Can anybody explain to me the concept of the toString() method, defined in the Object class? How is it used, and what is its purpose?
From the Object.toString docs:
Returns a string representation of the
object. In general, the toString
method returns a string that
"textually represents" this object.
The result should be a concise but
informative representation that is
easy for a person to read. It is
recommended that all subclasses
override this method.
The toString method for class Object
returns a string consisting of the
name of the class of which the object
is an instance, the at-sign character
`#', and the unsigned hexadecimal
representation of the hash code of the
object. In other words, this method
returns a string equal to the value
of:
getClass().getName() + '#' + Integer.toHexString(hashCode())
Example:
String[] mystr ={"a","b","c"};
System.out.println("mystr.toString: " + mystr.toString());
output:- mystr.toString: [Ljava.lang.String;#13aaa14a
Use of the String.toString:
Whenever you require to explore the constructor called value in the String form, you can simply use String.toString...
for an example...
package pack1;
import java.util.*;
class Bank {
String n;
String add;
int an;
int bal;
int dep;
public Bank(String n, String add, int an, int bal) {
this.add = add;
this.bal = bal;
this.an = an;
this.n = n;
}
public String toString() {
return "Name of the customer.:" + this.n + ",, "
+ "Address of the customer.:" + this.add + ",, " + "A/c no..:"
+ this.an + ",, " + "Balance in A/c..:" + this.bal;
}
}
public class Demo2 {
public static void main(String[] args) {
List<Bank> l = new LinkedList<Bank>();
Bank b1 = new Bank("naseem1", "Darbhanga,bihar", 123, 1000);
Bank b2 = new Bank("naseem2", "patna,bihar", 124, 1500);
Bank b3 = new Bank("naseem3", "madhubani,bihar", 125, 1600);
Bank b4 = new Bank("naseem4", "samastipur,bihar", 126, 1700);
Bank b5 = new Bank("naseem5", "muzafferpur,bihar", 127, 1800);
l.add(b1);
l.add(b2);
l.add(b3);
l.add(b4);
l.add(b5);
Iterator<Bank> i = l.iterator();
while (i.hasNext()) {
System.out.println(i.next());
}
}
}
... copy this program into your Eclipse, and run it... you will get the ideas about String.toString...
The toString() method returns a textual representation of an object. A basic implementation is already included in java.lang.Object and so because all objects inherit from java.lang.Object it is guaranteed that every object in Java has this method.
Overriding the method is always a good idea, especially when it comes to debugging, because debuggers often show objects by the result of the toString() method. So use a meaningful implementation but use it for technical purposes. The application logic should use getters:
public class Contact {
private String firstName;
private String lastName;
public Contact (String firstName, String lastName) {
this.firstName = firstName;
this.lastName = lastName;
}
public String getFirstName() {return firstName;}
public String getLastName() {return lastName;}
public String getContact() {
return firstName + " " + lastName;
}
#Override
public String toString() {
return "["+getContact()+"]";
}
}
It may optionally have uses within the context of an application but far more often it is used for debugging purposes. For example, when you hit a breakpoint in an IDE, it's far easier to read a meaningful toString() of objects than it is to inspect their members.
There is no set requirement for what a toString() method should do. By convention, most often it will tell you the name of the class and the value of pertinent data members. More often than not, toString() methods are auto-generated in IDEs.
Relying on particular output from a toString() method or parsing it within a program is a bad idea. Whatever you do, don't go down that route.
toString() returns a string/textual representation of the object.
Commonly used for diagnostic purposes like debugging, logging etc., the toString() method is used to read meaningful details about the object.
It is automatically invoked when the object is passed to println, print, printf, String.format(), assert or the string concatenation operator.
