In C++, there is a concept of a copy constructor. Similarly, Java also supports Copy constructors, but the difference is that in C++, the copy constructor is created by default, while in Java, we have to create the copy constructor on our own. In this article, we will see how to create a copy constructor and why it is necessary in our programs.<\/p>\n
Before learning about copy constructors, we need to understand the concept of constructors. Constructors are special methods that have the same name as the class. Using constructors, we can initialize global variables. Constructors do not have any return type. They are called automatically when an object is declared for that class. Constructors occupy space according to the variables present in them.<\/p>\n
Constructors that do not have a parameter list are known as a default constructor.<\/p>\n
Example of Default Constructor in Java:<\/strong><\/p>\n The output of the above code:<\/strong><\/p>\n These are the Constructors which have a parameter list.<\/p>\n Example of Parameterized Constructor in Java:<\/strong><\/p>\n The output of the above Code:<\/strong><\/p>\n Other than these two Constructors, Java also supports copy constructors. We will discuss it in detail now.<\/p>\n As the name suggests, the copy constructor is used to create a copy of a previously existing object present in the class. It is a special type of constructor that returns a duplicate copy of another object. The main purpose of a copy constructor is to create a copy of an object. This helps in the reusability of code. Sometimes, when there is a need to make changes to the attributes of the object without affecting the original object, we create a copy of the object. For such scenarios, a copy constructor is essential.<\/p>\n The steps to create a copy constructor are as follows:<\/strong><\/p>\n Step 1:<\/strong> We have to create a constructor that takes an object as a parameter. Make sure that the object belongs to the same class.<\/p>\n Step 2:<\/strong> Using getters, copy each variable of the copied object to the new instances.<\/p>\n To copy mutable type data, we have to create a deep copy instead of a shallow copy, which we did in the previous example.<\/p>\n Let us understand this with a code:<\/p>\n Code to understand Mutable data with copy constructor:<\/strong><\/p>\n Code to explain the working of the copy constructor in Java:<\/strong><\/p>\n The Output of the above Program:<\/strong><\/p>\n To understand the advantage of a copy constructor, let us try to copy an object without using a copy constructor.<\/p>\n Code for creating a copy of an object without using the copy constructor:<\/strong><\/p>\n The output of the above code:<\/strong><\/p>\n So, we can clearly see that the size of the code increased considerably for only 2 variables. Now, imagine if there are 50 such variables, how hard it will be to copy each variable separately. So, we can see why a copy constructor is an important concept to write optimal code.<\/p>\n When crafting copy constructors, here are some valuable guidelines to consider:<\/p>\n Clone() is also a method using which we can use to create a duplicate copy of a pre-existing object. But using a clone method is more difficult compared to a copy constructor, due to the following reasons:<\/p>\n 1. The clone method may throw an exception if the cloneable is not imported properly. Handling an exception is an extra task. While the Copy Constructor does not throw any such exceptions.<\/p>\n 2. Using Clone(), we cannot assign values to variables that are declared final. As we all know by now, the copy constructor allows us to change the value of final variables.<\/p>\n 3. In the Clone() method, typecasting is required, while in the copy constructor, typecasting is not required.<\/p>\n One very big disadvantage of the copy constructor is that the subclasses of the parent class that contain the copy constructor cannot access the copy constructor. The program will give a casting error when we try to compile it.<\/p>\n Code explaining the Problem with inheritance and copy constructor:<\/strong><\/p>\n The above code will give an error, as the compiler will not be able to recognize the variables ID and name. To fix this we have to typecast the variable with the subclass.<\/p>\n For Example:<\/strong><\/p>\n We can avoid typecasting by adding a new inherited method to both classes.<\/p>\n Code explaining how to avoid type casting in case of inherited copy constructor:<\/strong><\/p>\npackage com.DataFlair.CopyConstructor;\r\npublic class DefaultConstructor\r\n{ \r\n DefaultConstructor()\r\n {\r\n System.out.println(\"Object Created, so Constructor Called\");\r\n } \r\n public static void main(String args[])\r\n { \r\n DefaultConstructor d=new DefaultConstructor(); \r\n }\r\n}\r\n<\/pre>\n2. Parameterized Constructor in Java<\/h4>\n
