Constructor in Java
In Java, constructor is a block of codes similar to method. It is called when an instance of object is created and memory is allocated for the object.
It is a special type of method which is used to initialize the object.
When a constructor is called
Everytime an object is created using new() keyword, atleast one constructor is called. It is called a default constructor.
Note: It is called constructor because it constructs the values at the time of object creation. It is not necessary to write a constructor for a class. It is because java compiler creates a default constructor if your class doesn't have any.
Rules for creating java constructor
There are basically two rules defined for the constructor.
Types of java constructors
There are two types of constructors in java:
Java Default Constructor
A constructor is called "Default Constructor" when it doesn't have any parameter.
Syntax of default constructor:
Example of default constructor
Bike is created
Rule: If there is no constructor in a class, compiler automatically creates a default constructor.
Q) What is the purpose of default constructor?
Default constructor is used to provide the default values to the object like 0, null etc. depending on the type.
Example of default constructor that displays the default valuesTest it Now
0 null 0 null
Explanation:In the above class,you are not creating any constructor so compiler provides you a default constructor.Here 0 and null values are provided by default constructor.
Java parameterized constructor
A constructor which has a specific number of parameters is called parameterized constructor.
Why use parameterized constructor?
Example of parameterized constructor
In this example, we have created the constructor of Student class that have two parameters. We can have any number of parameters in the constructor.
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In Java, a constructor is just like a method but without return type. It can also be overloaded like Java methods.
Constructor overloading in Java is a technique of having more than one constructor with different parameter lists. They are arranged in a way that each constructor performs a different task. They are differentiated by the compiler by the number of parameters in the list and their types.
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There are many differences between constructors and methods. They are given below.
|Java Constructor||Java Method|
|Constructor is used to initialize the state of an object.||Method is used to expose behaviour of an object.|
|Constructor must not have return type.||Method must have return type.|
|Constructor is invoked implicitly.||Method is invoked explicitly.|
|The java compiler provides a default constructor if you don't have any constructor.||Method is not provided by compiler in any case.|
|Constructor name must be same as the class name.||Method name may or may not be same as class name.|
There is no copy constructor in java. But, we can copy the values of one object to another like copy constructor in C++.
There are many ways to copy the values of one object into another in java. They are:
In this example, we are going to copy the values of one object into another using java constructor.Test it Now
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We can copy the values of one object into another by assigning the objects values to another object. In this case, there is no need to create the constructor.Test it Now
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Ans:yes, that is current class instance (You cannot use return type yet it returns a value).
Yes, like object creation, starting a thread, calling method etc. You can perform any operation in the constructor as you perform in the method.