The default implementation of toString() in class Object returns a string consisting of the class name of this object followed by # sign and the unsigned hexadecimal representation of the hash code of this object using the following logic,
getClass().getName() + "#" + Integer.toHexString(hashCode())
For example, the following
public final class Coordinates {
private final double x;
private final double y;
public Coordinates(double x, double y) {
this.x = x;
this.y = y;
}
public static void main(String[] args) {
Coordinates coordinates = new Coordinates(1, 2);
System.out.println("Bourne's current location - " + coordinates);
}
}
prints
Bourne's current location - Coordinates#addbf1 //concise, but not really useful to the reader
Now, overriding toString() in the Coordinates class as below,
#Override
public String toString() {
return "(" + x + ", " + y + ")";
}
results in
Bourne's current location - (1.0, 2.0) //concise and informative
The usefulness of overriding toString() becomes even more when the method is invoked on collections containing references to these objects. For example, the following
public static void main(String[] args) {
Coordinates bourneLocation = new Coordinates(90, 0);
Coordinates bondLocation = new Coordinates(45, 90);
Map<String, Coordinates> locations = new HashMap<String, Coordinates>();
locations.put("Jason Bourne", bourneLocation);
locations.put("James Bond", bondLocation);
System.out.println(locations);
}
prints
{James Bond=(45.0, 90.0), Jason Bourne=(90.0, 0.0)}
instead of this,
{James Bond=Coordinates#addbf1, Jason Bourne=Coordinates#42e816}
Few implementation pointers,
You should almost always override the toString() method. One of the cases where the override wouldn't be required is utility classes that group static utility methods, in the manner of java.util.Math. The case of override being not required is pretty intuitive; almost always you would know.
The string returned should be concise and informative, ideally self-explanatory.
At least, the fields used to establish equivalence between two different objects i.e. the fields used in the equals() method implementation should be spit out by the toString() method.
Provide accessors/getters for all of the instance fields that are contained in the string returned. For example, in the Coordinates class,
public double getX() {
return x;
}
public double getY() {
return y;
}
A comprehensive coverage of the toString() method is in Item 10 of the book, Effective Java™, Second Edition, By Josh Bloch.
Whenever you access an Object (not being a String) in a String context then the toString() is called under the covers by the compiler.
This is why
Map map = new HashMap();
System.out.println("map=" + map);
works, and by overriding the standard toString() from Object in your own classes, you can make your objects useful in String contexts too.
(and consider it a black box! Never, ever use the contents for anything else than presenting to a human)
Correctly overridden toString method can help in logging and debugging of Java.
Coding:
public class Test {
public static void main(String args[]) {
ArrayList<Student> a = new ArrayList<Student>();
a.add(new Student("Steve", 12, "Daniel"));
a.add(new Student("Sachin", 10, "Tendulkar"));
System.out.println(a);
display(a);
}
static void display(ArrayList<Student> stu) {
stu.add(new Student("Yuvi", 12, "Bhajji"));
System.out.println(stu);
}
}
Student.java:
public class Student {
public String name;
public int id;
public String email;
Student() {
}
Student(String name, int id, String email) {
this.name = name;
this.id = id;
this.email = email;
}
public String toString(){ //using these toString to avoid the output like this [com.steve.test.Student#6e1408, com.steve.test.Student#e53108]
return name+" "+id+" "+email;
}
public String getName(){
return name;
}
public void setName(String name){
this.name=name;
}
public int getId(){
return id;
}
public void setId(int id){
this.id=id;
}
public String getEmail(){
return email;
}
public void setEmail(String email){
this.email=email;
}
}
Output:
[Steve 12 Daniel, Sachin 10 Tendulkar]
[Steve 12 Daniel, Sachin 10 Tendulkar, Yuvi 12 Bhajji]
If you are not used toString() in Pojo(Student.java) class,you will get an output like [com.steve.test.Student#6e1408, com.steve.test.Student#e53108].To avoid these kind of issue we are using toString() method.
Apart from what cletus answered with regards to debugging, it is used whenever you output an object, like when you use
System.out.println(myObject);
or
System.out.println("text " + myObject);
The main purpose of toString is to generate a String representation of an object, means the return value is always a String. In most cases this simply is the object's class and package name, but on some cases like StringBuilder you will got actually a String-text.
/**
* This toString-Method works for every Class, where you want to display all the fields and its values
*/
public String toString() {
StringBuffer sb = new StringBuffer();
Field[] fields = getClass().getDeclaredFields(); //Get all fields incl. private ones
for (Field field : fields){
try {
field.setAccessible(true);
String key=field.getName();
String value;
try{
value = (String) field.get(this);
} catch (ClassCastException e){
value="";
}
sb.append(key).append(": ").append(value).append("\n");
} catch (IllegalArgumentException e) {
e.printStackTrace();
} catch (SecurityException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
}
}
return sb.toString();
}
If you learn Python first and then Java. I think it plays the same role as __str__() method in Python, it is a magic method like __dict__() and __init__() but to refer to a string representing the the object.
the toString() converts the specified object to a string value.
Okay, before anything, I would like to mention that this is for school so please DO NOT write any code that fixes my code as that will not teach me anything. Instead what I am looking for is references, explanations and proper terminologies if I use incorrect terminology.
