Is it possible to call a constructor from another (within the same class, not from a subclass)? If yes how? And what could be the best way to call another constructor (if there are several ways to do it)?
Yes, it is possible:
public class Foo {
private int x;
public Foo() {
this(1);
}
public Foo(int x) {
this.x = x;
}
}
To chain to a particular superclass constructor instead of one in the same class, use super instead of this. Note that you can only chain to one constructor, and it has to be the first statement in your constructor body.
See also this related question, which is about C# but where the same principles apply.
Using this(args). The preferred pattern is to work from the smallest constructor to the largest.
public class Cons {
public Cons() {
// A no arguments constructor that sends default values to the largest
this(madeUpArg1Value,madeUpArg2Value,madeUpArg3Value);
}
public Cons(int arg1, int arg2) {
// An example of a partial constructor that uses the passed in arguments
// and sends a hidden default value to the largest
this(arg1,arg2, madeUpArg3Value);
}
// Largest constructor that does the work
public Cons(int arg1, int arg2, int arg3) {
this.arg1 = arg1;
this.arg2 = arg2;
this.arg3 = arg3;
}
}
You can also use a more recently advocated approach of valueOf or just "of":
public class Cons {
public static Cons newCons(int arg1,...) {
// This function is commonly called valueOf, like Integer.valueOf(..)
// More recently called "of", like EnumSet.of(..)
Cons c = new Cons(...);
c.setArg1(....);
return c;
}
}
To call a super class, use super(someValue). The call to super must be the first call in the constructor or you will get a compiler error.
[Note: I just want to add one aspect, which I did not see in the other answers: how to overcome limitations of the requirement that this() has to be on the first line).]
In Java another constructor of the same class can be called from a constructor via this(). Note however that this has to be on the first line.
public class MyClass {
public MyClass(double argument1, double argument2) {
this(argument1, argument2, 0.0);
}
public MyClass(double argument1, double argument2, double argument3) {
this.argument1 = argument1;
this.argument2 = argument2;
this.argument3 = argument3;
}
}
That this has to appear on the first line looks like a big limitation, but you can construct the arguments of other constructors via static methods. For example:
public class MyClass {
public MyClass(double argument1, double argument2) {
this(argument1, argument2, getDefaultArg3(argument1, argument2));
}
public MyClass(double argument1, double argument2, double argument3) {
this.argument1 = argument1;
this.argument2 = argument2;
this.argument3 = argument3;
}
private static double getDefaultArg3(double argument1, double argument2) {
double argument3 = 0;
// Calculate argument3 here if you like.
return argument3;
}
}
When I need to call another constructor from inside the code (not on the first line), I usually use a helper method like this:
class MyClass {
int field;
MyClass() {
init(0);
}
MyClass(int value) {
if (value<0) {
init(0);
}
else {
init(value);
}
}
void init(int x) {
field = x;
}
}
But most often I try to do it the other way around by calling the more complex constructors from the simpler ones on the first line, to the extent possible. For the above example
class MyClass {
int field;
MyClass(int value) {
if (value<0)
field = 0;
else
field = value;
}
MyClass() {
this(0);
}
}
Within a constructor, you can use the this keyword to invoke another constructor in the same class. Doing so is called an explicit constructor invocation.
Here's another Rectangle class, with a different implementation from the one in the Objects section.
public class Rectangle {
private int x, y;
private int width, height;
public Rectangle() {
this(1, 1);
}
public Rectangle(int width, int height) {
this( 0,0,width, height);
}
public Rectangle(int x, int y, int width, int height) {
this.x = x;
this.y = y;
this.width = width;
this.height = height;
}
}
This class contains a set of constructors. Each constructor initializes some or all of the rectangle's member variables.
As everybody already have said, you use this(…), which is called an explicit constructor invocation.
However, keep in mind that within such an explicit constructor invocation statement you may not refer to
any instance variables or
any instance methods or
any inner classes declared in this class or any superclass, or
this or
super.
As stated in JLS (§8.8.7.1).
Yes, any number of constructors can be present in a class and they can be called by another constructor using this() [Please do not confuse this() constructor call with this keyword]. this() or this(args) should be the first line in the constructor.
Example:
Class Test {
Test() {
this(10); // calls the constructor with integer args, Test(int a)
}
Test(int a) {
this(10.5); // call the constructor with double arg, Test(double a)
}
Test(double a) {
System.out.println("I am a double arg constructor");
}
}
This is known as constructor overloading.
Please note that for constructor, only overloading concept is applicable and not inheritance or overriding.
Using this keyword we can call one constructor in another constructor within same class.
