private String title;
private String format;
public boolean onLoan;
public String loanedTo;
public String dateLoaned;
MediaItem() {
onLoan = false;
loanedTo = null;
dateLoaned = null;
title = null;
format = null;
}
MediaItem(String title, String format) {
this.title = title;
this.format = format;
}
public String getTitle() {
return this.title;
}
public String setFormat(String format) {
this.format = format;
return this.format;
}
public String getFormat() {
return format;
}
void markOnLoan(String name, String date) {
if (onLoan == true) {
System.out.println(title + " is already on loan to " + loanedTo);
} else {
onLoan = true;
loanedTo = name;
dateLoaned = date;
}
}
void markReturned() {
}
}
import java.util.Scanner;
public class Library {
int numberOfItems = 0;
MediaItem[] items = new MediaItem[100];
int displayMenu() {
Scanner s = new Scanner(System.in);
System.out.println(
"1. Add an item \n2. Mark an item as on loan \n3. List all items \n4. Mark an item as returned \n5. Quit \n\nWhat would you like to do?");
int choice = s.nextInt();
return choice;
}
void addNewItem(String title, String format) {
items[numberOfItems] = new MediaItem(title, format);
numberOfItems++;
}
void markItemOnLoan(String title, String name, String date) {
for (int i = 0; i <= 100; i++) {
if (items[i].getTitle() == title) {
items[i].onLoan = true;
items[i].loanedTo = name;
items[i].dateLoaned = date;
}
}
}
boolean checkIfLoaned(String title) {
char loaned = 'N';
System.out.println(items[0].getTitle());
for (int i = 0; i < 100; i++) {
if (items[i].getTitle() != null && items[i].getTitle().equals(title)) {
if (items[i].onLoan) {
String personName = items[i].loanedTo;
System.out.println(title + " is already on loan to " + personName);
loaned = 'Y';
}
}
}
if (loaned == 'Y') {
return true;
} else {
return false;
}
}
}
import java.util.Scanner;
public class MainMethod {
public static void main(String[] args) {
int choice = 1;
String personName;
String mediaName;
String format;
String loanDate;
Scanner s = new Scanner(System.in);
Library l = new Library();
while (choice != 5) {
choice = l.displayMenu();
switch (choice) {
case 1:
System.out.println("What is the title?");
mediaName = s.nextLine();
System.out.println("What is the format?");
format = s.nextLine();
l.addNewItem(mediaName, format);
break;
case 2:
System.out.println("Which item (enter the title)?");
mediaName = s.nextLine();
System.out.println("Who are you loaning it to?");
personName = s.nextLine();
boolean loaned = l.checkIfLoaned(mediaName);
if (loaned == true) {
break;
}
System.out.println("When did you loan the item?");
loanDate = s.nextLine();
l.markItemOnLoan(mediaName, personName, loanDate);
}
}
}
}

I am assuming this is homework, so I will steer you in the right direction, as opposed to posting code.
You are iterating through a hard-coded value of 100 items in both markItemOnLoan and checkIfLoaned. Just because you declare an array of 100 items, does not mean they are all initialized. Consider altering your loops to iterate through numberOfItems instead of 100. You should also do a check in addNewItem to ensure you won't surpass 100 items.
Related to #1, you do not check if the actual item you are extracting from the array is null, so you're attempt to call a method getTitle on a null Object, naturally throws a NPE.
Side note, in markItemOnLoan, you're incorrectly comparing Strings. The way you compare titles in checkIfLoaned is correct.

it would be better if you post the callstack too. 3 potential errors I can see are :
items[i] might be null. Although you are checking for null on items[X].getTitle()... You don't perform a null check on items[i]
Your for loop goes from 0 to 100. Is it gauranteed that all elements in array from 0-99 index are populated ? Shouldn't you go from 0 to items.length ?
Your array is defined for 100 elements. You are trying to access 101st elements in the for loop (i<=100) should be i < 100.

