Develop Reference

Looking through class Arraylist objects without nested loops

I had this object oriented programming project last semester and it was all about simulating a simple dormitory.
1.There should have been a manager class in which the main method and like 80 percent of the other methods had to be there.
2.A dormitory class containing an arraylist of blocks.
3.A block class containing an arraylist of rooms.
4.A room class containing an arraylist of students.
5.All of these classes contained getters and setters for their private variables.
There were a few methods that we should've added to the program which has nothing to do with my question,so I will not write them in here,but the thing is,a few of theses methods needed to look through these arraylists to find a specific object.For example,a student with a specific student number.I created these methods with nested loops.But I know there is some way I could avoid heavy nesting.
Here is my reduced code.I will only let the manager class contain one nested loop method as an example:
import java.util.Scanner;
public class Manager {
private String name;
private String familyName;
private String userName;
private String passWord;
private static Scanner scan = new Scanner(System.in);
private Dormitory dorm = new Dormitory();
private static Menu menu = new Menu();
private Student tempStudent;
private Block tempBlock;
private Room room;
private boolean bool;
{
dorm.setDormManager(this);
}
public Manager(String managerName, String managerID) {
name = managerName;
userName = managerID;
}
//find student with its number
public void findStudent() {
//Taking the student number from the user.
System.out.println("Please enter the student number:");
String studentNum = scan.nextLine();
for (int i = 0; i < dorm.getBlockList().size(); i++)
for (int j = 0; j < dorm.getBlockList().get(i).getRooms().size(); j++)
for (int k = 0; k < dorm.getBlockList().get(i).getRooms().get(j).getRoomStudents().size(); k++)
if (dorm.getBlockList().get(i).getRooms().get(j).getRoomStudents().get(k).getStudentNumber().equals(studentNum)) {
tempStudent = dorm.getBlockList().get(i).getRooms().get(j).getRoomStudents().get(k);
break;
}
}
public void create() {
//Used loops for the original program.
Block block1 = new Block("1");
Block block2 = new Block("2");
dorm.getBlockList().add(block1);
dorm.getBlockList().add(block2);
Room room1 = new Room("1");
Room room2 = new Room("2");
dorm.getBlockList().get(0).getRooms().add(room1);
dorm.getBlockList().get(1).getRooms().add(room2);
Student student1 = new Student("12345678");
Student student2 = new Student("98765432");
dorm.getBlockList().get(0).getRooms().get(0).getRoomStudents().add(student1);
dorm.getBlockList().get(1).getRooms().get(0).getRoomStudents().add(student2);
}
public static void main(String[] args) {
Manager manager = new Manager("Dumbledore", "#1112");
manager.create();
}
}
public class Dormitory {
private int blocks;
private Manager dormManager;
private long allMembers;
private ArrayList<Block> blockList = new ArrayList<Block>();
}
public class Block {
private String blockNumber;
private ArrayList<Room> rooms = new ArrayList<Room>();
private Dormitory dorm = new Dormitory();
public Block(String blockNum) {
blockNumber = blockNum;
}
}
public class Room {
private String roomNumber;
private ArrayList<Student> roomStudents = new ArrayList<Student>();
private Block roomBlock;
private Student roomManager;
public Room(String roomNum) {
roomNumber = roomNum;
}
}
public class Student {
private String studentName;
private String studentFamilyName;
private String studentNumber;
private Room room;
public Student(String studentNum) { //Creates a student object using the student number.
studentNumber = studentNum;
}
}
I tried my best to reduce the code.
I searched a lot and asked a lot of people about this but I didn't get my desired answer.I'm not sure why I couldn't find anything about this,but I'd really appreciate it if you'd lend me a hand or give me the link of a related article.