Example :-
public class Example {
private String name;
public Example() {
this("Mahesh");
}
public Example(String name) {
this.name = name;
}
}
Yes it is possible to call one constructor from another. But there is a rule to it. If a call is made from one constructor to another, then
that new constructor call must be the first statement in the current constructor
public class Product {
private int productId;
private String productName;
private double productPrice;
private String category;
public Product(int id, String name) {
this(id,name,1.0);
}
public Product(int id, String name, double price) {
this(id,name,price,"DEFAULT");
}
public Product(int id,String name,double price, String category){
this.productId=id;
this.productName=name;
this.productPrice=price;
this.category=category;
}
}
So, something like below will not work.
public Product(int id, String name, double price) {
System.out.println("Calling constructor with price");
this(id,name,price,"DEFAULT");
}
Also, in the case of inheritance, when sub-class's object is created, the super class constructor is first called.
public class SuperClass {
public SuperClass() {
System.out.println("Inside super class constructor");
}
}
public class SubClass extends SuperClass {
public SubClass () {
//Even if we do not add, Java adds the call to super class's constructor like
// super();
System.out.println("Inside sub class constructor");
}
}
Thus, in this case also another constructor call is first declared before any other statements.
I will tell you an easy way
There are two types of constructors:
Default constructor
Parameterized constructor
I will explain in one Example
class ConstructorDemo
{
ConstructorDemo()//Default Constructor
{
System.out.println("D.constructor ");
}
ConstructorDemo(int k)//Parameterized constructor
{
this();//-------------(1)
System.out.println("P.Constructor ="+k);
}
public static void main(String[] args)
{
//this(); error because "must be first statement in constructor
new ConstructorDemo();//-------(2)
ConstructorDemo g=new ConstructorDemo(3);---(3)
}
}
In the above example I showed 3 types of calling
this() call to this must be first statement in constructor
This is Name less Object. this automatically calls the default constructor.
3.This calls the Parameterized constructor.
Note:
this must be the first statement in the constructor.
You can a constructor from another constructor of same class by using "this" keyword.
Example -
class This1
{
This1()
{
this("Hello");
System.out.println("Default constructor..");
}
This1(int a)
{
this();
System.out.println("int as arg constructor..");
}
This1(String s)
{
System.out.println("string as arg constructor..");
}
public static void main(String args[])
{
new This1(100);
}
}
Output -
string as arg constructor..
Default constructor..
int as arg constructor..
Calling constructor from another constructor
class MyConstructorDemo extends ConstructorDemo
{
MyConstructorDemo()
{
this("calling another constructor");
}
MyConstructorDemo(String arg)
{
System.out.print("This is passed String by another constructor :"+arg);
}
}
Also you can call parent constructor by using super() call
There are design patterns that cover the need for complex construction - if it can't be done succinctly, create a factory method or a factory class.
With the latest java and the addition of lambdas, it is easy to create a constructor which can accept any initialization code you desire.
class LambdaInitedClass {
public LamdaInitedClass(Consumer<LambdaInitedClass> init) {
init.accept(this);
}
}
Call it with...
new LambdaInitedClass(l -> { // init l any way you want });
Pretty simple
public class SomeClass{
private int number;
private String someString;
public SomeClass(){
number = 0;
someString = new String();
}
public SomeClass(int number){
this(); //set the class to 0
this.setNumber(number);
}
public SomeClass(int number, String someString){
this(number); //call public SomeClass( int number )
this.setString(someString);
}
public void setNumber(int number){
this.number = number;
}
public void setString(String someString){
this.someString = someString;
}
//.... add some accessors
}
now here is some small extra credit:
public SomeOtherClass extends SomeClass {
public SomeOtherClass(int number, String someString){
super(number, someString); //calls public SomeClass(int number, String someString)
}
//.... Some other code.
}
Hope this helps.
Yes it is possible to call one constructor from another with use of this()
class Example{
private int a = 1;
Example(){
this(5); //here another constructor called based on constructor argument
System.out.println("number a is "+a);
}
Example(int b){
System.out.println("number b is "+b);
}
You can call another constructor via the this(...) keyword (when you need to call a constructor from the same class) or the super(...) keyword
(when you need to call a constructor from a superclass).
However, such a call must be the first statement of your constructor. To overcome this limitation, use this answer.
The keyword this can be used to call a constructor from a constructor, when writing several constructor for a class, there are times when you'd like to call one constructor from another to avoid duplicate code.
Bellow is a link that I explain other topic about constructor and getters() and setters() and I used a class with two constructors. I hope the explanations and examples help you.
Setter methods or constructors
I know there are so many examples of this question but what I found I am putting here to share my Idea. there are two ways to chain constructor. In Same class you can use this keyword. in Inheritance, you need to use super keyword.
import java.util.*;
import java.lang.*;
class Test
{
public static void main(String args[])
{
Dog d = new Dog(); // Both Calling Same Constructor of Parent Class i.e. 0 args Constructor.
