We can limit the opportunities for NullPointerException’s by following good practice like –
- Don’t return null’s from methods
- Don’t pass null’s as arguments
Another tool is to use java.lang.Optional. Java8 optional formalises the approach used in other languages, and 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
Im following a similar style to my last post Java8 – 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;
}
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