Short answer: No, you should never loop through everything checking for getStudentNumber().equals(studentNum). This has linear time complexity O(N)
Long answer: You should index your data based on your queries
Eg: Indexing with HashMaps which have constant time complexity O(1). (Note: This code is not thread safe)
public class SchoolService {
private Map<String, Student> studentsById = new HashMap<>();
private Map<Long, Dorm> dormsById = new HashMap<>();
/// dormsByAreaCode is showing an example of an index which groups objects into lists
private Map<String, List<Dorm>> dormsByAreaCode = new HashMap<>();
public void addStudent(Student student) {
if (studentsById.containsKey(student.getName()) {
throw new IllegalStateException("Duplicate student " + student.getName());
}
studentsById.put(student.getId(), student);
}
public Student getStudentById(String studentId) {
Student student = studentsById.get(studentId);
if (student == null) {
throw new IllegalStateException("No such student " + studentId);
}
return student;
}
public void addDorm(Dorm dorm) {
// TODO: validation
dormsById.put(dorm.getId(), dorm);
List<Dorm> areaDorms = dormsByAreaCode.get(dorm.getAreaCode());
if (areaDorms == null) {
areaDorms = new ArrayList<>();
dormsByAreaCode.put(dorm.getAreaCode(), areaDorms);
}
areaDorms.add(dorm);
}
public Dorm getDormById(long dormId) {
Dorm dorm = dormsById.get(id);
// TODO: validation
return dorm;
}
public List<Dorm> getDormsByAreaCode(String areaCode) {
List<Dorm> areaDorms = dormsByAreaCode.get(areaCode);
// TODO: validation
return areaDorms;
}
// etc
}

The following quote is from tutorialspoint. This is the perfect use case of the forEach method from the Stream interface. The link I provided and further reading on Streams can help avoid repetitive code.
Using collections framework in Java, a developer has to use loops and make repeated checks. Another concern is efficiency; as multi-core processors are available at ease, a Java developer has to write parallel code processing that can be pretty error-prone.
dorm.getBlockList().stream().forEach((b) -> {
b.getRooms().stream().forEach((r) -> {
...
})
});
You can also read about parallelStreams from here.

arraylist in java to sort data [duplicate]

This question already has answers here:
Collections.sort with multiple fields
(15 answers)
Closed 7 years ago.
so i'm having issues making this work for me. what this code needs to do is have 3 different (string) fields that then sort them into alphabetical order i've had help before but it wont run on my netbeans. i am currently up to date with all updates as well.
heres the code i have so far
import java.util.Scanner;
import java.util.List;
import java.util.ArrayList;
import java.util.Comparator;
public class test {
private Scanner scan = new Scanner(System.in);
private List<LayoutOfScientist> scientistsNames = new ArrayList<LayoutOfScientist>();
private String name, field, idea;
private boolean continueLoop = true;
private int countTo3 = 0;
private void run() {
while(countTo3<3&&continueLoop) {
if(countTo3>0) {
System.out.println("Would you like to add another scientist? (Y/N)");
}
if(countTo3 == 0 || scan.nextLine().equalsIgnoreCase("y")) {
System.out.println("Please enter the scientist's name:");
name = scan.nextLine();
System.out.println("Please enter the scientist's field:");
field = scan.nextLine();
System.out.println("Please enter the scientist's idea:");
idea = scan.nextLine();
scientistsNames.add(new LayoutOfScientist(name, field, idea));
} else {
continueLoop = false;
}
countTo3++;
}
scientistsNames.sort(Comparator.comparing(LayoutOfScientist::getScientistName));
for(LayoutOfScientist lOS : scientistsNames) {
System.out.println(lOS.getScientistName() + ", " + lOS.getScientistField() + ", " + lOS.getScientistIdea());
}
}
private class LayoutOfScientist {
private String scientistName, scientistField, scientistIdea;
private LayoutOfScientist(String scientistName, String scientistField, String scientistIdea) {
this.scientistName = scientistName;
this.scientistField = scientistField;
this.scientistIdea = scientistIdea;
}
public String getScientistName() {
return scientistName;
}
public String getScientistField() {
return scientistField;
}
public String getScientistIdea() {
return scientistIdea;
}
}
public static void main(String[] args) {
new Test().run();
}
}

Your class name is test (lowercase t) and in your main method, you are calling Test().run().
You need to rename your class to be Test and that should work. Or if your file is test you need to change Test().run() to test().run() instead of public class test to public class test. However, it is good programming practice to name a ClassLikeThis.
If your error is something else entirely, tell us what the error is.

Java HashMaps why does this return null?