Why is there a NullPointerException when there is no null?
It cannot happen.
From this we deduce that there is a null. That is simple logic.
If you are going to succeed in debugging a program MUST NOT deny what the evidence tells you.
So where is the null?
if (items[i].getTitle() != null && items[i].getTitle().equals(title)) {
Based on this line alone there are four possible explanations:
items is null
items[i] is null
another thread is changing something ... and you got unlucky.
the getTitle() method is returning something that is not a String or that is not the same String each time.
We can eliminate the first and fourth explanations by reading the code, and the third one is extremely unlikely (even supposing that it is possible).
That leaves the second explanation as the "working hypothesis".

Trouble removing a time slot while removing course

I'm creating a scheduling and registration system for a class project. I'm required to be able to add and delete rooms and courses. Each course can only last an hour and may not happen at the same time as another course in the same room. I'm able to delete the course itself, but I'm having trouble deleting the time associated with that course.
What I've done is create an ArrayList of Rooms with each room being able to hold an ArrayList of Courses. Each of these courses has a specific hour which is checked if it's in use using an ArrayList of times. I was able to add the courses hour to the list and halt the user from creating another course with the exact time slot in the same room. However, whenever I remove the course I'm trying to remove the time as well so that another course that's created may use that time slot. Problem is, the time slot gets filled and stays filled even after removing the course and I'm not sure why.
Some guidelines:
Rooms can be added and deleted by the user. (To delete a room, no course should be scheduled in this room).
All courses can be deleted by the user.
The user can create courses by specifying a room number and the participants.
A room cannot hold more than one course at any one-hour slot.
To be honest I've been working for around 7 hours straight and I'm not to confident in my code, if I'm even doing things right, or even what I'm talking about really. I apologize if I'm not being specific enough or making any sense, please let me know if something needs clarification. If you have any other tips/pointers or see any other mistakes please let me know. Thanks in advance.
Course.java
package Schedule;
import java.util.ArrayList;
public class Course extends Room {
private String name;
private int roomNum, hour, students;
private static ArrayList < Course > courseList = new ArrayList < > ();
private static ArrayList < Integer > times = new ArrayList < > (24);
public Course() {}
public Course(String name, int hour, int roomNum, int students) { //Constructor
this.name = name;
if (hour > 7 && hour < 18) {
this.hour = hour;
} else {
System.out.println("Not a valid time slot. Time set to 6:00PM/1800 HOURS. ");
this.hour = 18;
}
this.students = students;
this.roomNum = roomNum;
boolean inUse = checkTime(hour, roomNum);
if (inUse == false) {
times.add(hour);
Room.addCourse(roomNum, this);
courseList.add(this);
}
}
public static void deleteCourse(int courseNum, int roomNum) {
boolean pass;
pass = Room.removeCourse(courseNum, roomNum);
if (pass == true) {
times.remove(courseNum);
courseList.remove(courseNum);
System.out.println("Course Removed ");
}
}
public static boolean checkTime(int hour, int roomNum) {
boolean exist = false;
for (int i = 0; i < courseList.size(); i++) {
if (courseList.get(i).hour == hour && courseList.get(i).roomNum == roomNum) {
exist = true;
System.out.println("Time already in use, course could not be added. ");
}
}
return exist;
}
}
Room.java
package Schedule;
import java.util.ArrayList;
public class Room {
private int number, numOfClasses;
private static int numOfRooms = 1000;
private static ArrayList < Room > roomList = new ArrayList < > ();
private ArrayList < Course > courseList = new ArrayList < > ();
public Room() {}
public Room(int number, int numOfClasses) { //Constructor
this.number = number;
this.numOfClasses = numOfClasses;
if (roomList.size() < numOfRooms) {
roomList.add(this);
System.out.println("Room added");
} else {
System.out.println("Room couldn't be added, not enough rooms available.");
