Java Tutorials - Herong's Tutorial Examples - Version 7.03, by Dr. Herong Yang

Java Tutorials - Herong's Tutorial Examples

∟Lambda Expressions and Method References

∟Lambda Expression Stream Pipeline Operations

The best place to use lambda expressions is probably in stream pipeline operations. See the example program, LambdaOnStream.java, below:

/* LambdaStream.java
 - Copyright (c) 2014, HerongYang.com, All Rights Reserved.
 */
import java.util.*;;
import java.text.*;;
import java.util.stream.Stream;
class LambdaStream {
   public static void main(String[] arg) {
      Calendar today = new GregorianCalendar();
      Stream
         .iterate(today, d->{
      	    Calendar n = (Calendar)d.clone(); 
      	    n.add(Calendar.DATE,1); 
      	    return n;
          })
         .filter(d->d.get(Calendar.DATE)==13
            && d.get(Calendar.DAY_OF_WEEK)==Calendar.FRIDAY
          )
         .limit(10)
         .map(d->d.getTime())
         .forEach(d->System.out.println( 
            new SimpleDateFormat("dd-MMM-yyyy").format(d) 
          ));
   }   
}

LambdaOnStream.java uses several pipeline operations from the "Stream" interface introduced in JDK 1.8 to manipulate a stream of calendar objects:

• The pipeline consists of 5 operations: iterate(), filter(), limit(), map() and forEach().
• At the beginning of the pipeline, iterate() is used to create a new Stream object with a Calendar object "today" as the first element and a lambda expression to generate the next element with the previous element.
• The lambda expression used in iterate(), "d->{ Calendar n = (Calendar)d.clone(); n.add(Calendar.DATE,1); return n; }", is returning the next day Calendar object cloned from the given Calendar object.
• The next operation in the pipeline is filter(), which removes elements that does not meet the given criteria out of the stream.
• The lambda expression used in filter(), "d->d.get(Calendar.DATE)==13 && d.get(Calendar.DAY_OF_WEEK)==Calendar.FRIDAY", is returning true if the given Calendar object is Friday the 13th.
• The next operation in the pipeline is limit(), which simply ends the stream after a given number of elements.
• The next operation in the pipeline is map(), which converts all element from one object type to another object type.
• The lambda expression used in map(), "d->d.getTime()", is returning a Date object from the given Calendar object.
• At the end of the pipeline, forEach() is used to apply final actions on each element.
• The lambda expression used in forEach(), "d->System.out.println( new SimpleDateFormat("dd-MMM-yyyy").format(d) )", is printing the date in console for each given Date object.

Overall, the program is trying to find the next 10 Friday the 13th dates. Here they are generated by the above program with JDK 1.8:

13-Jun-2014
13-Feb-2015
13-Mar-2015
13-Nov-2015
13-May-2016
13-Jan-2017
13-Oct-2017
13-Apr-2018
13-Jul-2018
13-Sep-2019

Year 2015 is pretty bad. It has 3 Friday the 13th dates!

Last update: 2014.

Table of Contents

 About This Book

 Installing JDK 1.8 on Windows

 Execution Process, Entry Point, Input and Output

 Primitive Data Types and Literals

 Bits, Bytes, Bitwise and Shift Operations

 Managing Bit Strings in Byte Arrays

 Reference Data Types and Variables

 StringBuffer - The String Buffer Class

 System Properties and Runtime Object Methods

 Generic Classes and Parameterized Types

 Generic Methods and Type Inference

►Lambda Expressions and Method References

 What Is Lambda Expression?

 LambdaCalculator.java - Lambda Expression Example

 Lambda Expression Syntax Options

 Lambda Expression as Method Reference

 Method Reference Example - LambdaMethodRefernce.java

►Lambda Expression Stream Pipeline Operations

 Execution Threads and Multi-Threading Java Programs

 ThreadGroup Class and "system" ThreadGroup Tree

 Synchronization Technique and Synchronized Code Blocks

 Deadlock Condition Example Programs

 Garbage Collection and the gc() Method

 Outdated Tutorials

 References

 PDF Printing Version

Lambda Expression Stream Pipeline Operations - Updated in 2018, by Dr. Herong Yang