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:
Constructor Reference Example: If you want reference to the constructor of wrapper class
STEP 2 - Now lets create a factory interface for employees called
Note - Since
STEP 3 - The client side code to invoke
Explanation of the code
(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;
}
}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);
}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);- The Constructor Reference of
Employeeis assigned to an instance ofEmployeeFactorycalledempFactory. This is possible because the function descriptorRead tutorial on Java 8 Function Descriptors ofEmployeeconstructor is same as that of the abstract method of the Functional InterfaceEmployeeFactoryi.e.(String, Integer) ->Employee. - Then the
getEmployeemethod ofempFactoryis called with John's name and age which internally calls the constructor ofEmployeeand a newEmployeeinstanceempis created.