I setup a test jar app to run for my other program I have been working on this for hours and i cant find a reason for why it returns null. Thanks for helping!
hashy.java
import java.util.HashMap;
public class hashy {
private static HashMap<String, Integer> targets = new HashMap<String, Integer>();
public static void main(String[] args) {
Hashymashy mash = new Hashymashy();
mash.hashyMash();
String name = "Bobby";
int num = 10;
targets.put(name, num);
if (targets.containsKey(name) == true) {
System.out.println("It contains a key!");
} else {
System.out.println("It does not contain a key!");
}
if (targets.containsValue(num) == true) {
System.out.println("It contains a value");
} else {
System.out.println("It does not contain a value!");
}
}
public HashMap<String,Integer> getTargets(){
return targets;
}
}
Hashymashy.java
public class Hashymashy {
public void hashyMash(){
hashy h = new hashy();
String name = "Bobby";
Integer fnum = h.getTargets().get(name);
System.out.println("Number is "+fnum+"!");
}
}

You are trying to retrieve the value associated with "Bobby" before adding it to the HashMap.
mash.hashyMash();
is called before targets.put(name, num);, so
Integer fnum = h.getTargets().get("Bobby");
will return null since there is no "Bobby" yet.
PS: seems like a bad design to me, since Hashymashy classes create instances of hashy classes & vice-versa.

(Library) checked out status returning all "true"

Instead of using boolean to see if a book was checked out I thought using a string to represent it was what I wanted but when I call the method to checkout a book the string applies to the entire array list of book objects. How can i change the status of just that book ?
import java.util.*;
public class Library
{
String owner;
int numBooks;
boolean isCheckedOut;
String status = "false";
ArrayList<Book> bookList = new ArrayList<Book>();
public void addBook(Book b)
{
bookList.add(b);
}
public String checkout(Book c) {
status = "true";
return status;
}
public Library(String o)
{
owner = o;
}
public String toString()
{
String s = "Owner: \t" + owner + "\nSize: \t" +
bookList.size()+"\nBooks: \t";
for( int i = 0;i < bookList.size();i++)
{
String title = bookList.get(i).getTitle();
//System.out.println(title);
s+="\n\t\t\t" +title +" Checked out: " + status; //bookList.get(i).getStatus();
}
return s;
}
public static void main(String [] args)
{
Library l = new Library("Jeremiah");
System.out.println(l);
}
}
i
public class Mainn
{
public static void main(String [] args)
{
Book book1 = new Book("Aambi","Aisney","Aalt","1942",1453);
Book book2 = new Book("Bambi","Bisney","Balt","1942",1453);
Book book3 = new Book("Cambi","Cisney","Calt","1942",1453);
Book book4 = new Book("Dambi","Disney","Dalt","1942",1453);
Book book5 = new Book("Eambi","Eisney","Ealt","1942",1453);
Book book6 = new Book("Fambi","Fisney","Falt","1942",1453);
Book book7 = new Book("Gambi","Gisney","Galt","1942",1453);
Book book8 = new Book("Hambi","Hisney","Halt","1942",1453);
Book book9 = new Book("Iambi","Iisney","Ialt","1942",1453);
Book book10 = new Book("Jambi","Jisney","Jalt","1942",1453);
Book book11 = new Book("Kambi","Kisney","Kalt","1942",1453);
Library l = new Library("Jeremiah");
// l.addBook(new Book("Xambi","Xisney","Xalt","1579",765));
l.addBook(book1);
l.addBook(book2);
l.addBook(book3);
l.addBook(book4);
l.addBook(book5);
l.addBook(book6);
l.addBook(book7);
l.addBook(book8);
l.addBook(book9);
l.addBook(book10);
l.addBook(book11);
// this changes all of the values to true instead of just this one
l.checkout(book1);
l.checkout(book11);
l.addBook(book1);
System.out.println(l);
}
}

You are storing the status against the Library, and not against each Book. To check out a single book, you'll need to put the status field into the book, and then set the book's status:
public class Book
{
public String status;
// etc...
}
public String checkout(Book c) {
c.status = "true";
return c.status;
}
Storing a boolean value as a string isn't usually the best idea - you'd normally be better off using an actual Boolean field for this.
A cleaner (more object oriented) implementation would look something like:
public class Book {
private boolean checkedOut = false;
public void checkout() {
checkedOut = true;
}
public void checkin() {
checkedOut = false;
}
public boolean isCheckedOut() {
return checkedOut;
}
}
This keeps the logic for managing a book with the book itself, and would allow you to easily add validation, such as preventing checkout if the field is already set to true.

