Zero argument constructor in java

Java Constructors

A constructor in Java is similar to a method that is invoked when an object of the class is created.

Unlike Java methods, a constructor has the same name as that of the class and does not have any return type. For example,

Here, Test() is a constructor. It has the same name as that of the class and doesn’t have a return type.

Example 1: Java Constructor

class Main < private String name; // constructor Main() < System.out.println("Constructor Called:"); name = "Programiz"; >public static void main(String[] args) < // constructor is invoked while // creating an object of the Main class Main obj = new Main(); System.out.println("The name is " + obj.name); >>

Constructor Called: The name is Programiz

In the above example, we have created a constructor named Main() . Inside the constructor, we are initializing the value of the name variable.

Notice the statement of creating an object of the Main class.

Here, when the object is created, the Main() constructor is called. And, the value of the name variable is initialized.

Hence, the program prints the value of the name variables as Programiz .

Types of Constructor

In Java, constructors can be divided into 3 types:

1. Java No-Arg Constructors

Similar to methods, a Java constructor may or may not have any parameters (arguments).

If a constructor does not accept any parameters, it is known as a no-argument constructor. For example,

Example 2: Java private no-arg constructor

class Main < int i; // constructor with no parameter private Main() < i = 5; System.out.println("Constructor is called"); >public static void main(String[] args) < // calling the constructor without any parameter Main obj = new Main(); System.out.println("Value of i: " + obj.i); >>

Constructor is called Value of i: 5

In the above example, we have created a constructor Main() . Here, the constructor does not accept any parameters. Hence, it is known as a no-arg constructor.

Notice that we have declared the constructor as private.

Once a constructor is declared private , it cannot be accessed from outside the class. So, creating objects from outside the class is prohibited using the private constructor.

Here, we are creating the object inside the same class. Hence, the program is able to access the constructor. To learn more, visit Java Implement Private Constructor.

However, if we want to create objects outside the class, then we need to declare the constructor as public .

Example 3: Java public no-arg constructors

class Company < String name; // public constructor public Company() < name = "Programiz"; >> class Main < public static void main(String[] args) < // object is created in another class Company obj = new Company(); System.out.println("Company name /java-programming/access-modifiers">Java Access Modifier
 
2. Java Parameterized Constructor
 
A Java constructor can also accept one or more parameters. Such constructors are known as parameterized constructors (constructor with parameters).
 
Example 4: Parameterized constructor
 
class Main < String languages; // constructor accepting single value Main(String lang) < languages = lang; System.out.println(languages + " Programming Language"); >public static void main(String[] args) < // call constructor by passing a single value Main obj1 = new Main("Java"); Main obj2 = new Main("Python"); Main obj3 = new Main("C"); >>
 
Java Programming Language Python Programming Language C Programming Language
 
In the above example, we have created a constructor named Main() . Here, the constructor takes a single parameter. Notice the expression,
 
Here, we are passing the single value to the constructor. Based on the argument passed, the language variable is initialized inside the constructor.
 
3. Java Default Constructor
 
If we do not create any constructor, the Java compiler automatically create a no-arg constructor during the execution of the program. This constructor is called default constructor.
 
Example 5: Default Constructor
 
Programming Language: Java Programming Language: Python
 
In the above example, we have two constructors: Main() and Main(String language) . Here, both the constructor initialize the value of the variable language with different values.
 
Based on the parameter passed during object creation, different constructors are called and different values are assigned.
 
It is also possible to call one constructor from another constructor. To learn more, visit Java Call One Constructor from Another.
 
Note: We have used this keyword to specify the variable of the class. To know more about this keyword, visit Java this keyword.
 
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Should we always have a zero-argument constructor in a Class?
 
Question: I am learning about classes and constructors in Java. A good rule of thumb for constructors is to pass an object its identity , not its state .
 
Should we always have a zero-argument constructor in a Class?
 
