Optional orElseGet() method in Java with examples

The orElseGet() method of java.util.Optional class in Java is used to get the value of this Optional instance if present. If there is no value present in this Optional instance, then this method returns the value generated from the specified supplier.

Syntax:

public T orElseGet(Supplier<T> supplier)

Parameters: This method accepts supplier as a parameter of type T to generate a value to return if there is no value present in this Optional instance.

Return supplier: This method returns the value of this Optional instance, if present. If there is no value present in this Optional instance, then this method returns the value generated from the specified supplier.

Exception: This method throws NullPointerException if there is no value present in this Optional instance.

Below programs illustrate orElseGet() method:
Program 1:

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// Java program to demonstrate
// Optional.orElseGet() method
  
import java.util.*;
import java.util.function.*;
  
public class GFG {
  
    public static void main(String[] args)
    {
  
        // create a Optional
        Optional<Integer> op
            = Optional.of(9455);
  
        // print supplier
        System.out.println("Optional: "
                           + op);
  
        // orElseGet supplier
        System.out.println("Value by orElseGet"
                           + "(Supplier) method: "
                           + op.orElseGet(
                                 () -> (int)(Math.random() * 10)));
    }
}

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Output:

Optional: Optional[9455]
Value by orElseGet(() -> (int)(Math.random() * 10)) method: 9455

Program 2:

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// Java program to demonstrate
// Optional.orElseGet() method
  
import java.util.*;
import java.util.function.*;
  
public class GFG {
  
    public static void main(String[] args)
    {
  
        // create a Optional
        Optional<Integer> op
            = Optional.empty();
  
        // print supplier
        System.out.println("Optional: "
                           + op);
  
        try {
  
            // orElseGet supplier
            System.out.println("Value by orElseGet"
                               + "(Supplier) method: "
                               + op.orElseGet(
                                     () -> (int)(Math.random() * 10)));
        }
        catch (Exception e) {
            System.out.println(e);
        }
    }
}

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Output:

Optional: Optional.empty
Value by orElseGet(() -> (int)(Math.random() * 10)) method: 4

Reference: https://docs.oracle.com/javase/9/docs/api/java/util/Optional.html#orElseGet-java.util.function.Supplier-



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