So a big problem I'm seeing here is that you don't have the "Status" attribute on each book separately, you just have the one status String for the entire library.
It's not storing a status for each book.
What you could do is add an attribute called status to the Book class, and in the method where you check it out, you would set that to false. When printing them, you would print the status of each book by accessing this variable within each book object.

I think, the following piece of code help you:
public class Book {
private boolean status;
public boolean getStatus() {
return this.status;
}
public void setStatus(boolean status) {
this.status = status;
}
}
public class Library {
public boolean checkout(Book c) {
c.setStatus(true);
return c.getStatus();
}
}
And I advice to you use a boolean primitive type instead a String object.

Problems when creating methods in my task

The goal is to produce this:
Picture of the task summary here
These are the errors I get when I try to compile:
screen shot
I have changed and fixed most of the more obvious errors I think which was mainly just stupid of me. Sorry.
I have this code
public class Ex5Program {
public void start() {
Tutor[] tutors = createTutorsArray();
printTutors(tutors);
printOnLeaveList(tutors);
updateTutorDetails(tutors[1]);
printNewTutorDetails(tutors[1]);
Tutor tutorWithMostPapers = getTutorWithMostPapers(tutors);
printTutorWithMostPapers(tutorWithMostPapers);
}
private Tutor[] createTutorsArray() {
String[] noPapers = {};
String[] introductoryPapers = {"CompSci101", "CompSci111"};
String[] coreStage1Papers = {"CompSci101", "CompSci105"};
String[] allStageOnePapers = {"CompSci111", "CompSci101", "CompSci105"};
String[] stageTwoPapers = {"CompSci210", "CompSci220", "CompSci225", "CompSci230"};
Tutor[] tutors = new Tutor[7];
tutors[5] = new Tutor("Sad Sack", 86302, introductoryPapers, false);
tutors[4] = new Tutor("Crystal Ball", 49123, introductoryPapers, false);
tutors[2] = new Tutor("Earl Lee Riser", 40879, allStageOnePapers, true);
tutors[3] = new Tutor("Tom Katt", 50876, stageTwoPapers, false);
tutors[1] = new Tutor("Candy Kane", 30869, noPapers, false);
tutors[0] = new Tutor("Carrie Oakey", 30987, coreStage1Papers, true);
tutors[6] = new Tutor("Sonny Day", 49586, stageTwoPapers, true);
return tutors;
}
private void printTutors(Tutor[] tutors) {
System.out.println("Current Tutors");
System.out.println("==============");
for (int i = 0; i < tutors.length; i++) {
System.out.print(i + 1 + ". ");
System.out.println(tutors[i].toString());
}
}
private void printOnLeaveList(Tutor[] tutors) {
System.out.println();
System.out.println("Tutors Currently on Leave");
System.out.println("=========================");
for (int i = 0; i < tutors.length; i++) {
if (tutors[i].isOnLeave()) {
System.out.println(tutors[i].getName());
}
}
}
private void updateTutorDetails(Tutor tutor) {
tutor.setName("Ali Katt");
tutor.setStaffId(23456);
String[] stage1Papers = {"CompSci101", "CompSci105", "CompSci111"};
tutor.setPapers(stage1Papers);
tutor.setOnLeave(true);
}
private void printNewTutorDetails(Tutor tutor) {
System.out.println();
System.out.println("Updated details");
System.out.println("===============");
System.out.println("Name: " + tutor.getName());
System.out.println("Id: " + tutor.getStaffId());
String[] papers = tutor.getPapers();
System.out.print("Papers: ");
if (papers.length > 0) {
for (int i = 0; i < papers.length; i++) {
System.out.print(papers[i] + " ");
}
} else {
System.out.print("None");
}
System.out.println();
if (tutor.isOnLeave()) {
System.out.println("Currently on leave");
}
}
private Tutor getTutorWithMostPapers(Tutor[] tutors) {
Tutor tutorWithMostPapersSoFar = tutors[0];
for (int i = 0; i < tutors.length; i++) {
if (tutors[i].teachesMorePapersThan(tutorWithMostPapersSoFar)) {
tutorWithMostPapersSoFar = tutors[i];
}
}
return tutorWithMostPapersSoFar;
}
private void printTutorWithMostPapers(Tutor tutorWithMostPapers) {
System.out.println();
System.out.println("Most papers");
System.out.println("===========");
System.out.println(tutorWithMostPapers.getName() + " teaches more papers than any other tutor.");
}
}
and I created this code here(It has been changed):
public class Tutor {
// instance variables
private String name;
private int staffId;
private String[] papers;
private boolean onLeave;
public Tutor(String name, int staffId, String[] papers, boolean onLeave) {
// Complete this constructor method
this.name = name;
this.staffId = staffId;
this.papers = papers;
this.onLeave = onLeave;
}
// Insert getName() method here
public String getName(){
return name;
}
// Insert setName() method here
public void setName(String name){
this.name = name;
}
// Insert getStaffId() method here
public int getStaff(){
return staffId;
}
// Insert setStaffId() method here
public void setStaffId(int StaffId){
this.staffId = staffId;
}
// Insert getPapers() method here;
public String[] getPapers(){
return papers;
}
// Insert setPapers() method here
public void setPapers(String[] papers){
this.papers = papers;
}
// Insert isOnLeave() method here
public boolean isOnLeave(){
return onLeave;
}
// Insert setOnLeave() method here
public void setOnLeave(boolean OnLeave){
this.onLeave = onLeave;
}
// Insert toString() method here
public String toString(){
return name + "(Staff id:"+staffId+")";
}
// Insert teachesMorePapersThan() method here
public Tutor teachesMorePapersThan(Tutor other){
return(papers.length>other.papers.length);
}
}