So I am having a few issues here. Here is what I need to do,
*Include the following methods in the Student class:
a. an accessor (i.e., getter) for each instance variable from part B1
b. a mutator (i.e., setter) for each instance variable from part B1
Note: All access and change to the instance variables of the Student class should be through accessor and mutator methods.
c. constructor using all of the input parameters
d. print() to print specific student data (e.g., student ID, first name, last name) using accessors (i.e., getters)
Create a student Roster class with the following methods that contain all ArrayList method calls:
a. public static void remove(String studentID) that removes students from the roster by student ID
Note: If the student ID doesn’t exist, the method should print an error message indicating that it is not found.
b. public static void print_all() that prints a complete tab-separated list of student data using accessor methods
Note: Tabs can be formatted as such: 1 [tab] First Name: John [tab] Last Name: Smith [tab] Age: 20 [tab] Grades: {88, 79, 59}. The print_all() method should loop through all the students in the student array list and call the print() method for each student.
c. public static void print_average_grade(String studentID) that correctly prints a student’s average grade by student ID
d. public static void print_invalid_emails() that verifies student e-mail addresses and displays all invalid e-mail addresses to the use*
That above is where I am having trouble, The arraylist, the constructor and calling the Student class in the Roster class.
here is my code.
Student Class
/**
* Write a description of class Student here.
*
* #author (Richard Talcik)
* #version (v1 3/5/17)
*/
public class Student {
// initialize instance variables
private String csFirstName; //I am using cs infront of the name because this is a class variable and a string
private String csLastName;
private String csEmail;
private int ciAge;
private int ciStudentID;
private int[] ciaGrades; //cia for class, integer, array;
public Student(String sFirstName, String sLastName, String sEmail, int iAge, int iStudentID, String[] grades) {
//doing the consturctor
this.setFirstName(sFirstName);
this.setLastName(sLastName);
this.setEmail(sEmail);
this.setAge(iAge);
this.setStudentID(iStudentID);
this.setGrades(grades);
}
/**he methods to get and then followed by set
*/
public String getFirstName()
{
// put your code here
return csFirstName; //returning the value of the first name when called.
}
public String getLastName()
{
// put your code here
return csLastName;
}
public String getEmail()
{
// put your code here
return csEmail;
}
public int getStudentID()
{
// put your code here
return ciStudentID;
}
public int getAge()
{
// put your code here
return ciAge;
}
public int[] getGrades()
{
// put your code here
return ciaGrades;
}
// calling the sets from here on out
public void setFirstName(String newFirstName)
{
// put your code here
csFirstName = newFirstName;
//setting it
}
public void setLastName(String newLastName)
{
// put your code here
csLastName = newLastName;
//setting it
}
public void setEmail(String newEmail)
{
// put your code here
csEmail = newEmail;
//setting it
}
public void setAge(int newAge)
{
// put your code here
ciAge = newAge;
//setting it
}
public void setStudentID(int newStudentID)
{
// put your code here
ciStudentID = newStudentID;
//setting it
}
public void setGrades(int[] newGrades)
{
// put your code here
ciaGrades = newGrades;
//setting it
}
}
This roster class is asking me to add arguments when I call Student stu = new Student(); But I don't understand what arguments to add into there?
I also dont really understand how I am going to incorporate my arraylist to add my methods in there from the student class? (sorry if i used wrong terminology, please correct me if needed)
Roster Class
import java.util.ArrayList;
/**
* Write a description of class Roster here.
*
* #author (Richard Talcik)
* #version (v1)
*/
public class Roster {
// instance variables - replace the example below with your own
/**
* Constructor for objects of class Roster
*/
public static void main(String args[]) {
Student stu = new Student("John", "Smith", "John1989#gmail.com", 20, 1, Grades[1]);
ArrayList<Student> myRoster = new ArrayList<Student>();
myRoster.add(new Student("Suzan", "Erickson","Erickson_1990#gmailcom", 19, 2, [91,72,85]));
}
public static void remove(String studentID)
{
// put your code here
}
public static void print_all()
{
//do something
}
public static void print_average_grade(String studentID)
{
//do something
}
public static void print_invalid_emails()
{
//do something
}
}
please any tutorials that will help explain this will be well worth it.
Also, I work third shift and my school is fully online. My mentor isn't or tutor isn't available during the hours that I am awake and emails are not really best method of communication as it takes days for a response.
