I'm trying to output a string from an Object that has been passed into a set. The Following line is where my problem lies. It outputs [alex, jane] but with correct formatting I believe it should be outputted at alex jane. i.e. without the comma separated value and the brackets from the array.
System.out.print(module.getStudents() + " ");
I've tried various solutions including:
System.out.prinf(%s, module.getStudents() + " ");
and
System.out.prinln(module.getStudents().[whatever Netbeans makes available] + " ");
To help you better understand the problem. The idea of the application so far is to allow a user to search for a mosule ans return all students connected to it. The full source code bar the driver is:
import java.util.*;
public class Control {
public void run() {
Student jane = new Student("jane");
Student alex = new Student("alex");
Set<Student> students = new HashSet<Student>();
students.add(jane);
students.add(alex);
Module ufce1 = new Module("UFCE1");
Module ufce2 = new Module("UFCE2");
Set<Module> modules = new HashSet<Module>();
modules.add(ufce1);
modules.add(ufce2);
jane.addModule(ufce1);
jane.addModule(ufce2);
alex.addModule(ufce2);
ufce1.addStudent(jane);
ufce2.addStudent(jane);
ufce2.addStudent(alex);
System.out.println("Search module code: ");
Scanner scan = new Scanner(System.in);
scan = new Scanner(System.in);
String searchModule = scan.nextLine().trim();
for (Module module : modules) {
if (searchModule.equalsIgnoreCase(module.getName())) {
Iterator it = students.iterator();
Student student = (Student) it.next();
if (student.getModules().contains(module)) {
System.out.print(student + " ");
}
}
}
}
}
Module Class:
import java.util.HashSet;
import java.util.Set;
public class Module {
private String name;
private Set<Student> students = new HashSet<Student>();
public Module(String name) {
this.name = name;
}
public String getName() {
return name;
}
public void addStudent(Student student){
students.add(student);
}
public Set<Student> getStudents() {
return students;
}
#Override
public String toString() {
return name;
}
}
Student Class:
import java.util.HashSet;
import java.util.Set;
public class Student {
private String name;
private Set<Module> modules = new HashSet<Module>();
public Student(String name) {
this.name = name;
}
public String getName() {
return name;
}
public void addModule(Module module){
modules.add(module);
}
public Set<Module> getModules() {
return modules;
}
#Override
public String toString() {
return name;
}
}
When you do this System.out.print(module.getStudents() + " "); you're implicitly calling the toString method on the HashSet instance. So to get the formatting you want, you have 2 choices:
iterate over the set and print it the way you want
Subclass HashSet and override toString to display the way you want it.
The problem is that getStudents returns a Set object (a HashSet, to be specific). HashSet, in turn, inherits a toString() method from AbstractCollection which behaves as follows:
Returns a string representation of this collection. The string representation consists of a list of the collection's elements in the order they are returned by its iterator, enclosed in square brackets ("[]"). Adjacent elements are separated by the characters ", " (comma and space). Elements are converted to strings as by String.valueOf(Object).
You'll need to write your own method for converting a set of students into the format you want, or else you can doctor up the value returned by the default toString implementation.
The brackets and the commas are coming from the toString() call on the Set. If you look in the source code of this method, you will see that it adds those. You can override the toString() method of the Set in your Module class, or just not printing the Set directly but manually looping over all elements and printing them one by one
#Chris gave an excellent solution in resolving your issues. There is however another one that I see more easy to implement and it is the following:
public String formatOutputString(){
String setStrings = module.getStudents();
// Get rid of opening bracket
String formatedString = setStrings.replace("[", "");
// Get rid of closing bracket
formatedString = setStrings.replace("]", "");
// Replace commas by spaces
formatedString = setStrings.replace(",", " ");
return formatedString;
}
Related
I'm new to Java and I'm trying to get my head around constructs, classes and objects. I apologise if any of this seems stupid to you.
I have been tasked with creating an ArrayList to hold items of type Data (my class). using the .add method - I should add a certain number of names and ages then output.