}
}
public static void numOfRooms(int r) {
numOfRooms = r;
}
public static void deleteRoom(int roomNum) { //Delete room
boolean exist = false;
for (int i = 0; i < roomList.size(); i++) {
if (roomList.get(i).getRoomNum() == roomNum) {
if (roomList.get(i).courseList.size() > 0) {
System.out.printf("%s%d%s%n", "Cannot delete room ", roomNum, " " + "There is currently a course in the room. ");
} else {
roomList.remove(i);
System.out.printf("%s%d%s%n", "Room ", roomNum, " Deleted");
}
exist = true;
}
}
if (exist == false) {
System.out.printf("%s%d%s%n", "Room ", roomNum, " does not exist, could not delete.");
}
}
public int getRoomNum() {
return number;
}
public static ArrayList < Room > getRoomList() {
return roomList;
}
public static void addCourse(int roomNum, Course c) { //Add Course to room.
boolean empty = true;
for (int i = 0; i < roomList.size(); i++) {
if (roomList.get(i).getRoomNum() == roomNum) {
roomList.get(i).courseList.add(c);
System.out.printf("%s%d%n", "Course added to room ", roomNum);
empty = false;
}
}
if (empty == true) {
System.out.println("No rooms with that room number. ");
}
}
public static boolean removeCourse(int courseNum, int roomNum) {
boolean exist = false;
try {
for (int i = 0; i < roomList.size(); i++) {
if (roomList.get(i).getRoomNum() == roomNum) {
roomList.get(i).courseList.remove(courseNum);
exist = true;
}
}
if (exist == false) {
System.out.println("Could not find course to delete. ");
}
} catch (IndexOutOfBoundsException e) {
System.out.println("Error: Could not find a room or course to delete. ");
}
return exist;
}
}
ScheduleDemo.java
//For adding rooms, create a room object and input the room number and number of courses.
//For adding courses, create a course object and input the Name, hour1, room number, and # of students.
//For Deleting rooms, type Room.deleteRoom("room number").
//For Deleting Courses, type Course.deleteCourse("Course number", "Room Number").
package Schedule;
public class ScheduleDemo {
public static void main(String[] args) {
Room.numOfRooms(100);
Room room0 = new Room(0, 1);
Room room1 = new Room(3, 1);
Room room2 = new Room(99, 1);
Course course0 = new Course("Course", 9, 3, 10);
Course course1 = new Course("Course2", 9, 99, 12);
Course.deleteCourse(0, 99);
Course course2 = new Course("Help", 9, 99, 122);
Room.deleteRoom(56);
Room.deleteRoom(99);
Course.deleteCourse(1, 99);
}
}
Output:
Room added
Room added
Room added
Course added to room 3
Course added to room 99
Course Removed
Time already in use, course could not be added.
Room 56 does not exist, could not delete.
Room 99 Deleted
Could not find course to delete.
BUILD SUCCESSFUL (total time: 0 seconds)
UPDATE:
I managed to fix the issues by removing course and room number completely and instead passed the name of the course. Since I was passing in the index (courseNum) of each course, I ended up deleting the wrong course which is why my times didn't delete properly. By searching the name of the course in both my Course list and my room course list, I was able to accurately delete the right course from both list. Here's what I fixed.
Main
Course course1 = new Course("Course2", 9, 99, 12); //Creates Course2 and time slot
Course.deleteCourse("Course2"); //Deletes Course2 and time slot
Course course2 = new Course("Help", 9, 99, 122); //Adds course Help into same hour
/*
New Output
Course added to room 99
Course Removed
Course added to room 99
*/
Course
public static void deleteCourse(String name) {
boolean pass;
pass = Room.removeCourse(name);
if (pass == true) {
for (int i = 0; i < courseList.size(); i++) {
if (courseList.get(i).getName().equals(name)) {
times.clear();
courseList.remove(i);
System.out.println("Course Removed ");
}
}
}
}
public String getName() {
return name;
}
Room
public static boolean removeCourse(String name) {
boolean exist = false;
try {
for (int j = 0; j < roomList.size(); j++) {
for (int i = 0; i < roomList.get(j).courseList.size(); i++) {
if (roomList.get(j).courseList.get(i).getName().equals(name)) {
roomList.get(j).courseList.remove(i);
exist = true;
}
}
}
if (exist == false) {
System.out.println("Could not find course to delete. ");
}
} catch (IndexOutOfBoundsException e) {
System.out.println("Could not find a room or course to delete. ");
}
return exist;
}
Now I can move forward to other things. Thanks!

