It works fine when constructors are not defined, but gives errors if I define a parameterized constructor and not a default one and not passing any values while creating an object. I thought constructors are predefined.
Why do I need to define a default constructor if I've defined a parameterized constructor? Ain't default constructor predefined?
A default (no-argument) constructor is automatically created only when you do not define any constructor yourself.
If you need two constructors, one with arguments and one without, you need to manually define both.
While all the answers above are correct, it's a bit difficult for new-comers to wrap it in their head. I will try to answer the question anew for new-comers.
The problem that Ayush was facing was in trying to instantiate an Object for a class via a no-arg constructor. This class however has one or more parameterized constructor and this results in a compile time error.
For example, let us create a class Student with a single parameterized constructor and try to instantiate it via the no-arg constructor.
public class Student {
private String name;
private int rollNo;
public Student(String name, int rollNo) {
this.name = name;
this.rollNo = rollNo;
}
public static void main(String[] args) {
// The line below will cause a compile error.
Student s = new Student();
// Error will be "The constuctor Student() is undefined"
}
}
Woha! But when we remove the public Student(String name, int rollNo)
constructor all-together, the error is gone and the code compiles.
The reason behind this seeming anomaly lies in the fact that Java only
provides us with the default (no-arg) constructor when we do not define any
constructor for that class on our own.
For example, the following class is supplied with a default contructor:
public class Student {
private String name;
private int rollNo;
}
becomes:
public class Student {
private String name;
private int rollNo;
//Default constructor added by Java.
public Student() {
super();
}
}
In other words, the moment we define any parameterized constructor,
we must also define a no-arg constructor if we want to instantiate
the object of that class via a no-arg constructor.
Also in case of inheritance, a sub-class with no constructors; is supplied one
default constructor. This default constructor supplied by Java as above calls the super class's no-arg constructor. If it can't find one, then it will throw an error.
So yes it's always a good choice to define a no-arg/default constructor.
Ref : Oracle Java Tutorial
A no-arg constructor is automatically inserted for you, if you don't write one. This means, if you write a constructor with some parameters, it will be the only constructor you have, so you must pass some values for those parameters to create an instance of it.
This is exactly the expected behavior.
Java automatically generates a default (no arguments constructors) for classes that don't have any constructor.
If you define another constructor (with arguments), default constructor will not be generated. If you still want one, you need to define it yourself.
Whenever you class is complied, if compiler does not find any valid constructor in class (default,parametrized) only then it will substitute auto generated default constructor for your class.
You must have noticed, you can create objects without any default constructor defined in your class, there comes the concept of auto-generated default constructor.As whenever object is created,default constructor is called.
But, If you define Parametrized constructor in your class, that means you restrict the objects to have that parameters
Example:Every employee must have an id.
So,at that time, Compiler will not insert any default constructor there as there is valid constructor in a class.If you need default constructor too, you have to define by yourself.
There is also one curious case when you must define non argument constructor.
As the other wrote, if you don't specify default constructor - Java will do it for you. It's good to understand how "default generated by Java" constructor looks like.
In fact it calls constructor of the super class and this is fine.
Let's now imagine one case.
You are creating Vehicle class:
public class Vehicle {
private String name;
private String engine;
public Vehicle(String name, String engine) {
this.name = name;
this.engine = engine;
}
public String makeNoise(){
return "Noiseee";
}
}
As we can see Vehicle class has got only one defined 2 arguments constructor.
Now let's create Car class which inheritates from Vehicle class:
public class Car extends Vehicle {
#Override
public String makeNoise() {
return "Wrrrrrrr....";
} }
Maybe it looks strange but only one reason why wouldn't it compile is fact that Java cannot create default constructor for Car class which call super Vehicle class. Vehicle class doesn't have no argument constructor and it cannot be generated automatically while 2 arg constructor already exists.
I know that it's very rare case, but I found it as a interesting to know.
Related
I have a record and want to add default constructor to it.
public record Record(int recordId) {
public Record {
}
}
But it created constructor with int param.
public final class Record extends java.lang.Record {
private final int recordId;
public Record(int);
//other method
}
How can we add a default constructor to a record?
To split hairs, you cannot ever define a default constructor, because a default constructor is generated by the compiler when there are no constructors defined, thus any defined constructor is by definition not a default one.