I have sort of done it, but I was wondering if I could use the functions like SetAge and SetAge from my class to use within my ArrayList to create a new person and then output it together like I have with the rest. Is there another way?
Thank you for your help & explanation.
Below is my code;
import java.util.ArrayList;
public class workingOn {
public static void main (String[] args)
{
Data Fred = new Data("Fred", 21);
Data Jo = new Data("Jo", 43);
Data Zoe = new Data("Zoe", 37);
ArrayList<Data> myArray = new ArrayList<Data>();
myArray.add(Fred);
myArray.add(Jo);
myArray.add(Zoe);
for (Data temp : myArray)
{
System.out.println(temp.toString());
}
}
}
Below is my class;
public class Data {
private String name;
private int age;
Data(String n,int a)
{
name = n;
age = a;
}
public String GetName()
{
return(name);
}
public void SetName(String n)
{
name = n;
}
public int GetAge()
{
return(age);
}
public void SetAge(int a)
{
age = a;
}
public void Print()
{
System.out.print(("("+ GetName() ));
System.out.print(",");
System.out.print(GetAge());
System.out.print(") ");
}
//i made this so I don't output the object id
public String toString() {
return (name + ", " + age);
}
}
Going through the comments, I am guessing you want to somehow use the setter methods (setName() and setAge()) for the class that you have designed. The easiest way would be:
Data d = new Data("tempName", 10);
myArray.add(d);
This basically creates an object of type Data and adds it to the arraylist. Let's assume that you want to change the name and age of this person you just added, then you should do this:
myArray.get(myArray.size() - 1).setName("newName");
myArray.get(myArray.size() - 1).setAge(18);
To add some explanation, when you add an object to an arraylist, it always adds in the end. We use size() method to get the number of objects in the Arraylist. Since index starts from 0, we use myArray.size() - 1. We use the .get() method to retrieve the object from the arraylist which takes a number as a parameter and return the object at that index.
Combining these two, we get the object at the last possible index, and call the setName() and setAge() function on it.
Hope this helps.
HashSet<Soldier> soldiers; // it has name, rank, description
#Override
public String toString(){
return "Team: " + teamName + "\n" + "Rank: " + getRanking(soldiers) + "\n"
+ "Team Members Names are: "+"\n" + soldiers.iterator().hasNext();
//last line doesn't work
// I also tried soldiers.forEach(System.out::println) but doesn't work
}
Can anyone please how I can print all the name from Hashset in overriden toString method. Thanks
If you use java 8. It's simple to do with stream API:
public class Test {
public static void main(String[] args) {
Set<String> strings = new HashSet<>();
strings.add("111");
strings.add("113");
strings.add("112");
strings.add("114");
String contactString = strings.stream().map(String::toString).collect(Collectors.joining(","));
}
}
If you want change a delimiter you should replace Collectiors.joining(",") code to what you need. See also documentation by StringJoiner
For your class Soldier which has method getName():
Set<Soldier> soldiers = new HashSet<>();
String soldierNames = soldiers.stream().map(Soldier::getName).collect(Collectors.joining("\n"));
You will get a next result:
Din
Mark
David
... values from the soldiers set
hasNext() does only return a boolean indicating if the Iterator is finished or not.
You still have to call next() (in a loop) to get the next value(s).
See: https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
Ok, now I think I've given up all hope of finding solution to what should to be a simple problem. Basically, I'm creating a students' record system that stores students' details in an ArrayList. I first created a constructor in the Student class to specify what entries each student will have. I then created an instance of the Student class in the main class (i.e. class with the main method) and then added the student object to the studentList ArrayList.
By the way, instead of hard-coding the student details, my initial aim was to let the user enter the details and then I'll use a Scanner or BufferedReader object to get the details stored in the Student object, and then to the ArrayList but I'm having trouble with that as well; so I'll probably tackle that problem as soon as I'm done with this one.
