Append or Prepend Items to a Stream

Learn to add items to a Java Stream. Remember that a Stream is not a data structure or collection that can store values. To add items to an existing Stream, we need to :

• Create a new Stream with items to be added
• Concatenate with the first stream to get a merged stream.

1. Concatenating Streams

The Stream.concat(stream1, stream2) is used to merge two streams into one stream which consists of all the elements of both streams.

• The concat(s1, s2) method creates a lazily concatenated stream whose elements are all the elements of the s1 followed by all the elements of the s2.
• The resulting stream is ordered if both of the input streams are ordered.
• The resulting stream is parallel if either of the input streams is parallel.

2. Examples of Adding Items

2.1. Appending Items

To append items at the start of a Stream, create a new stream of items and pass the new Stream as the first method argument in the concat() method.

Stream<Integer> stream = Stream.of(1, 2, 3, 4);
         
//Append 5 and 6 to the stream
Stream<Integer> appenededStream = Stream.concat(stream, Stream.of(5, 6));
 
//Verify Stream
appenededStream.forEach(System.out::print); //Prints 123456

2.2. Prepending Items

To prepend the items at the end of a Stream, create a new stream of the items and pass the new Stream as the second method argument in the concat() method.

Stream<Integer> stream = Stream.of(1, 2, 3, 4);
         
//Prepend 0 to the stream
Stream<Integer> prependedStream = Stream.concat(Stream.of(0), stream);
 
//Verify Stream
prependedStream.forEach(System.out::print); //Prints 01234

3. Conclusion

The Stream API provides lots of useful methods which can be used to solve many problems. In the above case, adding new objects to the Java stream has been demonstrated using the concat() API whose original purpose is to merge two streams.

Happy Learning !!

Sourcecode on Github