Dog cs = new Dog("Bite"); // Both Calling Same Constructor of Parent Class i.e. 0 args Constructor.
// You need to Explicitly tell the java compiler to use Argument constructor so you need to use "super" key word
System.out.println("------------------------------");
Cat c = new Cat();
Cat caty = new Cat("10");
System.out.println("------------------------------");
// Self s = new Self();
Self ss = new Self("self");
}
}
class Animal
{
String i;
public Animal()
{
i = "10";
System.out.println("Animal Constructor :" +i);
}
public Animal(String h)
{
i = "20";
System.out.println("Animal Constructor Habit :"+ i);
}
}
class Dog extends Animal
{
public Dog()
{
System.out.println("Dog Constructor");
}
public Dog(String h)
{
System.out.println("Dog Constructor with habit");
}
}
class Cat extends Animal
{
public Cat()
{
System.out.println("Cat Constructor");
}
public Cat(String i)
{
super(i); // Calling Super Class Paremetrize Constructor.
System.out.println("Cat Constructor with habit");
}
}
class Self
{
public Self()
{
System.out.println("Self Constructor");
}
public Self(String h)
{
this(); // Explicitly calling 0 args constructor.
System.out.println("Slef Constructor with value");
}
}
It is called Telescoping Constructor anti-pattern or constructor chaining. Yes, you can definitely do. I see many examples above and I want to add by saying that if you know that you need only two or three constructor, it might be ok. But if you need more, please try to use different design pattern like Builder pattern. As for example:
public Omar(){};
public Omar(a){};
public Omar(a,b){};
public Omar(a,b,c){};
public Omar(a,b,c,d){};
...
You may need more. Builder pattern would be a great solution in this case. Here is an article, it might be helpful
https://medium.com/#modestofiguereo/design-patterns-2-the-builder-pattern-and-the-telescoping-constructor-anti-pattern-60a33de7522e
Yes, you can call constructors from another constructor. For example:
public class Animal {
private int animalType;
public Animal() {
this(1); //here this(1) internally make call to Animal(1);
}
public Animal(int animalType) {
this.animalType = animalType;
}
}
you can also read in details from
Constructor Chaining in Java
Originally from an anser by Mirko Klemm, slightly modified to address the question:
Just for completeness: There is also the Instance initialization block that gets executed always and before any other constructor is called. It consists simply of a block of statements "{ ... }" somewhere in the body of your class definition. You can even have more than one. You can't call them, but they're like "shared constructor" code if you want to reuse some code across constructors, similar to calling methods.
So in your case
{
System.out.println("this is shared constructor code executed before the constructor");
field1 = 3;
}
There is also a "static" version of this to initialize static members: "static { ... }"
I prefer this way:
class User {
private long id;
private String username;
private int imageRes;
public User() {
init(defaultID,defaultUsername,defaultRes);
}
public User(String username) {
init(defaultID,username, defaultRes());
}
public User(String username, int imageRes) {
init(defaultID,username, imageRes);
}
public User(long id, String username, int imageRes) {
init(id,username, imageRes);
}
private void init(long id, String username, int imageRes) {
this.id=id;
this.username = username;
this.imageRes = imageRes;
}
}
class A {
A(int x) { System.out.println("constructor A"); } }
class B extends A {
B() { System.out.println(" constructor B"); } }
public class C {
public static void main(String args[]) { B b = new B(); } }
It says constructor from class A can't be applied for given types.
You have to call the constructor A(int x) explicitly in your constructor B(). I.e., you have to write
class B extends A {
B() {
super(<<< insert some int here>>>);
System.out.println(" constructor B");
}
}
If you do not add such a super call, then java will insert super(); for you which would try to call A(). As there is no constructor A() you receive the error that your constructor cannot be applied for the argument types.
Its always good to have a default constructor in a class if your writing a paramterized one.
class A {
A(){System.out.println("Default A");}
A(int x) { System.out.println("constructor A"); } }
class B extends A {
B() { System.out.println(" constructor B"); } }
public class C {
public static void main(String args[]) { B b = new B(); } }
If we don't specify a constructor to a class A, then a default constructor with no args will be associated to the class, if you specify one/more constructor to the class A, then you have to use one of them each time you want to create an object from this class.when a class B extend a class A, then constructors have to call one constructor of the parent class ( A ) by super() , if you don't specify a constructor to A then it's implecitly called be the constructor of B, if you explecitly defined constructor of A , then You have to call it when you create constructor of B like this :
B() { super(0)/*this call should be if the first line, you have to pass your default args if the constructor have args*/;
System.out.println(" constructor B");
}
what you are seeing is constructor chaining, read
snippet from the link.