Anyway, I'm expecting the output to print out the students' details but instead I get a memory location (i.e. [studentrecordsys.Student#15c7850]). I'm aware that I need to override the toString method but how exactly this is done is what I can't seem to get. I get syntax errors everywhere as soon as I enter the #Override code block for the toString method. Here's what I've tried:
import java.util.*;
class Student {
private String studentID;
private String studentName;
private String studentAddress;
private String studentMajor;
private int studentAge;
private double studentGPA;
Student (String studentID, String studentName, String studentAddress, String
studentMajor, int studentAge, double studentGPA){
this.studentID=studentID;
this.studentName=studentName;
this.studentAddress=studentAddress;
this.studentMajor=studentMajor;
this.studentAge=studentAge;
this.studentGPA=studentGPA;
}
}
public static void main(String[] args) {
Student ali = new Student("A0123", "Ali", "13 Bond Street", "BSc Software Engineering", 22, 3.79);
List<Student> studentList = new ArrayList<>();
studentList.add(ali);
#Override
String toString() {
StringBuilder builder = new StringBuilder();
builder.append(ali).append(studentList);
return builder.toString();
}
System.out.println(builder);
}
You need to implement the toString() on the Student object.
public class Student {
...
Your existing code
...
#Override
public String toString() {
return studentID + ", " + studentName + ", " + //The remaining fields
}
}
then in your main method, call
for (Student student : studentList) {
System.out.println(student.toString());
}
You to override toString method because it is going to give you clear information about the object in readable format that you can understand.
The merit about overriding toString:
Help the programmer for logging and debugging of Java program
Since toString is defined in java.lang.Object and does not give valuable information, so it is
good practice to override it for subclasses.
Source and Read more about overriding toString
public String toString() {
return "Studnet ID: " + this.studentID + ", Student Name:"
+ this.studentName+ ", Studnet Address: " + this.studentAddress
+ "Major" + this.studentMajor + "Age" + this.studentAge
+ GPA" + this.studentGPA ;
}
You get errors because you have to Override the toString method inside the class you want to use it for. i.e you have to put the method, with the #Override inside your Student class.
And you can call it like this:
System.out.println(studentA.toString());
System.out.println(studentB.toString());
or in a loop:
for(Student x : studentList)
System.out.println(x.toString());
and so on..
Also, in your code you create a method inside your main method. Of course you will get errors.
NOTE: I edited my code to how I think people are trying to tell me but it still doesn't give me my desired output. Now my output is "examples.search.Person#55acc1c2" however many times I enter new first and last names. At least it's making it through the code with out crashing lol
I am learning how to use ArrayLists and need to load an Array list with instances of an Object I created. I know how to do this with an array but for this assignment I need to do it with an ArrayList. Here's an example of what I need to do.
// my "main" class
package examples.search;
import java.util.ArrayList;
import dmit104.Util;
public class MyPeople {
public static void main(String[] args) {
ArrayList<Person> people = new ArrayList<Person>();
Person tempPerson = new Person();
String firstName;
String lastName;
char choice = 'y';
int count = 1;
// fill my ArrayList
do {
people.add(tempPerson);
// I have a Util class that has a prompt method in it
firstName = Util.prompt("Enter First Name: ");
lastName = Util.prompt("Enter Last Name: ");
tempPerson.setFirstName(firstName);
tempPerson.setLastName(lastName);
count++;
choice = Util.prompt(
"Enter another person? [y or n]: ")
.toLowerCase().charAt(0);
} while (choice == 'y');
// display my list of people
for(int i = 0; i < people.size(); i += 1) {
System.out.print(people.get(i));
}
}
}
// my Person class which I am trying to build from
public class Person {
// instance variables
private String firstName;
private String lastName;
// default constructor
public Person() {
}
public String getFirstName(){
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
}
I've tried it a number of ways but no matter what my ArrayList doesn't fill up. Like I mentioned I can do it no problem with an array or even if I had a loaded constructor method but I don't. In my actual assignment I am supposed to do it with the set methods.