If it makes no sense to create an instance of the class without supplying any information to the constructor then you need not have a zero-argument constructor.
 
A good example is java.awt.Color class, whose all ctors are argumented.
 
No, it doesn't make sense to always create zero argument constructors, the following scenarios are examples where it makes sense to provide at least a-some-argument-constructor 
 
 
Required dependencies that the class itself cannot create.
 
There are no senseful defaults for the properties.
 
 
Cases where you want to have/need a zero-argument constructor:
 
 
You want to comply to the JavaBeans specification (makes sense for simple data objects).
 
All fields can be initialized using senseful defaults.
 
You want to use a framework that needs it.
 
 
One of the mis-arguments for having a zero-argument constructor in my opinion is a long list of arguments. For that there are better solutions than accepting to initialize an object that isn't in a safe state after creation:
 
 
Using the Builder pattern.
 
Provide specialized container objects to configure an instance via the constructor.
 
Provide multiple constructors where the base arguments of each one are the required parameters that cannot have defaults assigned.
 
 
As Andy Thomas-Cramer has already noted, it is even impossible:
 
class NeedsToBeImmutable < // For a class to be immutable, its reachable state // MUST be reached through a final field private final String stuff; //!!Compile error!! public NeedsToBeImmutable()<>public NeedsToBeImmutable(String stuff) < this.stuff = stuff; >//getters. > 
 
No. However there are exceptions. For instance, if you intend your class to contain just static util methods or a singleton class or a class with just constants then you should create a private constructor with no arguments to prevent it from being explicitly instantiated.
 
Parameter 0 of constructor in required a bean of type, I am a bit confused. Many of my service classes do not have a public constructor even then I am able to create bean with the custom constructor. But I started getting an issue with one service class where it got solved by providing a default constructor. I am creating bean from config class where parameters value is …
 
Final parameters in a constructor in Java
 
I am learning about classes and constructors in Java. I messing around with the code in an example program and can't seem to figure out exactly what's going on.
 
This code won't compile which makes sense to me:
 
I'm trying to assign the original start Point object reference to another Point object by calling the constructor of the Point object. The final keyword is in conflict with this.
 
However when I remove the final keyword from the Point start parameter.
 
it doesn't seem to actually change the reference, the Point object that is passed to the Line constructor still seems to point to the original object and is unchanged by the code of the Line constructor. So what gives? Does this have something to do with the fact that the 'start' referred to is local in scope to the Line constructor?
 
Java don't use pass-by-reference, it uses ALWAYS pass-by-value. Actually, references type in Java are simply pointers and does not share at all the same meaning as References in C# for instance.
 
So when you do this statement in your constructor:
 
The original Point passing in argument IS NOT changed. However, the local variable (meaning the paramater) will point to the new Point defined by coordinates: 0,4, 0,4 .
 
For better understanding, read this article: Java is Pass-by-Value, Dammit!
 
 
it doesn't seem to actually change the reference
 
 
You can't change the reference of a parameter you use to call a method inside that method itself. The value of the parameter (which is a reference to an object) is copied into the method parameter, so you have a second reference to the same object. Inside the method you can only modify the second reference and can't change the original reference. The only thing you can do is the modifiy the referenced object.
 
When you do "start = new Point(0.4, 0.4);", you are referencing the start that was received on the constructor. As the constructor defines it as final, its illegal to assign a new value to the variable.
 
The detail is that as java uses the pass-by-value approach, even if you don't use final and change the value inside the constructor, the value of the original value (the one you passed as parameter to the method) is still the same.
 
That variable is local to the constructor. So, assigning it a new reference doesn't make sense outside the constructor because it doesn't exist outside the constructor.
 
One way to achieve that kind of effect is to wrap it up in another object. And pass reference to the mutable wrapper.
 
