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Issue 32

Java 8 Optional is not just for replacing a null value

Peter Lawrey
CEO @ Higher Frequency Trading Ltd
PROGRAMMING

In Java 8, you can return an Optional instead of return null; as you might do in Java 7.  This may or may not make a big difference depending on whether you tend to forget to check for null or whether you use static code analysis to check to nullalbe references.

However, there is a more compelling case which is to treat Optional like a Stream with 0 or 1 values.

Simple Optional Use Case

In the old days of Java 7 you would write something like

String text = something();

if (text != null) {

Note: Oracle Java 7 will be "End of Public Updates" in April 2015.

With Optional you can instead write

Optional  text = something();
if (text.isPresent()) {
    String text2 = text.get();

However, if you are being paranoid you might write.

Optional  text = something();
if (text != null && text.isPresent()) {
    String text2 = text.get();

If you have NullPointerException errors often in your project Optional might help, but otherwise it's not looking like it helps much. 

A more complex example

Let′s instead consider this example

static String getFirstSecondThird(Nested nested) {
    try {
          return ((Contained2) nested.first.second).get(0).third;
    } catch (NullPointerException \| 
             ClassCastException \| 
             IndexOutOfBoundsException ignored) {
          return null;
    }
}

This is really ugly.  Instead of catching exceptions, you can build a long list of condition check but it becomes really hard to see what you are trying to do.

Optional allows you to handle all the possible error conditions without an Exception or nested if/else logic.

static Optional  getFirstSecondThird(Optional  nested) {
    return nested // could be non-present
            .map(x -\> x.first) // could be null
            .map(x -\> x.second) // could be null
            // could be another type
            .map(x -\> x instanceof Contained2 ? (Contained2) x : null)
            .map(x -\> x.list) // could be null
            .filter(x -\> !x.isEmpty()) // could be empty
            .map(x -\> x.get(0)) // could be null
            .map(x -\> x.third); // could be null.
}

What we get is a series of mappings and filters which only progress if the value is non-null and present. If any value is null, or a filter is not true, the whole result is "not present".

Conclusion

Using Optional can be a powerful way to navigate a complex data structure in a safe way.  The purpose of lambdas is to reduce boiler plate code, and in the case it avoids all the checks or errors you have.

Additional

For your interest, here are the classes I used in the example above.

static class Nested {
    Contained first;
}

static class Contained {
    IContained2 second;
}

interface IContained2 {
}

static class Contained2 implements IContained2 {
    List  list;
}

static class Data {
    String third;
}

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