In Java, constructors play an important role in object creation. A constructor is a special block of code that is called when an object is created. Its main job is to initialize the object, to set up its internal state, or to assign default values to its attributes. This process happens automatically when we use the "new" keyword to create an object.
Characteristics of Constructors:
- Same Name as the Class: A constructor has the same name as the class in which it is defined.
- No Return Type: Constructors do not have any return type, not even void. The main purpose of a constructor is to initialize the object, not to return a value.
- Automatically Called on Object Creation: When an object of a class is created, the constructor is called automatically to initialize the object’s attributes.
- Used to Set Initial Values for Object Attributes: Constructors are primarily used to set the initial state or values of an object’s attributes when it is created.
Now, let us look at a simple example to understand how a constructor works in Java.
Example: This program demonstrates how a constructor is automatically called when an object is created in Java.
Java
// Java Program to demonstrate
// Constructor usage
import java.io.*;
// Driver Class
class Geeks {
// Constructor
Geeks()
{
super();
System.out.println("Constructor Called");
}
// main function
public static void main(String[] args)
{
Geeks geek = new Geeks();
}
}
Note: It is not necessary to write a constructor for a class. It is because the Java compiler creates a default constructor (constructor with no arguments) if your class doesn't have any.
Constructor vs Method in Java
The below table demonstrates the key difference between Java Constructor and Java Methods.
Features | Constructor | Method |
---|
Name | Constructors must have the same name as the class name | Methods can have any valid name |
---|
Return Type | Constructors do not return any type | Methods have the return type or void if does not return any value. |
---|
Invocation | Constructors are called automatically with new keyword | Methods are called explicitly |
---|
Purpose | Constructors are used to initialize objects | Methods are used to perform operations |
---|
Now let us come up with the syntax for the constructor being invoked at the time of object or instance creation.
class Geek
{
......
// A Constructor
Geek() {
}
.......
}
// We can create an object of the above class
// using the below statement. This statement
// calls above constructor.
Geek obj = new Geek();
The first line of a constructor is a call to super() or this(), (a call to a constructor of a super-class or an overloaded constructor), if you don't type in the call to super in your constructor the compiler will provide you with a non-argument call to super at the first line of your code, the super constructor must be called to create an object:
Note: If you think your class is not a subclass it actually is, every class in Java is the subclass of a class object even if you don't say extends object in your class definition.
Why Do We Need Constructors in Java
Constructors play a very important role, it ensures that an object is properly initialized before use.
What happens when we don't use constructors:
Without constructors:
- Objects might have undefined or default values.
- Extra initialization methods would be required.
- Risk of improper object state
Think of a Box. If we talk about a box class then it will have some class variables (say length, breadth, and height). But when it comes to creating its object (i.e Box will now exist in the computer's memory), then can a box be there with no value defined for its dimensions? The answer is No.
So, constructors are used to assign values to the class variables at the time of object creation, either explicitly done by the programmer or by Java itself (default constructor).
When Java Constructor is Called?
Each time an object is created using a new() keyword, at least one constructor (it could be the default constructor) is invoked to assign initial values to the data members of the same class. Rules for writing constructors are as follows:
- The constructor of a class must have the same name as the class name in which it resides.
- A constructor in Java can not be abstract, final, static, or Synchronized.
- Access modifiers can be used in constructor declaration to control its access i.e which other class can call the constructor.
So, we have learned constructors are used to initialize the object's state. Like methods , a constructor also contains a collection of statements (i.e. instructions) that are executed at the time of object creation.
Types of Constructors in Java
Now is the correct time to discuss the types of the constructor, so primarily there are three types of constructors in Java are mentioned below:
- Default Constructor
- Parameterized Constructor
- Copy Constructor
1. Default Constructor in Java
A constructor that has no parameters is known as default constructor. A default constructor is invisible. And if we write a constructor with no arguments, the compiler does not create a default constructor. Once you define a constructor (with or without parameters), the compiler no longer provides the default constructor. Defining a parameterized constructor does not automatically create a no-argument constructor, we must explicitly define if needed. The default constructor can be implicit or explicit.
- Implicit Default Constructor: If no constructor is defined in a class, the Java compiler automatically provides a default constructor. This constructor doesn’t take any parameters and initializes the object with default values, such as
0
for numbers, null
for objects. - Explicit Default Constructor: If we define a constructor that takes no parameters, it's called an explicit default constructor. This constructor replaces the one the compiler would normally create automatically. Once you define any constructor (with or without parameters), the compiler no longer provides the default constructor for you.
Example: This program demonstrates the use of a default constructor, which is automatically called when an object is created.
