introduction.md

Introduction

Optional

The Optional type was introduced in Java 8 as a way to indicate that a method may return a value.

In other words, there is a chance the method returns "no value" at all.

Creating an Optional object

Given an object of type Employee, an Optional object can be created using the static of method:

Employee employee = new Employee();
Optional<Employee> optionalEmployee = Optional.of(employee);

If the employee may be not present, the static ofNullable method must be used:

Employee nullableEmployee = new Employee();
Optional<Employee> nullableEmployee = Optional.ofNullable(employee);

optionalEmployee and nullableEmployee both are wrappers of an Employee object.

Basic methods

If a value is present, the isPresent method returns true and the get method returns the value.

Employee employee = new Employee("Tim", 45);
Optional<Employee> optionalEmployee = Optional.ofNullable(employee);
boolean isThereAnEmployee = optionalEmployee.isPresent();  // true
Employee employee = optionalEmployee.get();

Usage

In order to throw an exception when the value is not present, the orElseThrow method must be used.

public Optional<Employee> getEmployee(String name) {
    // Assume that getEmployeeByName returns an Optional<Employee>
    return getEmployeeByName(name)
           .orElseThrow(() -> new IllegalArgumentException("Employee not found"));
}

If a default value must be returned, the orElse method can be used.

public Optional<Employee> getEmployee(String name) {
    // Assume that getEmployeeByName returns an Optional<Employee>
    return getEmployeeByName(name)
           .orElse(new Employee("Daniel"));
}

Other commonly used method is ifPresentOrElse, which is used to handle both cases with the same method: the case where the value is present and the case where the value is empty.

public Optional<Employee> getEmployee(String name) {
    // Assume that getEmployeeByName returns an Optional<Employee>
    return getEmployeeByName(name)
           .ifPresentOrElse(
                   employee -> System.out.println(employee.getName()),
                   () -> System.out.println("Employee not found")
           );
}

Provided all the invoked methods return Optional objects, many methods can be chained without having to worry about null checking:

public Optional<Integer> getEmployeeAge(String name) {
    Optional<Employee> optionalEmployee = getEmployeeByName(name);
    return getEmployeeByName(name)
               .map(employee -> employee.getAge())
               .orElse("No employee found");
}