I think the main issue is you are deleting the CourseList using index. And I think you are assuming that RoomNum 99 is associated with course Num 0. In fact there is no concept of course number in your application. By default it becomes the index of the List.
private static ArrayList < Course > courseList = new ArrayList < > ();
courseList.add(object of type Course);
courseList.remove(courseNum); // Note course Num becomes an index here
So this removes the wrong entry. Course2 which is in Room number 99 still remains in the list thereby saying that the course is still running.
EDIT: This design is very confusing and will not work if you have to remember the relationship between course number and the course name. You have not modeled the relationship anywhere in your design.

When you declare something static it makes the member belong to the class instead of belonging to the instance. Therefore, let's look at your ArrayLists content when you're running the program.
Room room0 = new Room(0, 1);
Room room1 = new Room(3, 1);
Room room2 = new Room(99, 1);
Course course0 = new Course("Course", 9, 3, 10);
// courseList : {course0} | hours : {9}
Course course1 = new Course("Course2", 9, 99, 12);
// courseList : {course0,course1} | hours : {9,9}
Course.deleteCourse(0, 99);
// courseList : {course1} | hours : {9}
Since your Arraylist is static, your deleteCourse method doesn't delete the course 0 of the room 99. It removes the course1 from your Room courseList but not from the staticarray list !

Cannot read text file into array and write to text file again

This my whole code.
I want to make a simple program that will read a
text file and put it to array then write it to the
same text file,
also can add and delete the existing input and my input.
Problem
The delete and writer part seems not working, only blank text file when I run the code
These are the error after I select the exit.
java.lang.NullPointerException at ContactList.writer(ContactList.java:51) at
ContactListDriver.main(ContactListDriver.java:73) at
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at
sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) at
sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) at
java.lang.reflect.Method.invoke(Unknown Source) at
edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:27‌​2)
public class Contact {
//Each contact stores the name, phone number, and email address
private String name;
private String number;
private String email;
public Contact(String name, String number, String email)
{
this.name = name;
this.number = number;
this.email = email;
}
public String getName()
{
return name;
}
public String getNumber()
{
return number;
}
public String getEmail()
{
return email;
}
public void setName(String name)
{
this.name = name;
}
public void setNumber(String number)
{
this.number = number;
}
public void setEmail(String email)
{
this.email = email;
}
}
class for processing the inputs.
import java.io.*;
import java.lang.*;
import java.util.*;
public class ContactList {
public Contact[] myContacts;
public static final int MAX = 100;
private int numContacts;
public ContactList()
{
myContacts = new Contact[MAX];
numContacts = 0;
}
public void addContact(String name, String number, String email)
{
Contact c = new Contact(name, number, email);
myContacts[numContacts] = c;
numContacts++;
}
public void deleteContact(String name)
{
for ( int i = 0; i <= numContacts-1 ; i++){
if( name == myContacts[i].getName())
{
myContacts[i] = null;
break;
}
}
numContacts--;
}
public void writer(){
String x = "MyContacts.txt";
try {
PrintWriter outputs = new PrintWriter(x);
for( int i=0; i < myContacts.length; i++)
{
Contact c = myContacts[i];
if(c!=null){ // check if c is null before writing to file
outputs.println(""+c.getName()+" "+c.getNumber()+" "+c.getName());
outputs.flush();
}
}
outputs.close();
}catch (IOException e) {
e.printStackTrace();
}
catch(NullPointerException ex){
}
}
public void displayContacts()
{
int i;
for(i=0; i < myContacts.length; i++)
{
Contact c = myContacts[i];
if(null != c){
System.out.println("Name: " + c.getName());
System.out.println("Number: " + c.getNumber());
System.out.println("Email: " + c.getEmail());
System.out.println("------------------------------------");
}
}
}
}
/*
* To change this template, choose Tools | Templates
* and open the template in the editor.
*/
import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;
The Driver....
public class ContactListDriver {
public static void main(String[] args) throws FileNotFoundException
{
ContactList cList = new ContactList();
File in = new File("MyContacts.txt");
Scanner sc = new Scanner(in);
int option;
char again = 'n';
String name = null;
String number = null;
String email = null;
while(sc.hasNext())
{
//read one line from text file
String entry = sc.nextLine();
//System.out.println(entry);
String[] con = entry.split("\\s+");
//System.out.println(con[0] + " " + con[1] + " " + con[2]);
cList.addContact(con[0], con[1], con[2]);
}
Scanner userIn = new Scanner(System.in);
do{
displayOptions();
option = userIn.nextInt();
switch(option)
{
case 1:
System.out.println(" Name > ");
name = userIn.next();
System.out.println(" Number > ");
number = userIn.next();
System.out.println(" Email Address > ");
email = userIn.next();
cList.addContact(name, number, email);
break;
case 2:
//delete contact
System.out.println("Contact Name > ");
name = userIn.next();
cList.deleteContact(name);
break;
case 3:
//display contact
cList.displayContacts();
break;
case 4:
cList.writer();
System.out.println(" are you sure ? press y ");
String x = userIn.next();
again = x.charAt(0);
break;
}
}while( again == 'n' );
}
private static void displayOptions() {
System.out.println("(1) Add");
System.out.println("(2) Delete");
System.out.println("(3) Show Contacts");
System.out.println("(4) Exit");
}
}

