We can limit the opportunities for NullPointerExceptions by following good practice like:

• Don’t return nulls from methods
• Don’t pass nulls as arguments

Another tool is to use java.lang.Optional. Java 8 Optionals formalize the approach used in other languages (and that already existed in some Java libraries). Optional is simply an object wrapper. The key to its use is to use the methods in the Optional classes.

Examples

I'm following a similar style to my last post, A Java 8 Streams Cookbook, and have included a class with a number of examples using Optional:

import java.util.Optional;

public class OptionalExamples {
    // Populate Bike with Optional<Wheels>
    private static Bike colnagoBike = new Bike(Optional.of(new Wheels("mavic", 32)), "colnago");

    // Dont do this - use a Optional.ofNullable
    private static Bike nullBike = new Bike(null, "nowheels");

    // Use a Optional.ofNullable
    private static Bike ofNullableBike = new Bike(Optional.ofNullable(null), "nowheels");

    public static void main(String[] args) {
        emptyOptionals();
        nullObjectsInOptionals();
        workingWithValuesInOptionals();
    }

    private static void workingWithValuesInOptionals() {
        // Working with Values

        // Populated Optional - now begin populating with Object
        Optional<Bike> optionalColnagoBike = Optional.of(colnagoBike);
        if (optionalColnagoBike.isPresent()) {
            System.out.println("isPresent() - ok - but not much improvement on != null");
        }

        optionalColnagoBike.ifPresent(bike -> System.out.println("ifPresent(Consumer) returns " + bike.getBrand()));

        // Filtering and mapping in combination with Lambdas
        Bike orElseThrowBike = optionalColnagoBike.filter(b -> "colnago".equals(b.getBrand())).get();

        //
        Optional<String> brand = optionalColnagoBike.map(b -> b.getBrand());
        brand.ifPresent(b -> System.out.println("brand " + b));

        // flatMap to prevent Optional<Optional<Wheels>>
        Optional<Wheels> wheels = optionalColnagoBike.flatMap(b -> b.getWheels());
        wheels.ifPresent(w -> System.out.println("flatMap - Wheel Brand " + w.getBrand()));
    }

    private static void nullObjectsInOptionals() {
        // of - Populate with null object - Throws Exception
        try {
            Optional<Bike> optionalNullBike = Optional.of(null);
        } catch (java.lang.NullPointerException nfe) {
            System.out.println("Cant call Optional.of(null) - " + nfe.getMessage());
        }

        // ofNullable - allows null
        System.out.println("We can pass a null with ofNullable");
        Optional<Bike> optionalOfNullableBike = Optional.ofNullable(null);
        System.out.println("optionalOfNullableBike.isPresent() returns " + optionalOfNullableBike.isPresent());
    }

    private static void emptyOptionals() {
        // Empty Optional - empty container with no object
        Optional<Bike> optionalEmptyBike = Optional.empty();

        // call get() on empty object throws NoSuchElementException
        try {
            Bike emptyBike = optionalEmptyBike.get();
        } catch (java.util.NoSuchElementException e) {
            System.out.println("get() on empty Optional throws java.util.NoSuchElementException " + e.getMessage());
        }

        // isPresent - check if object is empty - but not much advantage over !=
        // null checks
        if (!optionalEmptyBike.isPresent()) {
            System.out.println("isPresent() - ok - but not much improvement on != null");
        }

        // Better Alternatives -

        // orElse - returns a default object if none set
        Bike orElseBike = optionalEmptyBike.orElse(colnagoBike);
        System.out.println("orElse - Optional is empty so return colnagoBike " + orElseBike.getBrand());

        // ifPresent(Consumer<? extends Bike>) - this prints nothing as Optional
        // is empty
        optionalEmptyBike.ifPresent(bike -> System.out.println("ifPresent(Consumer) returns " + bike.getBrand()));

        // orElseThrow - Throw Exception
        try {
            Bike orElseThrowBike = optionalEmptyBike.orElseThrow(NoBikeException::new);
        } catch (NoBikeException nbe) {
            System.out.println("orElseThrow NoBikeException");
        }
    }
}

class Bike {
    private Optional<Wheels> wheels;
    private String brand;

    public Bike(Optional<Wheels> wheels, String brand) {
        this.wheels = wheels;
        this.brand = brand;
    }

    public Optional<Wheels> getWheels() {
        return wheels;
    }

    public void setWheels(Optional<Wheels> wheels) {
        this.wheels = wheels;
    }

    public String getBrand() {
        return brand;
    }

    public void setBrand(String brand) {
        this.brand = brand;
    }

}

class Wheels {
    private String brand;
    private int spokes;

    public Wheels(String brand, int spokes) {
        this.brand = brand;
        this.spokes = spokes;
    }

    public String getBrand() {
        return brand;
    }

    public void setBrand(String brand) {
        this.brand = brand;
    }

    public int getSpokes() {
        return spokes;
    }

    public void setSpokes(int spokes) {
        this.spokes = spokes;
    }
}

class NoBikeException extends Exception {
    private static final long serialVersionUID = 1L;