Typo: toString() not tostring(), which results in Object.toString() is being invoked and the intended formatted string is not being returned. Change to:
#Override public String toString()
Using the #Override annotation would have produced a compiler error in the case of tostring() being the method name and alerted you to the error, because no method of that name exists in a superclass.
Several of the setter methods have missing parameters:
// Insert setPapers() method here
public void setPapers(){
this.papers = papers;
}
// Insert setOnLeave() method here
public void setOnLeave(){
this.OnLeave = OnLeave;
}

First error: setStaffID()
You are calling it using an int, but on your method you say it doesn't have any parameter.
Take a look that you have some others errors caused by the same mistake. Correct them first...

You need to look at the error text to find the problems. While a newbie may instinctively just dismiss the error messages as uselesss (as a result of years of clicking the x or cancel or whatever on windows dialogues), The error text is actually the most useful resource for figuring out what the error is, 90% of the time.
For instance, the first error reads
File: F:\course related stuff\101\Lab06\Ex5\Ex5Program.java [line: 54]
Error: method setStaffId in class Tutor cannot be applied to given types;
required: no arguments
found: int
reason: actual and formal argument lists differ in length
If you read it carefully, you can see it tells you the name of the file, the line number, the method call name, the class name containing the method, and some additional information about the exact type of error. It is even telling you what you did wrong in calling the method, by putting an "int" where "no arguments" were required, that the "actual and formal argument lists differ in length".
Read the other error messages, and you will see that they actually tell you what the problem is.
This code also needs newlines inserted to group blocks of stuff, comments added to explain exactly how it works, and a few java style violations fixed - some teachers grade for style and clarity as well as just functionality.
Also, if the reason you are failing your class is because you don't understand how to program, it may be because of excessive use of stack overflow to solve the problems. In the real world, if you can just use somebody else's code, that's great, but the point of a programming class is is to teach you how to come up with your own code, not how to use somebody else's.

Well it's not easy to help, because i think you don't know what you are doing. But first thing when you create a set method like this:
public void setPapers(){
this.papers = papers;
}
you should declare the arguments like this:
public void setPapers(String[] papers){
this.papers = papers;
}
and you should know that variables names is caseSensitive so :
private boolean onLeave;
public boolean isOnLeave(){
//return OnLeave; this variable is not declared
return onLeave;
}
I think you need to study a little more, because you can't read the compilation errors.