One problem I see is:
You have a extra break; statement inside deleteContact(String name) function
and String comparision name == myContacts[i].getName() should be name.equals(myContacts[i].getName())
public void deleteContact(String name)
{
for ( int i = 0; i <= numContacts-1; i++){
if( name.equals( myContacts[i].getName()))// string comparison uses equals();
{
myContacts[i] = null;
numContacts--; // this line should be inside of if condition
break;
}
// break; No need of breaking the loop here
}
}
Another problem is at writer() function
public void writer(){
String x = "MyContacts.txt";
try {
PrintWriter outputs = new PrintWriter(x);
for( int i=0; i < myContacts.length; i++)
{
Contact c = myContacts[i];
if(c!=null){ // check if c is null before writing to file
outputs.println(""+c.getName()+" "+c.getNumber()+" "+c.getName());
outputs.flush();
}
}
outputs.close();
}catch (IOException e) {
e.printStackTrace();
}
catch(NullPointerException ex){ // Or just catch the NPE
}

You have declared and initialized the Contact array of size MAX. but,it seems to be that you haven't initialized the elements though. i.e. c is null in the below code
Contact c = myContacts[i];
outputs.println(""+c.getName()+" "+c.getNumber()+" "+c.getName());
outputs.flush();
myContacts[i] should return a Contact instance. As said by Meno, there are lot of other problems in your code. You have to always cover all the possible scenarios while writing the code.

Most importantly you need to fix the ContactList class. It is inserting new elements into the last index, and deleting at any location using the name.
For example, let's say the ContactList has three elements in it at 0, 1 and 2 indexes. So numContacts is set to 3.
Now ContactList has elements as:
[0]C0, [1]C1, [2]C2, [3]null, ...
Then if the contact at 0 index is deleted (set to null), then numContacts is set to 2.
Now the ContactList has elements as:
[0]null, [1]C1, [2]C2, [3] null, ...
A new insert will be added to the index 2, and it will override the C2 value.
Simplest solution is to use an ArrayList instead of an array.
As others have mentioned there are few more issues to fix, but above is the most important in my opinion.

There are many issues with your code so not easy to say where to begin.
First: Your public void deleteContact(String name)-method is broken. It compares Strings using == instead of equals(). And worse: It creates null pointers mid in your array which will cause problems in your writer()-method.
Second: Why do you use arrays? You should use java.util.ArrayList which offers out-of-the-box implementations for adding, getting and deleting contacts.
Third: If you are missing your text file, you might have overlooked it because of missing path so you don't know where to look for this file. So please add a full path to file name.
Fourth: I would also use scanner.hasNextLine() instead of scanner.hasNext() if you then call scanner.nextLine().
Since you said you are not allowed to use ArrayList you should study its source code especially for removing elements. It does not only nullify the array bucket, but also to shift all following elements one index backwards so you don't have any null gap until the index given by element count. And two breaks in deleteContact()-method are really not necessary.

