Java Lambda Expression Parameter and Return Types

This post shows how to pass the type of parameters to and return from a Lambda expression in Java based on the functional interface abstract method definition. The types must be sub-types or exact match to those defined in the abstract method for the parameters and return type.

Using the Function Interface And Lambda Expression

Consider the following method.

...
private static void demoMethod(Person person, Function<Person, Person> function)
{
    Person rtn = function.apply(person);

    System.out.println("param person instanceof Person? " + (rtn instanceof Person));
    System.out.println("param person instanceof Student? " + (rtn instanceof Student));

    System.out.println("rtn instanceof Person? " + (rtn instanceof Person));
    System.out.println("rtn instanceof Student? " + (rtn instanceof Student));
}
...

...

private static void demoMethod(Person person, Function<Person, Person> function)

{

Person rtn = function.apply(person);

System.out.println("param person instanceof Person? " + (rtn instanceof Person));

System.out.println("param person instanceof Student? " + (rtn instanceof Student));

System.out.println("rtn instanceof Person? " + (rtn instanceof Person));

System.out.println("rtn instanceof Student? " + (rtn instanceof Student));

}

...

It accepts a Person object and a Function functional interface that accepts and returns a Person object.

Person class and its sub-class

The Student class extends the Person class.

class Person 
{
	
}

class Student extends Person 
{
	
}

class Person

{

}

class Student extends Person

{

}

Now Polymorphism allows us to pass or/and return Student objects instead of Person objects.

...
public static void main(String[] args) 
{
    demoMethod(new Person(), (str) -> new Person());
    demoMethod(new Student(), (str) -> new Student());
}
...

...

public static void main(String[] args)

{

demoMethod(new Person(), (str) -> new Person());

demoMethod(new Student(), (str) -> new Student());

}

...

The codes output

param person instanceof Person?true
param person instanceof Student?false
rtn instanceof Person?true
rtn instanceof Student?false
param person instanceof Person?true
param person instanceof Student?true
rtn instanceof Person?true
rtn instanceof Student?true

param person instanceof Person?true

param person instanceof Student?false

rtn instanceof Person?true

rtn instanceof Student?false

param person instanceof Person?true

param person instanceof Student?true

rtn instanceof Person?true

rtn instanceof Student?true

But…

When we assign a Lambda expression to a variable, we need to declare that variable using Generics.

...
Function<Student, Student> function = (str) -> new Student();
demoMethod(new Student(), function);
...

...

Function<Student, Student> function = (str) -> new Student();

demoMethod(new Student(), function);

...

This results in a compilation error.

Because

The method demoMethod(Person, Function<Person,Person>) 
in the type LambdaExpressionParamAndReturnTypes is not 
applicable for the arguments (Student, Function<Student,Student>)

The method demoMethod(Person, Function<Person,Person>)

in the type LambdaExpressionParamAndReturnTypes is not

applicable for the arguments (Student, Function<Student,Student>)

The Full Codes

import java.util.function.Function;

public class LambdaExpressionParamAndReturnTypes 
{
    public static void main(String[] args) 
    {
        demoMethod(new Person(), (str) -> new Person());
        demoMethod(new Student(), (str) -> new Student());
    }

    private static void demoMethod(Person person, Function<Person, Person> function)
    {
        Person rtn = function.apply(person);
        System.out.println("param person instanceof Person? " + (rtn instanceof Person));
        System.out.println("param person instanceof Student? " + (rtn instanceof Student));
		
        System.out.println("rtn instanceof Person? " + (rtn instanceof Person));
        System.out.println("rtn instanceof Student? " + (rtn instanceof Student));
    }
}


class Person { }
class Student extends Person { }