If you want a record to have a no-arg constructor, records do allow adding extra constructors or factory methods, as long as the "canonical constructor" that takes all of the record fields as arguments is called.
public record Record(int recordId) {
public Record() {
this(0);
}
}
Explicit constructor
In your case, you can explicitly specify a no-argument constructor with the delegation to the canonical constructor with a default value if you want to and this can be done as -
public Record(){
this(Integer.MIN_VALUE);
}
In short, any non-canonical constructor should delegate to one, and that should hold true for the data-carrying nature of these representations.
Compact Constructor
On the other hand, note that the representation you had used in your code.
public Record {}
is termed as a "compact constructor" which represents a constructor accepting all arguments and that can also be used for validating the data provided as attributes of the record. A compact constructor is an alternate way of declaring the canonical constructor.
We all Know that the JVM provides us a default constructor in the every java program.
But if we declare any other type of constructor then it does not provide the any type of default constructor.
So, my question is that is it compulsory to declare default constructor when we declare any other type of constructor in our program.
If YES then explain Why?
If NO then also explain Why?
Give the Solution with proper suitable example.
No, it's not compulsory at all. There are loads of classes with no default constructor, and there's nothing stopping you from writing your own. One that springs to mind is java.awt.Color.
Declaring the default constructor depends on the business requirement and technically its not compulsory.
If you want a class to be initialized only with a set of parameters, then you can skip the default constructor, which indeed forces you -- to give the required values to create the object
For instance,
public class ClassA {
String name;
ClassA(String name) {
this.name = name;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
For the above class, if you want to do,
ClassA obj = new ClassA();
This is not possible as there is not default constructor.
ClassA obj = new ClassA("name");
The above is the only way to create object, as name is the parameter should be given.
If you want both to be created, add default constructor as
ClassA() {}
Which provides way to create the object with out name.
No, it's not cumpolsory.
class Dog {
Dog(String name)
{
system.out.println("Dog :" + name);
}
public static void main(String[] args)
{
Dog d = new Dog("dollar"); // works fine
Dog d2 = new Dog() // error , no default constructor defined for Dog
}
}
So, my question is that is it compulsory to declare default
constructor when we declare any other type of constructor in our
program.
No, It's not necessary to have a default constructor.
If NO then also explain Why ?
Default Constructor will be provided by Compiler, only if you don't defined any no argument Constructor. But, Keep in mind the following, Check mode from JLS
8.8.9. Default Constructor
If a class contains no constructor declarations, then a default
constructor with no formal parameters and no throws clause is
implicitly declared.
If the class being declared is the primordial class Object, then the
default constructor has an empty body. Otherwise, the default
constructor simply invokes the superclass constructor with no
arguments.
It is a compile-time error if a default constructor is implicitly
declared but the superclass does not have an accessible constructor
(§6.6) that takes no arguments and has no throws clause.
It is not necessary to create the default constructor, but it is good practice to create the default constructor.
If your class is to be reused then not creating the default constructor will limit the re-usability of your class.
Like if a class is extending that class, then the derived class must have an explicit super call.
consider the following example:-
class Base
{
public Base(int x){
//some statements
}
/*
some methods
*/
}
class Derived
{
// only one of the following will be used
public Derived(){ // This will cause a compile-time error
//some statements
}
public Derived(){ // This will work fine
//some statements
super(x);
}
/*
some methods
*/
}
The reason behind this is, if the base class do not have default constructor than derived class must call the appropriate super() in all its constructor declarations. But if we have a default constructor in base class then the call to super() is not mandatory.
No, it's not compulsory!
But then if you dont have a default constructor(No-argument constructor) and if you want the to create the object of your class in this form
A ref = new A();
then you might not be able to do it.
No this is not necessary.
let me explain why this is not necessary
The question which you've asked is seems like constructor overloading.
If you create an or many parameterized constructor in java then you do need need to be worried for the default constructor.
Basically constructor is used to initialize the instance variables or class members or for performing the preliminary tasks and if you have already done with this by using the another constructor then there is no need for another but you can do if you want.
public class base {
int a;
public base(int x)
{
this.a=x;
System.out.println(x);
}
public base()
{
System.out.println("abc");
}
public static void main(String []a)
{
base b=new base();
b=new base(4);
}
}
The output is :-
abc
4
I was practicing dome Java OO principles, when I encountered with this. I create to POJOs and when trying to make objects from it, if an empty constructor isn't defined, it wont compile.
