Optional

 

Optional is a container object that may or may not contain a value. You can check if a value is present by calling the isPresent() method. If isPresent() returns true you can use the get() method to return the value.

Null pointer exception is a real problem in programming and that we must handle it with care.

How to create an Optional

String name = "Folau";
// name can't be null or else you will get a NullPointerException
Optional<String> opt = Optional.of(name);

If we expect null values, we can use the ofNullable method which does not throw a NullPointerException

String name = "Folau";
Optional<String> opt = Optional.ofNullable(name);

String name = null;
// this won't throw a NullPointerException
Optional<String> opt = Optional.ofNullable(name);

java8-optional-f2

Check if Optional is empty or not

The Optional.isPresent() method is used to check if an Optional has a value or not. It returns true if the Optional has value otherwise it returns false.

String name = "Folau";
Optional<String> opt = Optional.ofNullable(name);

if (opt.isPresent()) {
	System.out.println("Value available.");
} else {
	System.out.println("Value not available.");
}

The Optional.ifPresent() method is used to execute a lambda function (on the value) if Optional is not empty. if Optional is empty, do nothing.

opt.ifPresent(n -> {
	System.out.println(n);
});

Optional.orElse(T other) returns the value if present, otherwise return other of same data type
Optional.orElseGet(Supplier T) returns the value if present, otherwise invoke T and return the result of that call.

//when using orElse(), whether the wrapped value is present or not, the default object is created. 
//So in this case, we have just created one redundant object that is never used.
String n = opt.orElse(getRealName());
		
System.out.println(n);
		
String na = opt.orElseGet(() -> getRealName());
		
System.out.println(na);

....

private String getRealName() {
	System.out.println("getRealName()");
	return "Lisa";
}

// Run program result
getRealName()
Folau
Folau

Optional.filter(Predicate T) – If a value is present, and the value matches the given predicate, return an Optional describing the value, otherwise return an empty Optional.

Optional<Integer> ageOpt = Optional.ofNullable(20);

boolean underAgeForBeer = ageOpt.filter(age -> {
	return (age >= 21);
}).isPresent();

System.out.println("underAgeForBeer = " + underAgeForBeer);

Optional.map(Function mapper) – If a value is present, apply the provided mapping function to it, and if the result is non-null, return an Optional describing the result. Otherwise return an empty Optional.

Optional<String> optFileName = Optional.ofNullable("file.pdf");
		
Optional<File> optFile = optFileName.map(fileName -> new File(fileName));

 

 




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