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Java 8 - orElse vs orElseGet

Introduction

Java 8 has many new features, one of them is the Optional monad. Optional has two methods ‘orElse’ and ‘orElseGet’ they are very similar and can create confusion for the developers. In the article, we will try to understand the difference between the two.

Syntax

orElse

/**
 * If a value is present, returns the value, otherwise returns
 * {@code other}.
 *
 * @param other the value to be returned, if no value is present.
 *        May be {@code null}.
 * @return the value, if present, otherwise {@code other}
 */
public T orElse(T other) {
    return value != null ? value : other;
}

orElseGet

/**
 * If a value is present, returns the value, otherwise returns the result
 * produced by the supplying function.
 *
 * @param supplier the supplying function that produces a value to be returned
 * @return the value, if present, otherwise the result produced by the
 *         supplying function
 * @throws NullPointerException if no value is present and the supplying
 *         function is {@code null}
 */
public T orElseGet(Supplier<? extends T> supplier) {
    return value != null ? value : supplier.get();
}

Exploring the Difference

orElseGet: The 'orElseGet' will execute the Supplier function only if the value is not present in the Optional.

private static String none() {
    System.out.println("Inside none");
    return Constants.NONE; //”None”
}

public static void main(String[] args) {
    Optional<String> name = Optional.ofNullable("TEST");
    String value = name.orElseGet(() -> none());
    System.out.println(value);
}

The code above returns “TEST” and the ‘none()’ function will not be executed. Now instead of “TEST” pass null,

public static void main(String[] args) {
    Optional<String> name = Optional.ofNullable(null);
    String value = name.orElseGet(() -> none());
    System.out.println(value);
}

Since the value is missing in this case, the ‘none()’ function will be executed and a String ‘NONE’ returned, the output is

Inside None

NONE

The 'orElseGet' will execute the ‘none’ function only when the value is not present.

orElse: The ‘orElse’ will execute the ‘none’ function, irrespective of the value is present or not,

public static void main(String[] args) {
    Optional<String> name = Optional.ofNullable("TEST");
    String value = name.orElse(none());
    System.out.println(value);
}

The orElse, executes the ‘none’ function, although the value is present, the program.

Output in this case is,

Inside none

TEST

with null,

public static void main(String[] args) {
    Optional<String> name = Optional.ofNullable(null);
    String value = name.orElse(none());
    System.out.println(value);
}

Output

Inside none

NONE

If the Optional doesn’t contain a value, both orElse and orElseGet are the same, but if the value is present then orElse will execute the ‘none()’ function in both cases if we are working with List or any other collection with Optional, the execution of ‘orElse’ will be an expensive one if we are trying to create objects inside it, then it will create as many objects as the size of the Collection.

Thank You.


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