Optional orElseGet() Method in Java

The orElseGet() method of the Optional class returns the value stored in the Optional when it is present. When the Optional is empty, it obtains a replacement value from the specified Supplier.

• The Supplier generates the alternative value only when the Optional is empty.
• orElseGet() is useful when the default value requires computation or method execution.
• A NullPointerException occurs when the Optional is empty and the supplied Supplier is null.

Syntax

public T orElseGet(Supplier<? extends T> supplier)

• Parameters: supplier generates the value returned when the Optional is empty.
• Return Value: The method returns the value stored in the Optional. If no value is present, it returns the value generated by the Supplier.
• Exception: NullPointerException is thrown when the Optional is empty and the specified Supplier is null.

Examples of orElseGet() Method

The following examples demonstrate how orElseGet() behaves when the Optional contains a value and when it is empty.

1. Using orElseGet() With a Present Value

The following program uses orElseGet() with an Optional containing the value 9455. A Supplier is also provided to generate a random value when required.

import java.util.*;
import java.util.function.*;

public class Geeks {
    public static void main(String[] args)
    {

        Optional<Integer> op = Optional.of(9455);

        System.out.println("Optional: " + op);

        System.out.println("Value by orElseGet(Supplier) method: "+ op.orElseGet(() -> (int)(Math.random() * 10)));
    }
}

Optional: Optional[9455]
Value by orElseGet(Supplier) method: 9455

Explanation: The Optional contains 9455, so orElseGet() returns the existing value. The Supplier is not executed because a value is already present.

2. Using orElseGet() With an Empty Optional

The following program uses an empty Optional. A Supplier is provided to generate a random value if the Optional is empty.

import java.util.*;
import java.util.function.*;

public class Geeks {
    public static void main(String[] args) {

        Optional<Integer> op = Optional.empty();

        System.out.println("Optional: " + op);

        System.out.println("Value by orElseGet(Supplier) method: "
                + op.orElseGet(
                    () -> (int) (Math.random() * 10)
                ));
    }
}

Optional: Optional.empty
Value by orElseGet(Supplier) method: 2

Explanation: The Optional is empty, so orElseGet() executes the Supplier. Math.random() generates a value, which is returned as the default result.

Note: The random output may differ each time the program runs.