Firstly for a beginner, not bad :)
However when using the ArrayList object type in java, the type of the list must be of the object you whish to put inside the ArrayList, i.e. if you want to keep a list of students, then you have to write it as
AssrayList<Student> = new ArrayList<Student>();
you can also specify the size of the list as a param, but remember, ArraList grows dynamically as things are added and removed from the list.
As far as you Constructor goes, you have to add the variables you assign values to on the inside as the params of the constructor. also the question states that you have to access all class objects with the use of their accessor and mutator methods, having that the current way you have your constructor at the moment is incorrect as you are directly assigning values to the class onjects of Student, you can change that to be similar to the following:
constructor (paramType param) {
mutatorMethod(param)
}
so your constructor should be in the lines of
Student(String name, String surname) {
setName(name)
setSurname(surname)
}
Hope this helps :)
You have written Student class with a constructor that takes parameters. So, in Roaster class you have to call matching constructor of Student class. My guess is that you did not quite understand my answer.
I would suggest look for a java tutorial on oracle site and even better if you can get a copy of this book called The java programming language
I am trying to complete a program to help me learn java before we do it in class. But I am stuck trying to add a Member to the club, print it out and then count the number of members. I do not know where to start after I have added the basics into the code.
I am new to stack overflow and do not know how to format correctly so I am sorry. :)
Any tips and help would be greatly appreciated.
Thanks
package lab8.club;
import java.util.ArrayList;
public class Club
{
ArrayList<Membership> members;
/**
* Constructor for objects of class Club
*/
public Club()
{
members = new ArrayList<Membership>();
// Initialise any fields here ...
}
/**
* Add a new member to the club's list of members.
* #param member The member object to be added.
*/
public void join(Membership member)
{
members.add(member);
}
/**
* #return The number of members (Membership objects) in
* the club.
*/
public int numberOfMembers()
{
return members.size();
}
public static void main(String args[]){
Membership member1 = new Membership("test", 1, 2011);
System.out.println();
}
}
package lab8.club;
public class Membership
{
// The name of the member.
private String name;
// The month in which the membership was taken out.
private int month;
// The year in which the membership was taken out.
private int year;
public Membership(String name, int month, int year)
throws IllegalArgumentException
{
if(month < 1 || month > 12) {
throw new IllegalArgumentException(
"Month " + month + " out of range. Must be in the range 1 ... 12");
}
this.name = name;
this.month = month;
this.year = year;
}
public String getName()
{
return name;
}
public int getMonth()
{
return month;
}
public int getYear()
{
return year;
}
public String toString()
{
return "Name: " + name +
" joined in month " +
month + " of " + year;
}
}
package lab8.club;
public class ClubDemo
{
// instance variables - replace the example below with your own
private Club club;
/**
* Constructor for objects of class ClubDemo
*/
public ClubDemo()
{
club = new Club();
}
/**
* Add some members to the club, and then
* show how many there are.
* Further example calls could be added if more functionality
* is added to the Club class.
*/
public void demo()
{
club.join(new Membership("David", 2, 2004));
club.join(new Membership("Michael", 1, 2004));
System.out.println("The club has " +
club.numberOfMembers() +
" members.");
}
}
Your code is actually ok. What you really need is a main method to start the execution of your code. Create a new class such as the following Demo.java:
public class Demo {
public static void main(String[] args) {
ClubDemo demo = new ClubDemo();
demo.demo();
}
}
Output:
The club has 2 members.
You will see that in ClubDemo's demo() method that members are added via club.join which calls the ArrayList.add method to add member objects to the members ArrayList.
For more information about the main method signature, see the Java Hello World Tutorial:
In the Java programming language, every application must contain a
main method whose signature is:
public static void main(String[] args)
The modifiers public and static can be written in either order (public
static or static public), but the convention is to use public static
as shown above. You can name the argument anything you want, but most
programmers choose "args" or "argv".
In main I think you can just call Club club = new Club() to create a new Club object, then simply call club.join(member1) to add your new member into the club.
I would suggest changing the method name join because when you're reading the code, it's not really right that the CLUB is joining a MEMBER. It should be the other way around so unless you need to, I'd just take out the method and just use the ArrayList's add.
And lastly, if you want to use your join method (and not the ArrayList's add), I would consider setting members in Club to be private.