If a constructor does not explicitly invoke a superclass constructor, the Java compiler
automatically inserts a call to the no-argument constructor of the superclass. If the super
class does not have a no-argument constructor, you will get a compile-time error. Object
does have such a constructor, so if Object is the only superclass, there is no problem.
I ran into this block of code, and there is this one line I don't quit understand the meaning or what it is doing.
public Digraph(In in) {
this(in.readInt());
int E = in.readInt();
for (int i = 0; i < E; i++) {
int v = in.readInt();
int w = in.readInt();
addEdge(v, w);
}
}
I understand what this.method() or this.variable are, but what is this()?
This is constructor overloading:
public class Diagraph {
public Diagraph(int n) {
// Constructor code
}
public Digraph(In in) {
this(in.readInt()); // Calls the constructor above.
int E = in.readInt();
for (int i = 0; i < E; i++) {
int v = in.readInt();
int w = in.readInt();
addEdge(v, w);
}
}
}
You can tell this code is a constructor and not a method by the lack of a return type.
This is pretty similar to calling super() in the first line of the constructor in order to initialize the extended class. You should call this() (or any other overloading of this()) in the first line of your constructor and thus avoid constructor code duplications.
You can also have a look at this post: Constructor overloading in Java - best practice
Using this() as a function like that, essentially calls the Constructor of the class. This allows you to all the generic initializations in one constructor and have specializations in others. So in this piece of code for example, the call to this(in.readInt()) is calling the Digraph constructor that has one int argument.
This code snippet is a constructor.
This call to this calls another constructor of the same class
public App(int input) {
}
public App(String input) {
this(Integer.parseInt(input));
}
In the above example we have a constructor that takes an int and one that takes a String. The constructor that takes a String converts the String to an int and then delegates to the int constructor.
Note that a call to another constructor or a superclass constructor (super()) must be the first line in a constructor.
Maybe take a look at this for a more detailed explanation of constructor overloading.
It's nearly the same
public class Test {
public Test(int i) { /*construct*/ }
public Test(int i, String s){ this(i); /*construct*/ }
}
Calling this essentially calls the class Constructor.
For example, if you're extending something, than along with add(JComponent), you could do: this.add(JComponent).
An other constructor of the class Digraph with an int parameter.
Digraph(int param) { /* */ }
Constructor Overloading:
ex:
public class Test{
Test(){
this(10); // calling constructor with one parameter
System.out.println("This is Default Constructor");
}
Test(int number1){
this(10,20); // calling constructor with two parameter
System.out.println("This is Parametrized Constructor with one argument "+number1);
}
Test(int number1,int number2){
System.out.println("This is Parametrized Constructor with two argument"+number1+" , "+number2);
}
public static void main(String args[]){
Test t = new Test();
// first default constructor,then constructor with 1 parameter , then constructor with 2 parameters will be called
}
}
this(); is constructor which is used to call another constructor in a class,
for example:-
class A{
public A(int,int)
{ this(1.3,2.7);-->this will call default constructor
//code
}
public A()
{
//code
}
public A(float,float)
{ this();-->this will call default type constructor
//code
}
}
Note:
i did not use this() constructor in default constructor because it will lead to deadlock state.
Hope this will help you:)
I am learning java. The book I'm reading has a question which asks what is wrong with the following code? I have typed the code in NetBeans and I can see the error but why is this error caused and how is it resolved?
The error is highlighted over the code public A(int t) and it says
Constructor B in class B cannot be applied to given types, require int, found no arguments, reason actual and formal arguments lists differ in length.
Here is the code:
public class Test {
public static void main(String[] args) {
B b = new B(5);
}
}
class A extends B {
public A(int t) {
System.out.println("A's constructor is invoked");
}
}
class B {
public B(int k) {
System.out.println("B's constructor is invoked");
}
}
when your super class has an args constructor and doesn't have a no-args constructor you have to explicitly invoke it using a super(args) call from sub-class constructor
class A extends B {
public A(int t) {
super(t);
System.out.println("A's constructor is invoked");
}
}
Class B has defined constructor, so it does not have public implicit default constructor (with no arguments). All subclass constructors have to explicitly call superclass constructors, via super(t), if zero argument superclass constructor is not available.
The class B has only one constructor which takes an int argument. On the other hand, the constructor you have in class A (implicitly) tries to call a constructor in A's super class (namely B) that takes no arguments. This is the cause of the error. There are at least two ways to fix the problem:
Create a no-arg constructor in class B.
Explicitly call the constructor in class B which takes an int as a parameter. You can do this using the super keyword.
You need to call the constructor of the super class as the first statement in A(int):
public A(int t) {
super(t);
System.out.println("A's constructor is invoked");
}