Find the first element of a Stream in Java

Input: Stream = {"Geek_First", "Geek_2", "Geek_3", "Geek_4", "Geek_Last"} Output: Geek_First Input: Stream = {1, 2, 3, 4, 5, 6, 7} Output: 1

1. Using Stream.reduce() Method: The reduce method works on two elements in the stream and returns the element as per the required condition. Therefore this method can be used to reduce the stream so that it contains only the first element. Approach:
   • Get the stream of elements in which the first element is to be returned.
   • To get the first element, you can use the reduce() method to ignore the second element, repeatedly, till there is no second element.

     Stream.reduce((first, second) -> first)
     

   • This reduces the set of elements in a Stream to a single element, which is first.
   • Hence the only single element will be remain in the stream which is the first element.
   Below is the implementation of the above approach: Example: Java

   // Java program to find first
   // element of a Stream in Java
   
   import java.util.*;
   import java.util.stream.*;
   
   public class GFG {
   
       // Function to find the
       // first_elements in a Stream
       public static <T> T
       firstElementInStream(Stream<T> stream)
       {
           T first_element
               = stream
   
                     // reduce() method reduces the Stream
                     // to a single element, which is first.
                     .reduce((first, second) -> first)
   
                     // if stream is empty
                     // null is returned
                     .orElse(null);
   
           return first_element;
       }
   
       // Driver code
       public static void main(String[] args)
       {
   
           Stream<String> stream
               = Stream.of("Geek_First", "Geek_2",
                           "Geek_3", "Geek_4",
                           "Geek_Last");
   
           // Print the first element of a Stream
           System.out.println(
               "First Element: "
               + firstElementInStream(stream));
       }
   }
   

   Output:

   First Element: Geek_First
   

2. Using Stream findFirst() Method: The findFirst() method will returns the first element of the stream or an empty if the stream is empty. Approach:
   • Get the stream of elements in which the first element is to be returned.
   • To get the first element, you can directly use the findFirst() method.

     Stream.findFirst()

   • This will return the first element of the stream.
   Below is the implementation of the above approach: Example: Java

   // Java program to find first
   // element of a Stream in Java
   
   import java.util.*;
   import java.util.stream.*;
   
   public class GFG {
   
       // Function to find the
       // first_elements in a Stream
       public static <T> T
       firstElementInStream(Stream<T> stream)
       {
           T first_element
               = stream
   
                     // findFirst() method returns
                     // the first element of stream
                     .findFirst()
   
                     // if stream is empty
                     // null is returned
                     .orElse(null);
   
           return first_element;
       }
   
       // Driver code
       public static void main(String[] args)
       {
   
           Stream<String> stream
               = Stream.of("Geek_First", "Geek_2",
                           "Geek_3", "Geek_4",
                           "Geek_Last");
   
           // Print the first element of a Stream
           System.out.println(
               "First Element: "
               + firstElementInStream(stream));
       }
   }
   

   Output:

   First Element: Geek_First