I am trying to write an if condition to check a value exists in a list containing many objects,
Here is my code:
List<TeacherInfo> teacherInfo=ServiceManager.getHelperService(TeacherManagementHelper.class, request, response).getTeacherInfoId();
if(teacherInfo.contains(inputParam))
{
out2.println("<font color=red>");
out2.println("Id Not Available");
out2.println("</font>");
}
else
{
out2.println("<font color=green>");
out2.println("Id Available");
out2.println("</font>");
}
after executing 1st sentence getTeacherInfoId() method successfully returns a list of objects, in those objects I want to check any object has a value same as inputParam. Is my above code right ? if wrong please help me .
contains(Object o) is internally based on equals between objects of your list and your input, as stated by the doc.
Since you said that inputParam is an integer, then the current state of your code can't work because you compare an integer to TeacherInfo objects, so they won't ever be equal. I believe you want to compare inputParam to one particular field of TeacherInfo objects.
If you're using Java 8, you can use the stream API instead of contains():
List<TeacherInfo> teacherInfo=ServiceManager.getHelperService(TeacherManagementHelper.class, request, response).getTeacherInfoId();
if (teacherInfo.stream().anyMatch(ti -> ti.getId() == inputParam)) {
// contains the id
} else {
// does not contain the id
}
For previous java versions, an alternative to contains() would be to iterate over your list and compare manually your integer to the TeacherInfo's field:
private static boolean containsTeacherId(List<TeacherInfo> teacherInfos, int id) {
for (TeacherInfo ti : teacherInfos) {
if (ti.getId() == inputParam) { // I used getId(), replace that by the accessor you actually need
return true;
}
}
return false;
}
Then:
List<TeacherInfo> teacherInfo=ServiceManager.getHelperService(TeacherManagementHelper.class, request, response).getTeacherInfoId();
if (containsTeacherId(teacherInfo, inputParam)) {
// contains the id
} else {
// does not contain the id
}
Note: If you don't need other information than the ID itself, I'd rather suggest to return the list of IDs from a method called getTeacherIds(), especially if this information comes from a DB.
No it won't work at all. you should iterate the 'teacherInfo' list and you need to override the compare()and hashvalue() of object class.
You would need to iterate over the list teacherInfo and compare each element of that list with inputParam.
Below is a small demo code that might help you.
I have created a testerInfo analogous to your teacherInfo and param analogous to your inputParam.
I hope it helps.
Tester.java
/**
*
*/
package com.demo;
/**
* #author Parul
*
*/
public class Tester {
private int id;
private String name;
/**
* #return the id
*/
public int getId() {
return id;
}
/**
* #param id the id to set
*/
public void setId(int id) {
this.id = id;
}
/**
* #return the name
*/
public String getName() {
return name;
}
/**
* #param name the name to set
*/
public void setName(String name) {
this.name = name;
}
public Tester(int id, String name) {
this.id = id;
this.name = name;
}
public Tester() {
}
}
Demo.java
/**
*
*/
package com.demo;
import java.util.ArrayList;
import java.util.List;
/**
* #author Parul
*
*/
public class Demo {
public static void main(String [] args){
List<Tester> testerInfo=new ArrayList<Tester>();
testerInfo.add(new Tester(1,"Java"));
testerInfo.add(new Tester(2,"C++"));
testerInfo.add(new Tester(3,"Python"));
testerInfo.add(new Tester(4,"C"));
Tester tester=null;
int param=2;
for(int i=0;i<testerInfo.size();i++){
tester=testerInfo.get(i);
if(tester.getId()==param){
System.out.println("param found: "+tester.getName());
break;
}
}
}
}
OUTPUT
param found: C++
I need to have the teams sortable by order of points, from most to least, i have that i have to write a new method and can make changes to the class header.
I was thinking of using this as my method, but it would put the teams in order of least to most :S so i dont know what way to fix it, also i dont know what changes i would need to make to the class header if any!
public void orderPoints()
{
List<String> points = new ArrayList<String>();
Collections.sort(points);
}
I know i can use Collections.max and min but unsure how i would filter the rest of the points in
Code for rest of the class
public class League
{ /* instance
variables */
private Team name;
private int points;
/**
* Constructor for objects of class League.
*/ public League(Team aname)
{
super();
name = aName;
points = 0; }
/**
* Returns the receiver's name Team
*/ public Team getName() {
return this.name; }
/**
* Returns the receiver's points
*/ public int getPoints() {
return points; }
/**
* Sets the receiver's points
*/ public void setPoints(int aPoints) {
this.points = aPoints; }
I'm not sure I understand what you're asking, but if you want to reverse the sorting of your List you can use Collections.sort(points, Collections.reverseOrder());