I find this weird because I used to do that and the JVM included a default one for me. Is it new in Java 7? Am I missing something?
Here is the sample code I made
public class Dog {
String name;
String race;
int age;
/*
public Dog() {
If this isn't included, it doesn't compile if I try to
create no-args objects.
}*/
public Dog (String _name) {
this.name = _name;
}
public Dog (String _name, String _race) {
this.name = _name;
this.race = _race;
}
public Dog (String _name, String _race, int _age) {
this.name = _name;
this.race = _race;
this.age = _age;
}
}
Using Dog newDog = new Dog(); will not work in your current code, because you have not defined it.
A default constructor will only be automatically generated if no other constructors are present.
You don't have to provide any constructors for your class, but you must be careful when doing this. The compiler automatically provides a no-argument, default constructor for any class without constructors. This default constructor will call the no-argument constructor of the superclass. In this situation, the compiler will complain if the superclass doesn't have a no-argument constructor so you must verify that it does. If your class has no explicit superclass, then it has an implicit superclass of Object, which does have a no-argument constructor.
Source: http://docs.oracle.com/javase/tutorial/java/javaOO/constructors.html
If you define constructors with arguments, you also have to define a default one with no arguments.
In this case, trying to create a new Dog (Dog d = new Dog()) can't be done because the constructor with no args is not defined.
"If a class defines an explicit constructor, it no longer has a default constructor to set the state of the objects.
If such a class requires a default constructor, its implementation must be provided. Any attempt to call the default constructor will be a compile time error if an explicit default constructor is not provided in such a case."
http://www.javabeginner.com/learn-java/java-constructors
I'm trying to learn Java and I've got everything setup and I'm running. I created a constructor for my first model and went to setup a constructor function that accepts a parameter. I added this function:
public Guest(String name) {
this.name = name;
this.signingDate = new Date(System.currentTimeMillis());
}
Netbeans threw an error saying I needed a default constructor. The error went away when I changed it to:
public Guest() {
}
public Guest(String name) {
this.name = name;
this.signingDate = new Date(System.currentTimeMillis());
}
I can't imagine that this is how you create optional parameters within Java. So it brings up 2 questions:
Why do I need a default constructor?
How do I create optional parameters?
An entity class must have a no-argument public or protected constructor.
Another scenario where you'd need no-arg constrcutors is when you have subclasses.
For example,
class Car {
Engine engine;
Car(Engine e)
{
// do something
}
}
And you have a subclass,
class Benz extends Car
{
Benz()
{
}
}
Now you can't instantiate a Benz object. The reason is, when a new Benz() instantiation is attempeted, the parent constructor is first invoked.
i.e,
class Benz extends Car
{
Benz()
{
super(); //implicitly added
}
}
By default, it tries to invoke a no-arg constructor of the super-class. So you'd need one, unless you have an explicit super(arg) call like this:
class Benz extends Car
{
Benz()
{
super(new Engine());
}
}
Note: A simple Java rule to remember
A no-arg constructor is provided by default as long as you don't have another constructor with argument.
i.e class Car{} is equivalent to class Car{ Car() }
But when you write,
class Car{ Car(Engine e){} } there is no no-arg constructor provided by default. You'll have to explicitly code class Car{ Car(){} Car(Engine e)} if you needed it.
Yes. Java supports method / constructor overloading.
You don't "need" the default constructor. However, NetBeans is being helpful to you, and informing you that you need one because it assumes you are creating a bean - which requires the option to be created without a constructor requiring parameters - because the calling code wouldn't know what to populate into these.
For "optional" parameters, you can define as many constructors as you would need - with each one omitting one or more of the "optional" parameters.
Why do I need a default constructor?
I think you are trying to call new Guest() and have only one constructor in Guest class that is Guest(String arg). And that's why it is showing error that you need to create a constructor with no arguments that is default constructor of a class.
How do I create optional parameters?
If you want to make a constructor with 0 more arguments you need to use ... in argument.