I have looked everywhere and cannot find the solution for my problem and being friday my instructor isn't in.
Thank you so much in advance
Leo
You'll have to create a Person and then add it to the ArrayList.
for (int i = 0; i < count; i++) {
Person person = new Person();
person.setFirstName("Foo");
person.setLastName("Bar");
people.add(person);
}
Its crashing because your line people.get(i).setFirstName(firstName); is first trying to what is at index i, but you have not set anything yet.
Either first set people[i] to a empty Person, or make a person using firstName and lastName, and add it to people using people.add(person);
You have an ArrayList<Person>, but that alone only defines a list of potential Person instances. But so far, each of the list entries is null. The get(i) returns null, and the following null.setFirstName(..) causes a NullPointerException.
So you need to create the instances of Person that are supposed to go into the list:
firstName = Util.prompt("Enter First Name: ");
Person p = new Person(); //create the instance
people.add(p); //add the instance to the list
p.setFirstName("..."); //set any values
Now you are storing the Person Object into an ArrayList and printing that Object.
To print the firstname and lastName when you print the Person object, you will have to override toString method.
Add the following code in your Person class
public String toString(){
return String.format("[Personn: firstName:%s ,lastName: %s]", firstName,lastName);
}
As for the second question you had, you have to override the toString() method in the Person class. The outputs you are getting, such as examples.search.Person#55acc1c2 is the default toString() method from the Object class, which is defined as class#hashCode
I am trying to get this program to get the passwords from an array list.
import java.util.ArrayList;
public class CompanyDatabase {
public ArrayList<Person> getPeople() {
ArrayList<Person> people = new ArrayList<Person>();
String[] u = {"Joe","Stan","Leo","John","Sara","Lauren"};
String[] p = {"pass4321", "asdfjkl", "genericpw", "13579", "helloworld", "companypass"};
for(int j = 0; j < u.length; j++){
Person temp = new Person(u[j],p[j]);
people.add(temp);
}
return people;
}
}
import java.util.ArrayList;
import java.util.Scanner;
public class CompanyDatabaseDriver {
private static Scanner scan = new Scanner( System.in ) );
public static void main(String args[]) {
CompanyDatabase bcData = new CompanyDatabase();
ArrayList<Person> people = bcData.getPeople();
// what i tried
System.out.println(bcData.getPeople());
// also tried this
System.out.println(people.get(1));
}
}
The output is
[Person#1c9b9ca, Person#c4aad3, Person#1ab28fe, Person#105738, Person#ce5b1c, Person#1bfc93a]
or just
Person#1995d80
for the 2nd thing I tried.
The specific number / letter combination seems to change each time the program is run. Is there a way to specify which string to display from the array list?
Override toString() in the Person class
What you are seeing is the String returned by Object's default toString() method which is the name of the class followed by its hashcode. You will want to override this method and give the Person class a decent toString() method override.
e.g.,
// assuming Person has a name and a password field
#Override
public String toString() {
return name + ": " + password; // + other field contents?
}
Edit: if you only want to display one field in your output, then use Dave Newton's good solution (1+).
Yes; print the object property you want to see:
out.println(people.get(0).getFirstName());
the default implementation when you print List is to call toString for all objects in this List. and because you don't override toString method, it will call the default toString from Object class, that will print objects hashCode in hexadecimal notation, so you get this result:
Person#1c9b9ca ( classname#hashcode) , and it can be changed every time you execute the application because this hashcode come from memory address of the object).
so one option, is to override toString in your class
#Override
public String toString() {
return String.format("First name %s, Last name %s", firstName, lastName);
}
and call
System.out.println(yourList); // this will print toString for each object
the other option, is to print these attributes when you iterate on the List
for(Person person: persons) {
System.out.println("Person first name: " + person.getFirstName() + " , Last Name: " + person.getLastName());
}
In the first print statement you are trying to print the object..that is why you always see different number/letter combination..