Demo: making findById return Optional

Written Explanation

Step 1: the starting point

The repository from the generics module has a findById that returns whatever Map.get returns:

public T findById(ID id) {
    return store.get(id);          // null when the id is unknown
}

Files used throughout (in one folder): Contact.java with public record Contact(String name, String email) { }, InMemoryRepository.java with save, findById, findAll, delete as before, and a Main.java that uses them:

public class Main {
    public static void main(String[] args) {
        InMemoryRepository<Contact, Integer> repo = new InMemoryRepository<>();
        repo.save(1, new Contact("Ada", "ada@example.com"));

        Contact c = repo.findById(1);
        System.out.println(c.name());

        Contact missing = repo.findById(9);
        System.out.println(missing.name());
    }
}

Compile with javac -g *.java and run java Main:

Ada
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "Contact.name()" because "missing" is null
	at Main.main(Main.java:10)

That is the problem from the first lesson. Nothing in the code warned us.

Step 2: change the return type

Two edits in InMemoryRepository.java: add import java.util.Optional;, and change the method:

public Optional<T> findById(ID id) {
    return Optional.ofNullable(store.get(id));
}

We use ofNullable, not of, because Map.get returns null for a missing key, and Optional.of(null) would throw inside the repository itself. The repository's other methods stay unchanged: findAll already returns a list, which is empty when nothing is stored (the rule from the previous lesson), and delete returns a boolean.

Step 3: let the compiler find the callers

Compile again without touching Main:

Main.java:6: error: incompatible types: Optional<Contact> cannot be converted to Contact
        Contact c = repo.findById(1);
                                 ^

The compiler lists every caller that still treats the result as a plain Contact. (In our run it stopped at the first file with problems; fix the errors it shows and compile again until it is quiet.) This is the main benefit of the change: you cannot forget a caller, because the old style no longer compiles. In a large code base, the list of errors is your to-do list.

Repeat this loop: compile, read the error, fix the caller, compile again.

Step 4: choose a policy for each caller

For each broken line, ask what a missing id means there. Do not mechanically add .get(): that would hide the question.

Here is a Main2 that uses all three:

public class Main2 {
    public static void main(String[] args) {
        InMemoryRepository<Contact, Integer> repo = new InMemoryRepository<>();
        repo.save(1, new Contact("Ada", "ada@example.com"));

        String n1 = repo.findById(1).map(Contact::name).orElse("unknown");
        String n2 = repo.findById(9).map(Contact::name).orElse("unknown");
        System.out.println(n1 + " / " + n2);

        Contact c = repo.findById(1).orElseThrow(
            () -> new java.util.NoSuchElementException("No contact 1"));
        System.out.println(c.email());

        repo.findById(9).ifPresentOrElse(
            x -> System.out.println("found " + x.name()),
            () -> System.out.println("contact 9 not found"));
    }
}

Output:

Ada / unknown
ada@example.com
contact 9 not found

Step 5: the planned mistake, an unchecked get()

A caller in a hurry fixes the compile error with .get():

System.out.println(repo.findById(1).get().name());
System.out.println(repo.findById(9).get().name());

It compiles, and for id 1 it even works. For the missing id:

Ada
Exception in thread "main" java.util.NoSuchElementException: No value present
	at java.base/java.util.Optional.get(Optional.java:143)
	at Main3.main(Main3.java:7)

The compiler is satisfied, the crash has a new name, and the message No value present does not say what was missing. This is the lab's acceptance rule in action: no .get() without a check.

Step 6: fix it with a helpful message

int id = 9;
System.out.println(repo.findById(id).orElseThrow(
    () -> new NoSuchElementException("No contact " + id)).name());

Ada
Exception in thread "main" java.util.NoSuchElementException: No contact 9
	at Main4.lambda$main$1(Main4.java:12)
	at java.base/java.util.Optional.orElseThrow(Optional.java:403)
	at Main4.main(Main4.java:11)

Read the message: it names the entity and the id. In a log file at 3 a.m. that is the difference between a five-second diagnosis and a long search. (The id variable is effectively final, meaning it is assigned once, which a lambda requires.)

Step 7: an audit

To finish, search your project for .get() on Optional values. In an IDE this is a text search for .get(), and menus differ by product. A plain search tool also works (for example grep -n "\.get()" in many terminals). Look at every hit: calls on Optional must be replaced or sit right behind a check. Calls to Supplier.get() or List.get(i) are different methods and are fine.