I know that the Javadocs says:
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.
But when should I spend time overriding the toString method for my classes? Should it be one of the first things I do along with overriding equals and hashCode? Or should I wait until it's actually needed?
I know Eclipse can auto generate toString methods for you, so should I just have Eclipse auto generate them once I know the fields for my class?
Josh Bloch gives a good explanation in Effective Java, in item 10.
[...] providing a good toString implementation makes your class much more pleasant to use.
It really makes it easier to output debugging traces, or makes better logging messages, since you can use the object's string representation provided by toString() directly; you don't have to manually build a string that gives the information needed on the object.
As stated in the book, you should include all the interesting information in the resulting String. You should also document properly your method; you may document the resulting String format or not, but you should at least document your intent (whether the format is subject to change, or not likely to change).
In the end, it is up to you (and your company's standards) to decide if overriding it in every class should be part of your habits or not. Personally, I don't override toString () in every classes, only in the ones which are most at risk of being used in a debuging trace.
I would implement toString() method on any class that holds human understandable non confidential data. Ex: Transfer Object, Beans, Data Object, Wrappers. For such classes just go on to implement 'toString()' method.
Classes that represent a service, process with transient states need not implement the method.Here, You can wait until it is actually needed.
Make sure you do not expose any variables with "transient" keyword in 'toString()'!
Typically, I override it when I want to assign a default format of displaying an object, often formatting a compact/digestable display of relevant attributes. So that I can simply, for example, display it in debug or log by doing:
MyClass myClsInst = new MyClass();
...
System.out.println(myClsInst);
In general, It's used to show or see what the object has.
For instance, Let's say there is a Student class and you created objects.
Student class has age, grade, gpa, name, country, address.
class Student{
private int age;
private int grade;
private double gpa;
private String name;
private String country;
private String address;
Student(...){
// ...
}
public String toString(){
String str = "age is "+age+ ", grade is " + grade + ...
return str;
}
}
And you created A student, and B student ( and maybe more )
You just need to 'toString()' for checking its inside like this:
System.out.println(aStudent.toString());
System.out.println(bStudent.toString());
or You can just write the object name, it automatically calls 'toString()'
System.out.println(aStudent);
System.out.println(bStudent);
It removes the redundant works & faster.
Then, you will see like this:
Output:
age is 13, grade is 3, ...
age is 15, grade is 5, ...
It's useful when you see what A student or B student has when you debug.
And also, It's useful when you make your own form like JSON.
It will be easier to manipulate its data with JSON format.
I am trying to demonstrate in simple way.
package com.web.doamin;
public class User {
String name;
long id;
/*#Override
public String toString() {
return "User [user = " + name + ", id="+ id + "]";
}*/
public static void main(String[] args) {
User user = new User();
System.out.println(" : user : " + user );
}
}
If we did't Override toString() method we will get Object Hash code in sysout
O/P without Override toString() method - com.web.doamin.User#7852e922
if We Override the ToString() method we will get O/P - User [user = null, id=0]
Note - It is a good idea to override toString() as we get get proper output when an object is used in System.out.println();
Related
Is there a way to avoid calling this.field for every field in a class ?
public class Test {
private String name;
private String email;
public Test(String name, String email) {
// I want to avoid this
this.name = name;
this.email = email;
}
public Test(Test test) {
// Something like this would be perfect, setting both name and email to test
this(test);
}
}
The use of this is only required in cases of name collisions, to resolve the ambiguity.
Some programmers like me prefer using the this. prefix routinely, whereas other use only where necessary.
See Answer by Wasserman for an example of how to avoid naming collision.
Use the IDE, Luke
Your IDE will generate constructors, accessors (getters/setters), equals & hashCode, toString, and so on. So you need not type this.; let the machine do the typing.
Use custom settings to control whether you want the IDE to include or omit this. prefixes.
record
You may be interested in using the records feature, new in Java 16+. A record is a brief way to write a class whose main purpose is to communicate data transparently and immutably.
With a record, by default, the compiler implicitly writes the constructor, getters, equals & hashCode, and toString. The implicitly created constructor populates each and every member field on your behalf. You write none of that code.
Here is your entire example class when written as a record. No this required. All your member fields are automatically assigned.
public record Test ( String name , String email ) {}
Be cautious in using records. The reason for their invention was not writing less code. The reason was to provide an explicit mechanism for transmitting immutable data transparently, a “nominal tuple” in academic-speak. Less boilerplate coding is merely a nice side-effect. I highly recommend reading JEP 395 for more explanation.
Tip: You can combine the two points of this Answer. Ask your IDE to generate a full-blown class by starting with a record.
Write a record with all your member fields listed in the parentheses.
Invoke your IDE to convert from a record to a class.
