Constructor References Java 8 Simplified Tutorial with examples

This tutorial explains the new Java 8 feature known as constructor reference. It starts off with explaining what is a constructor reference by showing its structure and an example. Next, the tutorial shows an example scenario where constructor references can be applied for instantiating objects from an object factory.
(Note – If you are new to the concept of method references then I would recommend that first refer the method references tutorialRead Method References Tutorial.)

What are Constructor References: Constructor Introduced in Java 8, constructor references are specialized form of method references which refer to the constructors of a class. Constructor References can be created using the Class Name and the keyword new with the following syntax –
Syntax of Constructor References: <ClassName>::new 

Constructor Reference Example: If you want reference to the constructor of wrapper class Integer, then you can write something like this –
Supplier<Integer> integerSupplier = Integer::new

Example usage of Constructor References in code
STEP 1 – Let us define an Employee class with a constructor having 2 parameters as shown below –
public class Employee{
 String name;
 Integer age;
 //Contructor of employee
 public Employee(String name, Integer age){;

STEP 2 – Now lets create a factory interface for employees called EmployeeFactory. getEmployee() method of EmployeeFactory will return an employee instance as per the normal design of factory pattern
Read tutorial on Factory Design Pattern

public Interface EmployeeFactory{
 public abstract Employee getEmployee(String name, Integer age);
Note – Since EmployeeFactory interface has a single abstract method hence it is a Functional InterfaceRead Tutorial on Functional Interfaces.

STEP 3 – The client side code to invoke EmployeeFactory to create Employee instances would be as follows –

Client-side code for invoking Factory interface
EmployeeFactory empFactory=Employee::new;
Employee emp= empFactory.getEmployee("John Hammond", 25);
Explanation of the code

  • The Constructor Reference of Employee is assigned to an instance of EmployeeFactory called empFactory. This is possible because the function descriptorRead tutorial on Java 8 Function Descriptors of Employee constructor is same as that of the abstract method of the Functional Interface EmployeeFactory i.e.
    (String, Integer) ->Employee.
  • Then the getEmployee method of empFactory is called with John’s name and age which internally calls the constructor of Employee and a new Employee instance emp is created.

Summary: In the above tutorial we understood constructor references by first understanding their definition/structure. Next we saw examples for defining a constructor reference for actual usage and then saw an example practical scenario where constructor references can be used.


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