Java
// Java Program to demonstrate
// Default Constructor
import java.io.*;
// Driver class
class Geeks{
// Default Constructor
Geeks() {
System.out.println("Default constructor");
}
// Driver function
public static void main(String[] args)
{
Geeks hello = new Geeks();
}
}
OutputDefault constructor
Note: Default constructor provides the default values to the object like 0, null, false etc. depending on the type.
2. Parameterized Constructor in Java
A constructor that has parameters is known as parameterized constructor. If we want to initialize fields of the class with our own values, then use a parameterized constructor.
Example: This program demonstrates the use of a parameterized constructor to initialize an object's attributes with specific values.
Java
// Java Program for Parameterized Constructor
import java.io.*;
class Geeks {
// data members of the class
String name;
int id;
Geeks(String name, int id) {
this.name = name;
this.id = id;
}
}
class GFG
{
public static void main(String[] args)
{
// This would invoke the parameterized constructor
Geeks geek1 = new Geeks("Sweta", 68);
System.out.println("GeekName: " + geek1.name
+ " and GeekId: " + geek1.id);
}
}
OutputGeekName: Sweta and GeekId: 68
Remember: Does constructor return any value?
There are no "return value" statements in the constructor, but the constructor returns the current class instance. We can write 'return' inside a constructor.
3. Copy Constructor in Java
Unlike other constructors copy constructor is passed with another object which copies the data available from the passed object to the newly created object.
Note: Java does not provide a built-in copy constructor like C++. We can create our own by writing a constructor that takes an object of the same class as a parameter and copies its fields.
Example: This example, demonstrates how a copy constructor can be used to create a new object by copying the values of another object's attributes.
Java
// Java Program for Copy Constructor
import java.io.*;
class Geeks {
// data members of the class
String name;
int id;
// Parameterized Constructor
Geeks(String name, int id)
{
this.name = name;
this.id = id;
}
// Copy Constructor
Geeks(Geeks obj2)
{
this.name = obj2.name;
this.id = obj2.id;
}
}
class GFG {
public static void main(String[] args)
{
// This would invoke the parameterized constructor
System.out.println("First Object");
Geeks geek1 = new Geeks("Sweta", 68);
System.out.println("GeekName: " + geek1.name
+ " and GeekId: " + geek1.id);
System.out.println();
// This would invoke the copy constructor
Geeks geek2 = new Geeks(geek1);
System.out.println(
"Copy Constructor used Second Object");
System.out.println("GeekName: " + geek2.name
+ " and GeekId: " + geek2.id);
}
}
OutputFirst Object
GeekName: Sweta and GeekId: 68
Copy Constructor used Second Object
GeekName: Sweta and GeekId: 68
Constructor Overloading
This is a key concept in OOPs related to constructors is constructor overloading. This allows us to create multiple constructors in the same class with different parameter lists.
Example: This example, demonstrates constructor overloading, where multiple constructors perform the same task (initializing an object) with different types or numbers of arguments.
Java
// Java Program to illustrate constructor overloading
// using same task (addition operation ) for different
// types of arguments
import java.io.*;
class Geeks {
// constructor with one argument
Geeks(String name)
{
System.out.println("Constructor with one "
+ "argument - String: " + name);
}
// constructor with two arguments
Geeks(String name, int age)
{
System.out.println(
"Constructor with two arguments: "
+ " String and Integer: " + name + " " + age);
}
// Constructor with one argument but with different
// type than previous
Geeks(long id)
{
System.out.println(
"Constructor with one argument: "
+ "Long: " + id);
}
}
class GFG {
public static void main(String[] args)
{
// Creating the objects of the class named 'Geek'
// by passing different arguments
// Invoke the constructor with one argument of
// type 'String'.
Geeks geek2 = new Geeks("Sweta");
// Invoke the constructor with two arguments
Geeks geek3 = new Geeks("Amiya", 28);
// Invoke the constructor with one argument of
// type 'Long'.
Geeks geek4 = new Geeks(325614567);
}
}
OutputConstructor with one argument - String: Sweta
Constructor with two arguments: String and Integer: Amiya 28
Constructor with one argument: Long: 325614567
To know deep down about constructors refer to this article Constructor Chaining
Common Mistakes to Avoid
Some common mistakes to avoid when working with constructors in Java are listed below:
- Forgetting super() in Child Classes: Always call the parent constructor (super()) if the parent class has no default constructor, or it will lead to compilation errors.
- Excessive Work in Constructors: Keep constructors simple and focused on initialization, avoiding heavy logic that slows down object creation.
- Not Handling Null Checks: Always validate parameters to avoid NullPointerException when constructing objects.