Voilà, you have a complete class with constructor, accessors, equals & hashCode, and toString all written out with an absolute minimum of typing by you.
For example, in IntelliJ 2022, choosing Convert record to class from the light-bulb icon menu turns this:
public record Test ( String name , String email ) {}
… into this:
package work.basil.example.recs;
import java.util.Objects;
public final class Test
{
private final String name;
private final String email;
public Test ( String name , String email )
{
this.name = name;
this.email = email;
}
public String name ( ) { return name; }
public String email ( ) { return email; }
#Override
public boolean equals ( Object obj )
{
if ( obj == this ) { return true; }
if ( obj == null || obj.getClass() != this.getClass() ) { return false; }
var that = ( Test ) obj;
return Objects.equals( this.name , that.name ) &&
Objects.equals( this.email , that.email );
}
#Override
public int hashCode ( )
{
return Objects.hash( name , email );
}
#Override
public String toString ( )
{
return "Test[" +
"name=" + name + ", " +
"email=" + email + ']';
}
}
Caveat: That result may not be the default. I may have altered the settings in IntelliJ.
Sorry, the only way to avoid this is to have different names for your constructor parameters and for your class fields.
public Test(String _name, String _email) {
// I want to avoid this
name = _name;
email = _email;
}
That said, you might have better luck using Java 16+'s record syntax.
As suggested, using records is the easiest way:
public record Test (String name, String email) {
}
That's all you need. What you then get:
A constructor that takes all arguments, in the same order as the field list
A method for each field. This does not start with get. In this case, the methods are name() and email().
equals, hashCode and toString implementations that use all fields.
There is no need for a copy constructor, because every field is automatically final.
If you want, you can add extra constructors. However, they must delegate to the automatically generated constructor, because that's the one that sets the fields. Adding additional utility methods is also fine.
And if needed, you can add validation to the generated constructor. There's special syntax that allows you to omit all the field names:
public record Test (String name, String email) {
public Test {
Objects.requireNonNull(name);
Objects.requireNonNull(email);
}
}
The assignments are done for you, there's no need to type those either.
You need this.x everytime, if there are 2 or more variables, which are called x and you want to call the attribute variable x.
The this keyword is used, to point on an attribute variable of the created instance (object) of the class.
There could be an attribute, that is called x, and a local variable which is called x too.
I am a beginner in Java (and programming), and there should be a simple answer to this, but I could not find it. I want to write a code that would print the value of the reference name of an instance variable. For example:
Public class Person {
Person() {
//attributes, height, weight, etc.
}
Person Person1 = new Person();
}
I would like to write a line of code that would produce something to the tune of
"The attribute of Person1 is..."
Something to the tune of System.out.println("The attribute of Person1 " +(????)+" is ....")
I was unable to find or create a method that would return the name Person1.
"Person1" is not the name of the instance, but instead is the name of a variable that holds the reference to the instance. The instance itself has no name. You will have to give it an attribute of name if you care to keep it.
public class Person {
private final String name;
public Person(String name) {
this.name = name;
}
public String getName() {
return this.name;
}
}
then you can do:
Person person1 = new Person("Jeff");
System.out.println("Person1's name is: " + person1.getName());
The output of that will be:
Person1's name is: Jeff
It is not generally possible to do exactly what you want. I don't think variable names are even compiled into the classes.
It can be done with compile time annotations but it's not trivial.
To be able to do that you should override the toString() function
Something like this
/* Returns the string representation of your class object .*/
#Override
public String toString() {
return "The attributes of this person are :height - " + height + " weight - " + weight + // you are print out all your properties.
}
Now whenver you want to print the object you can do
System.out.println(person1);
I'm not sure how familiar you are with programming languages in general. I see you say you are new to Java, so I'll start there. Java, like many object-oriented languages, uses inheritance when you create classes. In Java, when you define a class you can use the "extends" keyword to sub-class and use or override any methods in the parent class. Now, in Java, ALL classes automatically inherit from this class called Object.
This is very useful to know, because Object contains a few useful methods, most notably of them is "toString()". You do not need to use extends to get these methods btw. Now, toString on its own is not useful, but you can override it to print out what you want.
public class Person
{
String name;
int age;
Person(String name, int age)
{
this.name = name;
this.age = age;
}
#Override
public String toString()
{
return "Name is: " + name + "and age is: " + age";
}
}
Notice the toString() method I defined there? Anytime you call this method on an object, you will get that string printed out. So for instance, in your example:
Person person1 = new Person("Ford Prefect", 42);
System.out.println(person1.toString()); //Will print what we defined in toString.
You don't even need the .toString(), just person1 because the JVM will realize you meant to use toString. If you use IntelliJ IDE, you can do Alt + Insert and select toString() to override it. IDEs are wonderful tools to help you be more efficient. Good luck!
