Develop Reference

Java printing Class objects in ArrayList

Sailor class
public class Sailor {
private String name;
private String email;
public Sailor(String name, String email) {
}
public String getName() {
return name;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
}
Crew class
import java.util.ArrayList;
public class Crew {
private ArrayList<Sailor> sailorList = new ArrayList<>();
public Crew() {
}
public void addCrewMember(Sailor sailor) {
sailorList.add(sailor);
}
public String toString() {
String str = "";
for(int i = 0; i < sailorList.size(); i++) {
str = sailorList.get(i).getName() + sailorList.get(i).getEmail();
}
return str;
}
}
Main program
public class ObjectsSailorProgram {
public static void main(String[] args) {
Sailor firstSailor = new Sailor("Frank", "frank#mail.com");
Sailor secondSailor = new Sailor("Susan", "susan#mail.com");
Sailor thirdSailor = new Sailor("John", "john#sailors.com");
Sailor fourthSailor = new Sailor("Ann", "ann#sailors.com");
Crew firstCrew = new Crew();
Crew secondCrew = new Crew();
firstCrew.addCrewMember(firstSailor);
firstCrew.addCrewMember(secondSailor);
firstCrew.addCrewMember(fourthSailor);
secondCrew.addCrewMember(thirdSailor);
secondCrew.addCrewMember(secondSailor);
System.out.println("=== First crew ===\n" + firstCrew);
System.out.println("=== Second crew ===\n" + secondCrew);
secondSailor.setEmail("Susan#sailors.com");
System.out.println("=== Second crew ===\n" + secondCrew);
}
}
I am having trouble printing the crews and I'm not sure if the addCrewMember is correct.
I've tried reading other similar posts but i haven't been able to use the solutions here. So i need help with the addCrewMember and toString methods

You aren't setting the variables neither in constructor nor by calling setters in your main.
You can change the constructor as below:
public Sailor(String name, String email) {
this.name = name;
this.email = email;
}
In your toString() method, you're overwriting the str. Use StringBuilder to append strings into the resulting string. You can modify your toString() method as follows:
public String toString() {
StringBuilder str = new StringBuilder();
for(int i = 0; i < sailorList.size(); i++) {
str.append(i+1)
.append(". ")
.append(sailorList.get(i).getName())
.append(" ")
.append(sailorList.get(i).getEmail())
.append("\n");
}
return str.toString();
}
And your program will give the output:
=== First crew ===
1. Frank frank#mail.com
2. Susan susan#mail.com
3. Ann ann#sailors.com
=== Second crew ===
1. John john#sailors.com
2. Susan susan#mail.com
=== Second crew ===
1. John john#sailors.com
2. Susan Susan#sailors.com

0> sailor's constructor is wrong-> you're not initializing the data
public Sailor(String name, String email) {
setEmail(email); //or this.email=email;
this.name=name; //you may make a setter also for this
}
1> private ArrayList<Sailor> sailorList = new ArrayList<Sailor>();
2> the to-string is also wrong: it prints just the last sailor, use += instead of =
str +=" "+ sailorList.get(i).getName() + sailorList.get(i).getEmail();
This is just personal taste: I prefer "for-each" syntax.
for(Sailor s: sailorList){
out += s.getName()+" "+s.getEmail();
}

As others have noted, you are not assigning the values in your Sailor constructor so do that. You should also add any missing get/set methods.
public Sailor(String name, String email) {
this.name = name;
this.email = email;
}
The addCrewMember(Sailor) method in Crew looks correct but your toString() method is flawed.
You are trying to build a String but you are re-assigning it on each iteration rather then concatenating elements on each iteration.
Except in very simple cases, you should avoid String concatenation when compose Strings.
The code is pushing all of the elements together with no separation which means you will have an unreadable blob of characters (e.g. bobsmithbsmith#boat.comsamsmithssmith#boat.com)
I would suggest that you write a toString() in Sailor class. This example separates the elements of Sailor by commas and wraps the entire entity in curly braces (e.g. "{Sam Smith, ssmith#boat.com}"). Note the use of String.format(..) rather than concatenation.
public class Sailor {
... stuff ...
#Override public String toString() {
return String.format("{%s, %s}", name, email);
}
}
The Crew.toString() method can employ this to compose its String. The following makes use of the StringJoiner class to compose a comma-separated list of elements wrapped in square braces to represent the crew members (e.g. "[{Al Almond, aalmond#boat.com},{Bob Bobson, bbobson#boat.com}]");
public class Crew {
... stuff ...
#Override public String toString() {
StringJoiner sj = new StringJoiner(",", "[", "]");
for (Sailor s : sailorList) {
sj.add(s.toString()); // Sailor.toString() is explicitly invoked here but you could remove that call and it should still be invoked implicitly
}
return sj.toString();
}
}