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Stack Class in JavaThe Java Collection framework provides a Stack class, which implements a Stack data structure. The class is based on the basic principle of LIFO (last-in-first-out). Besides the basic push and pop operations, the class also provides three more functions, such as empty, search, and peek. The Stack cl
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Set in JavaThe Set Interface is present in java.util package and extends the Collection interface. It is an unordered collection of objects in which duplicate values cannot be stored. It is an interface that implements the mathematical set. This interface adds a feature that restricts the insertion of duplicat
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Java HashSetHashSet in Java implements the Set interface of Collections Framework. It is used to store the unique elements and it doesn't maintain any specific order of elements. Can store the Null values.Uses HashMap (implementation of hash table data structure) internally.Also implements Serializable and Clon
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TreeSet in JavaTreeSet is one of the most important implementations of the SortedSet interface in Java that uses a Tree(red - black tree) for storage. The ordering of the elements is maintained by a set using their natural ordering whether or not an explicit comparator is provided. This must be consistent with equ
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Java LinkedHashSetLinkedHashSet in Java implements the Set interface of the Collection Framework. It combines the functionality of a HashSet with a LinkedList to maintain the insertion order of elements. Stores unique elements only.Maintains insertion order.Provides faster iteration compared to HashSet.Allows null el
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Queue Interface In JavaThe Queue Interface is a part of java.util package and extends the Collection interface. It stores and processes the data in order means elements are inserted at the end and removed from the front. Key Features:Most implementations, like PriorityQueue, do not allow null elements.Implementation Class
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PriorityQueue in JavaThe PriorityQueue class in Java is part of the java.util package. It implements a priority heap-based queue that processes elements based on their priority rather than the FIFO (First-In-First-Out) concept of a Queue.Key Points:The PriorityQueue is based on the Priority Heap. The elements of the pri
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Deque Interface in JavaDeque Interface present in java.util package is a subtype of the queue interface. The Deque is related to the double-ended queue that supports adding or removing elements from either end of the data structure. It can either be used as a queue(first-in-first-out/FIFO) or as a stack(last-in-first-out/
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Map Interface in JavaIn Java, the Map Interface is part of the java.util package and represents a mapping between a key and a value. The Java Map interface is not a subtype of the Collections interface. So, it behaves differently from the rest of the collection types.Key Features:No Duplicates in Keys: Keys should be un
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HashMap in JavaIn Java, HashMap is part of the Java Collections Framework and is found in the java.util package. It provides the basic implementation of the Map interface in Java. HashMap stores data in (key, value) pairs. Each key is associated with a value, and you can access the value by using the corresponding
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Java LinkedHashMapLinkedHashMap in Java implements the Map interface of the Collections Framework. It stores key-value pairs while maintaining the insertion order of the entries. It maintains the order in which elements are added.Stores unique key-value pairs.Maintains insertion order.Allows one null key and multiple
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Hashtable in JavaHashtable class, introduced as part of the Java Collections framework, implements a hash table that maps keys to values. Any non-null object can be used as a key or as a value. To successfully store and retrieve objects from a hashtable, the objects used as keys must implement the hashCode method an
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Java Dictionary ClassDictionary class in Java is an abstract class that represents a collection of key-value pairs, where keys are unique and used to access the values. It was part of the Java Collections Framework and it was introduced in Java 1.0 but has been largely replaced by the Map interface since Java 1.2.Stores
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SortedSet Interface in Java with ExamplesThe SortedSet interface is present in java.util package extends the Set interface present in the collection framework. It is an interface that implements the mathematical set. This interface contains the methods inherited from the Set interface and adds a feature that stores all the elements in this
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Java Comparator InterfaceThe Comparator interface in Java is used to sort the objects of user-defined classes. The Comparator interface is present in java.util package. This interface allows us to define custom comparison logic outside of the class for which instances we want to sort. The comparator interface is useful when
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Java Comparable InterfaceThe Comparable interface in Java is used to define the natural ordering of objects for a user-defined class. It is part of the java.lang package and it provides a compareTo() method to compare instances of the class. A class has to implement a Comparable interface to define its natural ordering.Exam
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Java Comparable vs ComparatorIn Java, both Comparable and Comparator interfaces are used for sorting objects. The main difference between Comparable and Comparator is:Comparable: It is used to define the natural ordering of the objects within the class.Comparator: It is used to define custom sorting logic externally.Difference
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Java IteratorAn Iterator in Java is an interface used to traverse elements in a Collection sequentially. It provides methods like hasNext(), next(), and remove() to loop through collections and perform manipulation. An Iterator is a part of the Java Collection Framework, and we can use it with collections like A
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