Logic of deleting in Java

I cant figure out how to start a method to delete a specific entry stored in an array...
I used to do this:
public void deleteEntry() {
SName = JOptionPane.showInputDialog("Enter Name to delete: ");
for (int i = 0; i < counter; i++) {
if (entry[i].getName().equals(SName)) {
JOptionPane.showMessageDialog(null, "Found!");
entry[i] = null;
}
}
}
but I was advised not to assign the entry[i] to null because it will ruin my entries...
I have no idea how to code it in another way...
What should I need to do is:
I need to delete a specific entry from an array
please help...
also... its output was error it says:
Exception in thread "main" java.lang.NullPointerException
at AddressBook.viewAll(AddressBook.java:62)
at AddressBook.main(AddressBook.java:36)
Java Result: 1
This is my code in my main program:
public class AddressBook {
private AddressBookEntry entry[];
private int counter;
private String SName;
public static void main(String[] args) {
AddressBook a = new AddressBook();
a.entry = new AddressBookEntry[100];
int option = 0;
while (option != 5) {
String content = "Choose an Option\n\n"
+ "[1] Add an Entry\n"
+ "[2] Delete an Entry\n"
+ "[3] Update an Entry\n"
+ "[4] View all Entries\n"
+ "[5] View Specific Entry\n"
+ "[6] Exit";
option = Integer.parseInt(JOptionPane.showInputDialog(content));
switch (option) {
case 1:
a.addEntry();
break;
case 2:
a.deleteEntry();
break;
case 3:
a.editEntry();
break;
case 4:
a.viewAll();
break;
case 5:
a.searchEntry();
break;
case 6:
System.exit(1);
break;
default:
JOptionPane.showMessageDialog(null, "Invalid Choice!");
}
}
}
public void addEntry() {
entry[counter] = new AddressBookEntry();
entry[counter].setName(JOptionPane.showInputDialog("Enter name: "));
entry[counter].setAdd(JOptionPane.showInputDialog("Enter add: "));
entry[counter].setPhoneNo(JOptionPane.showInputDialog("Enter Phone No.: "));
entry[counter].setEmail(JOptionPane.showInputDialog("Enter E-mail: "));
counter++;
}
public void viewAll() {
String addText = " NAME\tADDRESS\tPHONE NO.\tE-MAIL ADD\n\n";
for (int i = 0; i < counter; i++) {
addText = addText + entry[i].getInfo() + "\n";
}
JOptionPane.showMessageDialog(null, new JTextArea(addText));
}
public void searchEntry() {
int notfound = 0;
SName = JOptionPane.showInputDialog("Enter Name to find: ");
for (int i = 0; i < counter; i++) {
if (entry[i].getName().equals(SName)) {
JOptionPane.showMessageDialog(null, entry[i].getInfo2());
break;
} else {
notfound++;
}
}
if (notfound != 0) {
JOptionPane.showMessageDialog(null, "Name Not Found!");
}
notfound = 0;
}
public void editEntry() {
int notfound = 0;
SName = JOptionPane.showInputDialog("Enter Name to edit: ");
for (int i = 0; i < counter; i++) {
if (entry[i].getName().equals(SName)) {
entry[i] = new AddressBookEntry();
entry[i].setName(JOptionPane.showInputDialog("Enter new name: "));
entry[i].setAdd(JOptionPane.showInputDialog("Enter new add: "));
entry[i].setPhoneNo(JOptionPane.showInputDialog("Enter new Phone No.: "));
entry[i].setEmail(JOptionPane.showInputDialog("Enter new E-mail: "));
break;
} else {
notfound++;
}
}
if (notfound != 0) {
JOptionPane.showMessageDialog(null, "Name Not Found!");
}
notfound = 0;
}
public void deleteEntry() {
SName = JOptionPane.showInputDialog("Enter Name to delete: ");
for (int i = 0; i < counter; i++) {
if (entry[i].getName().equals(SName)) {
JOptionPane.showMessageDialog(null, "Found!");
entry[i] = null;
break;
}
}
}
}