Q: Player(class) java

Have to code a player class in java eclipse following these requirements
a) The Player class should have a default constructor and two custom constructors - one
that accepts a Name object, and another that accepts both a Name and PairOfDice object.
b) There should be get and set methods for its Name and a get method for PairOfDice. It
should have a method called rollDice and getDiceScore that both simply delegate to the
PairOfDice class, which already has this functionality. You should also have an
appropriate toString() method.
c) Add a further void method setFullPlayerName(String) that accepts a single String
argument (e.g. “Joe Bloggs”) and then uses this to set the first and family name
individually by extracting the relevant information and then calling the respective setter
methods of the Name class.
So far I have this
public class Player {
//Fields of the app
private String firstName;
private String lastName;
private Die red;
private Die blue;
//PlayerName and dice pair Default Constructor
public Player() {
firstName = "";
lastName = "";
red = new Die();
blue = new Die();
}
public Player(String firstName, String lastName) {
this.firstName = firstName;
this.lastName = lastName;
}
public Player(String firstName, String lastName,Die red,Die blue) {
this.firstName = firstName;
this.lastName = lastName;
this.red = red;
this.blue = blue;
}
// Methods
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public void setlastName(String lastName) {
this.lastName = lastName;
}
public String getFirstName() {
return firstName;
}
public String getlastName() {
return lastName;
}
#Override
public String toString() {
return "PairOfDice:[red=" + red + ", blue=" + blue + "]";
}
public void rollDice() {
red.roll();
blue.roll();
}
public int getDiceScore() {
return red.getScore() + blue.getScore();
}
public Die getRed() {
return red;
}
public Die getBlue() {
return blue;
}
public String setFullName() {
if (firstName.equals("") && lastName.equals("")) {
return "";
} else {
return firstName + " " + lastName;
}
}
}
Is my code correct? if not what changes do i have to make to correct it

There are several problems:
Dice not Die: private Die red;
It's better to name your variable using a noun: private Dice redDice;
Should have a space after a comma: , Dice redDice, Dice blueDice
setFullName should accept 1 string and call setFirstName and setLastName to set the name
Example of implementation:
void setFullName(String fullName) {
/// split full name into lastName and firstName
setFirstName(firstName);
setLastName(lastName);
}

Everything seems fine, but I think you forgot to define the roll() function. Besides that, everything else seems fine :)

I see a couple of issues with this code:
a) The Player class should have a default constructor and two custom constructors - one that accepts a Name object, and another that accepts both a Name and PairOfDice object.
b) There should be get and set methods for its Name and a get method for PairOfDice. It should have a method called rollDice and getDiceScore that both simply delegate to the PairOfDice class, which already has this functionality. You should also have an appropriate toString() method.
If it's telling you that you need a Name object and a PairOfDice object, you can't just put in two Strings and two Dies and call that the same thing. You need to actually use the Name and PairOfDice classes.
c) Add a further void method setFullPlayerName(String) that accepts a single String argument (e.g. “Joe Bloggs”) and then uses this to set the first and family name individually by extracting the relevant information and then calling the respective setter methods of the Name class
Instead of setFullPlayerName, you made a method called setFullName that does not do what it's supposed to do. It's supposed to accept a String and return void, and instead it accepts no parameters and returns a String. It looks like a getter instead of a setter.

Let's get this straight: This portion of the code:
public String setFullName() {
if (firstName.equals("") && lastName.equals("")) {
return "";
} else {
return firstName + " " + lastName;
}
}
it's misleading due to the fact that the method name it's saying that it's setting the name, BUT you're returning the name instead
So change it to this:
public void setFullPlayerName(String name) {
String[] splitted = name.split(" ");
firstname = splitted[0];
lastname = splitted[1]
}
And now it's unnecessary to add the method setFullPlayerName
BUT I see that you may need a method to return a full name, so:
public String getFullPlayerName() {
return firstname + " " + lastname;
}
Hopefully that'll resolve your issue :D

How to access class field variable inside all methods from toString twin toStrung?