Java - getConstructor with no parameters, I can't seem to use getConstructor for constructors with no parameters. I keep getting the following exception: java.lang.NoSuchMethodException: classname.<init>() Here is the code: inter
 
Constructor Parameters
 
In general, what is the maximum number of parameters a class constructor should accept? I'm developing a class that requires a lot of initialization data (currently 10 parameters). However, a constructor with 10 parameters doesn't feel right. That leads me to believe I should create a getter/setter for each piece of data. Unfortunately, the getter/setter pattern doesn't force the user to enter the data and without it characterization of the object is incomplete and therefore useless. Thoughts?
 
With that many parameters, it's time to consider the Builder pattern. Create a builder class which contains all of those getters and setters, with a build() method that returns an object of the class that you're really trying to construct.
 
Example:
 
public class ReallyComplicatedClass < private int int1; private int int2; private String str1; private String str2; // . and so on // Note that the constructor is private private ReallyComplicatedClass(Builder builder) < // set all those variables from the builder >public static class Builder < private int int1; private int int2; private String str1; private String str2; // and so on public Builder(/* required parameters here */) < // set required parameters >public Builder int1(int newInt) < int1 = newInt; return this; >// . setters for all optional parameters, all returning 'this' public ReallyComplicatedClass build() < return new ReallyComplicatedClass(this); >> > 
 
ReallyComplicatedClass c = new ReallyComplicatedClass.Builder() .int1(myInt1) .str2(myStr2) .build(); 
 
See pages 7-9 of Effective Java Reloaded [pdf], Josh Bloch's presentation at JavaOne 2007. (This is also item 2 in Effective Java 2nd Edition, but I don't have it handy so I can't quote from it.)
 
You can decide when enough is enough and use Introduce Parameter Object for your constructor (or any other method for that matter).
 
Code Complete 2 recommends a fairly sensible limit of seven parameters to any method.
 
Try to establish sensible default values for some of the members. This will let you use getter/setters without worrying that the characterization is incomplete.
 
I don't think you can say that an appropriate number is "seven, no more" or "five".
 
A good rule of thumb for constructors is to pass an object its identity , not its state . Those parameters you pass in are ones that are essential for the existence of the object, and without which most operations of the object may not be possible.
 
If you genuinely have a class with a very complicated natural identity, hence requiring many parameters then consider the design of your class.
 
An example of a bad constructor is:
 
public NightWatchman(int currentFloor, int salary, int hapiness)
 
Here the NightWatchman is being constructed with some default values that will almost certainly change in a short time. It seems funny that the object is told about their values one way, and then has them in a different way (via their setters) in future.
 
An example of a better constructor is:
 
public GateWatchman(Gate watchedGate, boolean shootOnSight)
 
The gate the watchman is watching is required information for one to exist. In the class I would mark it private final . I've chosen to pass the shootOnSight variable into the constructor, because here it was important that at all times the object knew whether to shoot burglars. Here, identity is being used as type.
 
I could have a class called ShootingGateWatchman and PoliceCallingGateWatchman - i.e the parameter is interpreted as part of the objects identity.
 
Java Constructor, Java Default Constructor A constructor is called "Default Constructor" when it doesn't have any parameter. Syntax of default constructor: () <> Example of default constructor In this example, we are creating the no-arg constructor in the Bike class. It will be invoked at the time of object creation.
 
Parameterized constructor in java
 
Parameterized constructor Java
 
class Main < String languages; // constructor accepting single value Main(String lang) < languages = lang; System.out.println(languages + " Programming Language"); >public static void main(String[] args) < // call constructor by passing a single value Main obj1 = new Main("Java"); Main obj2 = new Main("Python"); Main obj3 = new Main("C"); >>
 
Java Constructors, Constructor Parameters Constructors can also take parameters, which is used to initialize attributes. The following example adds an int y parameter to the constructor. Inside the constructor we set x to y (x=y). When we call the constructor, we pass a parameter to the constructor (5), which will set the value of x to 5: Example
 
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