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 –

Employee.java
public class Employee{
 String name;
 Integer age;
 //Contructor of employee
 public Employee(String name, Integer age){
  this.name=name;
  this.age=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
.

Interface EmployeeFactory.java
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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