Understanding Optional

Optional #

• Since Java 8, the Optional<T> class has been used to prevent NullPointerException (NPE).
• It's a wrapper class that packages objects of type T, similar to Integer or Double classes.
• It's a Wrapper class that encapsulates values that might be null, helping to prevent NPEs even when referenced. In other words, it allows unexpected NPE exceptions to be easily avoided using provided methods, enabling null-related exceptions to be handled without complex conditional statements.

public final class Optional<T> {
 
  // If non-null, the value; if null, indicates no value is present
  private final T value;
   
  ...
}

Optional Usage #

Creating Optional Objects #

• Objects can be created using the of() method or the ofNullable() method.
• If a value might be null, it should be created via ofNullable() to prevent an NPE.
• In this case, if the specified value is null, an empty Optional object is returned.

And you can safely retrieve values even if they are null by using the orElse or orElseGet methods.

// The value of Optional can be present or null.
Optional<String> optional = Optional.ofNullable(getName());
String name = optional.orElse("anonymous"); // If no value, returns "anonymous"

Accessing Optional Objects #

• The get() method allows access to the Optional object.
• If the value stored in the Optional object is null, a NoSuchElementException is thrown.
• Therefore, it's good practice to check if a value is present in the object using the isPresent() method before calling get().

Optional<String> opt = Optional.ofNullable("자바 Optional 객체");

if(opt.isPresent()) {

    System.out.println(opt.get());

}

orElse~~() #

The following methods allow you to specify an alternative value instead of null.

1. orElse(): Returns the specified value if it exists; otherwise, returns the value passed as an argument.
2. orElseGet(): Returns the specified value if it exists; otherwise, returns the result of the lambda expression passed as an argument.
3. orElseThrow(): Returns the specified value if it exists; otherwise, throws the exception passed as an argument.

Difference between orElse and orElseGet orElse is called whether the value is null or not. It takes a value as a parameter. orElseGet: Is called only when the value is null. It takes a Supplier as a parameter.

In JpaRepository, the findById() method returns an Optional.