Method Reference
Method reference is an abbreviated syntax used with lambda expressions for invoking a method.
Double colon operator (::) is used to represent a method reference.
The syntax for the same is :
ClassName::MethodName
ClassName::instanceMethodName represents a method reference for an instance method of a class.
This creates a one-parameter lambda that invokes the instance method on the lambda’s argument and returns the method’s result.
objectName::instanceMethodName represents a method reference for an instance method that should be called on a specific object.
This creates a one-parameter lambda that invokes the instance method on the specified object—passing the lambda’s argument to the instance method—and returns the method’s result.
ClassName::staticMethodName represents a method reference for a static method of a class.
This creates a one-parameter lambda in which the lambda’s argument is passed to the specified a static method and the lambda returns the method’s result.
Example of using Method reference in lambda expression
We can loop through a List using a lambda forEach loop as follows :
List<String> countries = Arrays.asList("Germany", "England", "China",
"Denmark", "Brazil", "France", "Australia");
countries.forEach(System.out::println);
This example prints the country names as :
Germany
England
China
Denmark
Brazil
France
Australia
Example of converting List of Strings to uppercase using method reference
List<String> countries = Arrays.asList("Germany", "England", "China",
"Denmark", "Brazil", "France", "Australia");
countries.stream().map(String::toUpperCase)
.forEach(System.out::println);
Output :
GERMANY
ENGLAND
CHINA
DENMARK
BRAZIL
FRANCE
AUSTRALIA
Constructor reference
Similar to method reference, we can use constructor reference instead of the traditional “new” syntax to create an instance.
ClassName::new represents a constructor reference.
This creates a lambda that invokes the no-argument constructor of the specified class to create and initialize a new object of that class.
Example : no-argument constructor
package com.topjavatutorial;
public class Employee {
public Employee(){
}
}
Supplier<Employee> supplier = Employee::new; Employee e = supplier.get();
Example : one-parameter constructor
package com.topjavatutorial;
public class Employee {
private String name;
public Employee(String name){
this.name = name;
}
}
Function<String,Employee> function = Employee::new;
System.out.println(function.apply("John Doe"));
ClassName[]::new is a shortcut for creating an Array using the method reference notation.
This creates an array of provided type.
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