Assigning the values to null is going to be the easiest practice. If you're really picky, you could resize the array, but that would be rather pointless. Just keep a separate size counter and decrement it each time you set something to null.
Another reason you're getting a null pointer exception is that you have to consider what's happening when you're replacing values in your array with null but still iterating by counter. You're going to be left with holes in your array upon deletion. The first solution would be to bypass null values altogether, and just shift your array down (somewhat of an expensive operation). The second would be to alter your methods to take those null values into consideration. Example:
public void viewAll() {
String addText = " NAME\tADDRESS\tPHONE NO.\tE-MAIL ADD\n\n";
int nonNull = 0;
for (int i = 0; i < entry.length; i++) {
if (entry[i] != null) {
addText = addText + entry[i].getInfo() + "\n";
nonNull++;
}
if (nonNull == counter) break;
}
JOptionPane.showMessageDialog(null, new JTextArea(addText));
}
I don't have a compiler on this computer, so consider it more of psuedo-code. But the idea is that the counter is only keeping track of how many non-null values you have in your address book, and that these null values could be in random places of your array. I added the nonNull integer as a local counter to keep track of how many values you've encountered that aren't null (so you aren't forced to run through the entire address book). Then, I added the if statement to ensure that the value at entry[i] isn't a null value (trying to invoke getInfo() on a null value is what's giving you that error). Lastly, I added the if statement to break the loop if you've encountered all of the non-null values you have stored. Hope this helps. (Also it may be worth considering a LinkedList to eliminate the null values all together).
Actually, for simplicity's sake, you probably are much better off using a LinkedList, unless you are required to use an array, since you would need to alter all of your methods to take null spaces in your array into account. Assuming you're familiar with LinkedLists of course.

Arrays are immutable. You can change the value for a particular index in the array but you can't change the array size itself. To "delete", you could do:
myArray[index] = null;
And just treat null values as unset/deleted entries.

Assigning to null (currently what you are doing) is the proper thing to do. That will eliminate the reference to the object at that index and allow it to be garbage collected.

Replace entry[i] = null; with this:
System.arraycopy(entry, i + 1, entry, i, counter - i - 1);
--counter;
entry[counter] = null; // optional; helps with garbage collection
--i; // required to not skip the next element
(I'm assuming here that counter is the number of valid entries in entry. This will leave no null entries among the first counter elements of entry (assuming that there weren't any to start with).
Further thought: If you need the array length to always match the number of valid entries, you'll have to re-allocate the array and copy the values over. Just use arraycopy to copy entries from 0 through i-1 and from i+1 to counter-1 into the new array and then assign it to entry. This isn't particularly efficient and is best avoided if possible.

Better to this is List which has remove() method. But if you really want use Array I recommend you change Array to List and then remove all values, after it you can always change List to Array

import javax.swing.JOptionPane;
public class Test {
private static User[] entry = new User[] { new User("Gil"),
new User("Bil"), new User("John") };
public static void main(String... args) {
final Test test = new Test();
test.deleteEntry();
for (int index = 0; index < entry.length; index++) {
User user = entry[index];
if (user != null)
System.out.println(entry[index]);
}
}
public void deleteEntry() {
String SName = JOptionPane.showInputDialog("Enter Name to delete: ");
for (int index = 0; index < entry.length; index++) {
if (entry[index].getName().equals(SName)) {
JOptionPane.showMessageDialog(null, "Found!");
entry[index] = null;
break;
}
}
}
private static class User {
private String name;
public User(String name) {
this.name = name;
}
/**
* #return the name
*/
public String getName() {
return name;
}
#Override
public String toString() {
return name;
}
}
}