Ex:
public class Testing {
public Testing(String... str) {
}
public static void main(String[] args) {
Testing testing = new Testing();
Testing testing1 = new Testing("1");
Testing testing2 = new Testing("2","1");
}
}
If you're using a POJO (Plain Old Java Object) than you need a default constructor only if you create an instance of this object by invoking no-argument constructor like Guest g = new Guest().
That's pretty obvious - you want to use no-argument constructor - you need to have one.
If you don't specify any constructor, the compiler will generate one (default, no-argument) for you. If you specify at least one constructor by yourself - the compiler won't generate any as it assumes that you know what you're doing.
However, if you're creating a JPA entity (so you have #Entity at the top of your class) than you need a default constructor as it's a JPA requirement.
It's because instances of your class are created by the JPA provider when you fetch them from the database. The JPA provider has to have a way to instantiate your Entity.
If you'd have only parametrized constructor - how the JPA would know what parameters should it pass?
There are no optional parameters in Java like there are in i.e. PHP world where you can define myMethod($var1, $var2 = null, $var3 = 2);. In Java world we use overloaded methods and varargs.
I program on netbeans ubuntu java standart project (test preparation).
when I create AccountStudent.java I get error.
Account.java
public abstract class Account {
protected double _sum;
protected String _owner;
protected static int accountCounter=0;
public Account(String owner){
this._sum=0;
this._owner=owner;
accountCounter++;
}
}
AccountStudent.java- error: cannot find symbol: constructor Account()
public class AccountStudent extends Account{
}
Fix for problem- add empty Account constructor:
Account.java
public abstract class Account {
protected double _sum;
protected String _owner;
protected static int accountCounter=0;
public Account(){
}
public Account(String owner){
this._sum=0;
this._owner=owner;
accountCounter++;
}
}
Why should i create empty constructor Account if already he exist because he inherit Object class?
Thanks
Why should i create empty constructor
Account if already he exist because he
inherit Object class?
Constructors are not inherited. If a class has no explicit constructor, hte compiler silently adds a no-argument default constructor which does nothing except call the superclass no-argument constructor. In your case, that fails for AccountStudent because Account does not have a no-argument constructor. Adding it is one way to resolve this. Another would be to add a constructor to AccountStudent that calls the existing constructor of Account, like this:
public class AccountStudent extends Account{
public AccountStudent(String owner){
super(owner);
}
}
Every class in Java must have a constructor, if you don't define one, the compiler does that for you and create a default constructor (the one with no parameters). If you create yourself a constructor then the compiler doesn't need to create one.
So even if it inherit from Object, that doesn't mean that it'll have a default constructor.
When you instantiate an AccountStudent, you will need to call the parent constructor.
By default if you don't specify yourself a parent constructor to call, it will call the default constructor.
If you want to explicitly call a parent constructor, you'll do it with super().
There are three way to avoid the error :
Call the parent constructor with a parameter you get from the child constructor :
public class AccountStudent extends Account{
public AccountStudent(String owner){
super(String owner);
}
}
Call the parent constructor with a parameter you create yourself :
public class AccountStudent extends Account{
public AccountStudent(){
super("Student");
}
}
Call the default parent constructor but you need to create one because the compiler will not create one if a non-default constructor already exists. (the solution you gave)
JLS 8.8.9 Default Constructor
If a class contains no constructor declarations, then a default constructor that takes no parameters is automatically provided.
If the class being declared is the primordial class Object, then the default constructor has an empty body.
Otherwise, the default constructor takes no parameters and simply invokes the superclass constructor with no arguments.
In this case the AccountStudent class does not have any constructor so the compiler adds a default constructor for you and also adds a call to superclass constructor. So your child class effectively looks like:
class AccountStudent extends Account{
AccountStudent() {
super();
}
}
An object of an extended class contains state variables (fields) that
are inherited from the superclass as well as state variables defined
locally within the class. To construct an object of the extended
class, you must correctly initialize both sets of state variables. The
extended class's constructor can deal with its own state but only the
superclass knows how to correctly initialize its state such that its
contract is honored. The extended class's constructors must delegate
construction of the inherited state by either implicitly or explicitly
invoking a superclass constructor.
The Java™ Programming Language, Fourth Edition
By Ken Arnold, James Gosling, David Holmes