Once upon modifying the above with static modifiers, line 16 requires the following syntax:
getLegs();toStrung();
//I think this is essentially printing the last called method updating class field variable toString. For example, to do setLegs();toStrung(); prints setLegs()'s toString.
Question: How should one access a shared field within methods? What if I included it into the constructor? Ideally, I want the code to look like getLegs().toStrung() and for toString to be a clean slate for every method.
My answer: I think a seperate instance of String toString inside each method works to get a clean slate appeal, but the syntax doesn't make sense. I know it is about my design. I think a solution would be a new class, but this returns to the same conflict that relates to the class field variable.
public class Dog{
public String toString;
public Dog(String name){
this.name = name;
}
public int getLegs(){
toString = "Dog has " + legs + " legs.";
return legs;
}
public int setLegs(int legs){
toString = getName() + "'s legs have changed from "
+ getLegs() + " to " + legs + ".";
this.legs = legs;
return this.legs;
}
public void toStrung(){
System.out.println(Dog.toString);
}
public static void main(String[] args){
Dog Dundt = new Dog("Dundt");
Dundt.getLegs();
Dundt.toStrung();
}

1) toString() should not be a static member.
2) getLegs() should not have the side effect of changing the member String toString.
3) There should not be a member variable String toString.
4) toString() should return a String.
5) name needs to be a member.
6) legs needs to be a member.
7) toString() should generate the string from the members at run time.
8) You do not need to explicitly call toString() in main. Simply passing the instance of Dog to println will call it for you.
9) It is good practice to annotate methods you are overriding with the #Override annotation. toString() is a member of Object and you are overriding Object.
public class Dog{
private String name;
private int legs = 4;
public Dog(String name){
this.name = name;
}
public int getLegs(){
return legs;
}
public int setLegs(int legs){
this.legs = legs;
return this.legs;
}
#Override
public String toString(){
return "Dog is called " + name + " it has " + legs + " legs.";
}
public static void main(String[] args){
Dog dundt = new Dog("Dundt");
System.out.println(dundt);
}
}

how can I return a String?

package book1;
import java.util.ArrayList;
public abstract class Book {
public String Book (String name, String ref_num, int owned_copies, int loaned_copies ){
return;
}
}
class Fiction extends Book{
public Fiction(String name, String ref_num, int owned_copies, String author) {
}
}
at the moment when i input values into the variable arguments and call them with this :
public static class BookTest {
public static void main(String[] args) {
ArrayList<Book> library = new ArrayList<Book>();
library.add(new Fiction("The Saga of An Aga","F001",3,"A.Stove"));
library.add(new Fiction("Dangerous Cliffs","F002",4,"Eileen Dover"));
for (Book b: library) System.out.println(b);
System.out.println();
}
}
i get a return value of this:
book1.Fiction#15db9742
book1.Fiction#6d06d69c
book1.NonFiction#7852e922
book1.ReferenceBook#4e25154f
how can i convert the classes to return a string value instead of the object value? I need to do this without changing BookTest class. I know i need to use to string to convert the values. but i don't know how to catch the return value with it. could someone please point me in the right direction on how to convert this output into a string value?

You need to overwrite the toString() Method of your Book class. In this class you can generate a String however you like. Example:
#Override
public String toString() {
StringBuilder sb = new StringBuilder();
sb.append(this.author).append(": ").append(this.title);
return sb.toString();
}

You need to override the toString() method in your Book or Fiction class. The method is actually declared in the Object class, which all classes inherit from.
#Override
public String toString(){
return ""; // Replace this String with the variables or String literals that you want to return and print.
}
This method is called by System.out.println() and System.out.print() when they receive an object in the parameter (as opposed to a primitive, such as int and float).
To reference the variables in the method, you'll need to declare them in the class and store them via the class's constructor.
For example:
public abstract class Book {
private String name;
private String reference;
private int ownedCopies;
private int loanedCopies;
public Book (String name, String reference, int ownedCopies, int loanedCopies) {
this.name = name;
this.reference = reference;
this.ownedCopies = ownedCopies;
this.loanedCopies = loanedCopies;
}
#Override
public String toString(){
return name + ", Ref:" + reference + ", OwnedCopies: " + ownedCopies + ", LoanedCopies: " + loanedCopies; // Replace this String with the variables or String literals that you want to return and print.
}
}

The classes you have defined, don't store any values. It is in other words useful to construct a new book. You need to provide fields:
public abstract class Book {
private String name;
private String ref_num;
private int owned_copies;
private int loaned_copies;
public String Book (String name, String ref_num, int owned_copies, int loaned_copies) {
this.name = name;
this.ref_num = ref_num;
this.owned_copies = owned_copies;
this.loaned_copies = loaned_copies;
}
public String getName () {
return name;
}
//other getters
}
Now an object is basically a set of fields. If you want to print something, you can access and print one of these fields, for instance:
for (Book b: library) System.out.println(b.getName());
In Java, you can also provide a default way to print an object by overriding the toString method:
#Override
public String toString () {
return ref_num+" "+name;
}
in the Book class.