I created a HashMap where the keys are Integers and the values are of the Employee class. Employee contains the employee's first name, last name and address. I am having issues with printing out the values. Here is what i tried.
employees.put(5, e1);
String test=employees.get(5).toString();
System.out.println(employees.toString());
System.out.println(test);
output:
{5=identitymanagement.Employee#6da264f1}
identitymanagement.Employee#6da264f1
What am I doing wrong?
A look at your code
String test=employees.get(5).toString();
This will grab the item with the key 5 in your hashmap, then call that object's toString method. The way your object is behaving at the moment implies you have no overridden that method, which is why it is printing out the objects address in memory.
System.out.println(employees.toString());
This will attempt to print out the HashMap object. In the same vain as your Employee class, HashMap does not override it's toString method, so it simply prints out the objects reference in memory.
A solution
The convention, when outputting the details of a class, is to override the toString() method. This would look something like this:
public String toString()
{
return "name: " + name;
}
When you place this method in your class, you can call the toString method and it won't just print out the memory address of the object, which is what it is doing at the moment :)
When using this code, all you have to do is pass the object to the System.out.println method and it will do the rest:
Employee e = employees.get(5);
System.out.println(e);
The correct way is
Employee e = employees.get(5); // return's Employee object stored in map with key 5
String firstName = e.firstName;
String lastName = e.lastName;
String address = e.address;
I have a HashMap. I am trying to retrieve the value and print it using the key from the user-code.
The code is:
lib.addbook(book2.getISBN(), book2);
Book ret = lib.getbook("978-81-291-1979-7");
System.out.println(ret);
Current Output:
O/P: LibraryPackage.Book#527c6768
I want the output to be a string and to display the actual value not the address of the book.
You have to implement (and override) the toString() method in your Book class, and specify what you want the output to be. E.g.:
#Override
String toString()
{
return this.author+": " + this.title;
}
commons-lang has a great utility for this if you don't want to override the .toString() method, or need to represent it differently in different situations:
Here's a call to build a string based on reflection:
String str = ToStringBuilder.reflectionToString(object);
In fact, this is a great way to implement the .toString() method itself. Another alternative use of this class would be a field by field creation of the string:
String str = new ToStringBuilder(object)
.append("field1", field1)
.append("field2", field2)
.toString();
Is there any way in which I can automatically convert a Custom Class Object into a human readable string?
e.g. consider the following class:
class Person {
String Name;
int Salary;
...
}
Person p = new Person();
p.setName("Tony");
p.setSalary(1000);
I need to get something like:
Person: Name="Tony", Salary=1000
Importing Commons Lang you could use ToStringBuilder
Check method reflectionToString(java.lang.Object), this will create automatically the representation you are expecting.
This code:
Person p = new Person();
p.setName("Tony");
p.setSalary(1000);
System.out.println(ToStringBuilder.reflectionToString(p));
results this string:
Person#64578ceb[Name=Tony,Salary=1000]
sure you can override the toString method of class.
as follow:
class Person {
String name;
int salary;
...
#Override public String toString() {
return "Person: Name='" + name + "', Salary=" + salary;
}
}
refer for more details https://blogs.oracle.com/CoreJavaTechTips/entry/writing_tostring_methods_tech_days
This is basically what toString is for. But given you want this done automatically, you can create some general service that can do it. Use reflection to iterate all fields, and then print each one's name and value. Simplest way to print their values would be by using their toString, but you can also pass them into that printing service recursively on some cases (you'll have to find the halt condition, of course).
For example, on some class PrintUtils have:
public static void printFields(Object o) {
System.out.print(o.getClass.getSimpleName() + ": ");
for (Field field : o.getClass().getDeclaredFields()) {
field.setAccessible(true); // you also get non-public fields
System.out.print(field.getName() + " = " + field.get(o) + ", ");
}
}
You'll have to handle exceptions etc. and possibly better format the output, of course. Also, this only print fields declared in the current class. If you want fields declared higher in the inheritance hierarchy, you'll have to work a bit more. Lastly, using reflection is much slower than just having a regular toString. If using toString is possible, it is preferable.
I think you could use ToStringBuilder that is part of commons-lang.
One way to do it is to rely on Apache Commons BeanUtils.describe. This will produce a Map of bean's properties, which converts to a string nicely via Map.toString. If you want something more custom, you'll need to dig into the reflection API.
You can use message format from java :
https://docs.oracle.com/javase/tutorial/i18n//message.html
seperate variable by - and there you have a humanreadable string of your class!
class Person {
String Name;
int Salary;
...
#Override
public String toString() {
return "Person: Name = " + Name + "," +
"Salary="+Salary;
}
}
Person p = new Person();
p.setName("Tony");
p.setSalary(1000);
System.out.println(p.toString());