package com.DataFlair.CopyConstructor;\r\npublic class ParameterConstructor\r\n{\r\n int id; \r\n String name;\r\n ParameterConstructor(int i,String n){ \r\n id = i; \r\n name = n; \r\n } \r\n void display()\r\n {\r\n System.out.println(id+\" \"+name);\r\n }\r\n public static void main(String args[]){ \r\n ParameterConstructor P = new ParameterConstructor(1,\"Arka\");\r\n P.display(); \r\n } \r\n} \r\n<\/pre>\nWhat is a Copy Constructor in Java?<\/h3>\n
\nIn the C language, the copy constructor is declared automatically, but in Java, we have to create the method separately.<\/p>\nWhy is it required to use the Java Copy Constructor?<\/h3>\n
Creating a Copy Constructor in Java<\/h3>\n
public class Interns \r\n{ \r\nprivate int ID; \r\nprivate String name; \r\npublic Interns(Interns interns)\/\/copy constructor \r\n{ \r\n\/\/getters \r\n} \r\n} \r\n<\/pre>\npublic class Interns \r\n{ \r\nprivate int ID; \r\nprivate String name; \r\npublic Interns(Interns interns)\/\/copy constructor \r\n{ \r\nthis.ID=interns.ID;\/\/getters \r\nthis.name=interns.name;\/\/getters\r\n} \r\n}\r\n<\/pre>\nHow to Copy Mutable Type data in Java?<\/h3>\n
public class Interns\r\n{\r\n private int ID;\r\n private String name;\r\n private Date InternDate;\/\/Mutable Data\r\n\r\n public Interns( Interns interns )\r\n {\r\n this.roll = interns.id;\r\n this.name = interns.name;\r\n this.InternDate = new Date(interns.InternDate.getTime());\/\/Deep Copy\r\n }\r\n}\r\n<\/pre>\npackage com.DataFlair.CopyConstructor;\r\npublic class Interns\r\n{ \r\n private int ID; \r\n private String Name; \r\n Interns(int id, String name)\/\/parameterized constructor\r\n {\r\n ID=id;\r\n Name=name;\r\n }\r\n Interns(Interns interns)\/\/copy constructor \r\n { \r\n System.out.println(\"The Copy Constructor Starts here\");\r\n ID=interns.ID;\/\/getters \r\n Name=interns.Name;\/\/getters\r\n } \r\n int showID()\r\n {\r\n return ID;\r\n }\r\n String showName()\r\n {\r\n return Name;\r\n }\r\n public static void main(String[] args)\r\n {\r\n Interns I=new Interns(1,\"Arka\");\r\n System.out.println(\"ID of Intern: \"+ I.showID()+ \" Name of Intern:\"+ I.showName());\r\n Interns IC=new Interns(I);\r\n System.out.println(\"ID of Intern: \"+ I.showID()+ \" Name of Intern:\"+ I.showName());\r\n }\r\n}<\/pre>\n
\nThe Copy Constructor Starts here
\nID of Intern: 1 Name of Intern:Arka<\/div>\nAdvantages of Using Copy Constructor in Java<\/h3>\n
\n
package com.DataFlair.CopyConstructor;\r\npublic class Intern\r\n{\r\n private int ID;\r\n private String name;\r\n Intern(int id, String Name)\r\n { \r\n ID = id;\r\n name= Name;\r\n }\r\n \r\n Intern()\r\n { \r\n }\r\n int printID()\r\n {\r\n return ID;\r\n }\r\n String printName()\r\n {\r\n return name;\r\n }\r\n public static void main(String[ ] args)\r\n {\r\n Intern I1 = new Intern(1, \"Arka\");\r\n System.out.println(\"ID of Intern: \"+ I1.printID());\r\n System.out.println(\"Name of Intern: \"+ I1.printName());\r\n\r\n Intern I2 = new Intern();\r\n I2.ID= I1.ID;\r\n I2.name= I1.name;\r\n\r\n System.out.println(\"Copied ID of Intern: \"+ I2.printID());\r\n System.out.println(\"Copied Name of Intern: \"+ I2.printName());\r\n}\r\n}\r\n<\/pre>\n
\nName of Intern: Arka
\nCopied ID of Intern: 1
\nCopied Name of Intern: Arka<\/div>\nBest Practices for Java Copy Constructors<\/h3>\n
\n
Difference Between Java Copy Constructor and Clone() Method:<\/h3>\n
Inheritance Issues in Java Copy Constructor:<\/h3>\n
package com.DataFlair.CopyConstructor;\r\nimport java.util.List;\r\n\r\npublic class TechWriter extends Interns\r\n{\r\nprivate List<Interns> Articles;\r\npublic TechWriter(TechWriter techwriter)\r\n{\r\n super(techwriter.ID, techwriter.name);\r\n this.Articles = Articles.stream().collect(Collectors.toList());\r\n}\r\n}\r\n<\/pre>\nInterns clone = new TechWriter((Techwriter)interns);\r\n<\/pre>\n
public class Interns\r\n{\r\npublic Interns copy()\r\n{\r\n return new Interns(this);\r\n }\r\n}\r\n \r\npublic class Techwriter extends Interns\r\n{\r\n@Override\r\n public Interns copy() \r\n{\r\n return new Techwriter(this);\r\n }\r\n}\r\n<\/pre>\nQuiz on Java Copy Constructor<\/h3>\n