Need to give your object Book a ToString() override.
http://www.javapractices.com/topic/TopicAction.do?Id=55
Example:
#Override public String toString()
{
return name;
}
Where name, is a string in the Class.

I am hoping that you have assigned the passed arguments to certain attributes of the classes. Now, once you are done with that, you can override the toString() method in Book to return your customized string for printing.

Java code output is weird? [duplicate]

This question already has answers here:
How do I print my Java object without getting "SomeType#2f92e0f4"?
(13 answers)
Closed last month.
I do not understand why my output is not what I expected, instead of showing the persons information, the output displays: examples.Examples#15db9742
Am I doing something wrong in my code?
package examples;
public class Examples {
String name;
int age;
char gender;
public Examples(String name, int age, char gender){
this.name = name;
this.age = age;
this.gender = gender;
}
public static void main(String[] args) {
Examples[] person = new Examples[10];
person[0] = new Examples("Doe",25,'m');
System.out.println(person[0]);
}
}

Add a toString() method to your class:
public class Examples {
String name;
int age;
char gender;
public Examples(String name, int age, char gender){
this.name = name;
this.age = age;
this.gender = gender;
}
#Override
public String toString() {
StringBuilder result = new StringBuilder();
result.append(this.name + " ");
result.append(this.age + " ");
result.append(this.gender + " ");
return result.toString();
}
public static void main(String[] args) {
Examples[] person = new Examples[10];
person[0] = new Examples("Doe",25,'m');
System.out.println(person[0]);
}
}

When you say
System.out.println(person[0]);
java doesn't automatically know what you want printed out. To tell it, you write a method in your Examples class called toString() which will return a string containing the info you want. Something like:
#Override
public String toString() {
return "Name: " + name +
" Age: " + String.valueOf(this.age) +
" Gender: " + String.valueOf(this.gender);
}

Java has no way of knowing what you want it to print. By default, the toString() method is called when you use System.out.println() with an object.
Your Examples class should have its own toString() method so you can decide what to print. The default toString() returns a representation of the object in memory.
For example, to print out the object's name:
package examples;
public class Examples {
...
#Override
public String toString() {
return name;
}
}

Your output is right, when you print an object the method toString() of the object is called; by default it returns what you see (the class and a memory direction).
Override the method toString() of the class to make him return a descriptive String. E.g.:
public class Examples {
// The same ...
public String toString(){
return "My name is " + name + " and I have " + age + " years."
}
// The same ...
}
If you do that you will get a more descriptive String when calling toString() and so when printing an object of class Examples.
New output is
My name is Dow and I have 25 years.

person is an array of type Examples, so by acessing person[0] you are telling it to print an Examples instance. Since the Examples class does not implement an toString() method it will call the parent Object.toString() method that produces the output you are seeing.
Add the following method to your Examples class
public String toString() {
return "[name="+this.name+", age="+this.age+", gender="+this.gender+"]";
}

You have explicitly to create a method which outputs the persons data or override the toString() method to do the same thing:
public class Person
{
String name;
int age;
char gender;
public Person(String name, int age, char gender)
{
this.name = name;
this.age = age;
this.gender = gender;
}
//Override the toString() method
//is a usual programming technique
//to output the contents of an object
public String toString()
{
return "Name: " + this.name + "\nAge: " + this.age + "\nGender: "
+ this.gender;
}
//You can also write something like this
public void showInfo()
{
System.out.printf("Persons Info:\n\nName: %s\nAge: %s\nGender: %s", this.name, this.age, this.gender);
}
public static void main(String[] args)
{
Person p = new Person("bad_alloc", 97, 'm');
//System.out.println("Persons info:\n" + p.toString());
//If you want directly to "output the object" you have to override the toString() method anyway:
//System.out.println(p);//"Outputting the object", this is possible because I have overridden the toString() method
p.showInfo();
}
}