Java 8 Concepts

Note

Java 8 was released in 18th March 2014, so it’s high time to look into Java 8 Features. here we will look into Java 8 features with examples

Some of the important Java 8 features are;

1. forEach() method in Iterable interface -- go
2. default and static methods in Interfaces
3. Functional Interfaces and Lambda Expressions
4. Java Stream API for Bulk Data Operations on Collections
5. Java Time API
6. Collection API improvements
7. Concurrency API improvements
8. Java IO improvements
9. Miscellaneous Core API improvements

1. forEach() method in Iterable interface

Whenever we need to traverse through a Collection, we need to create an Iterator whose whole purpose is to iterate over and then we have business logic in a loop for each of the elements in the Collection. We might get ConcurrentModificationException if iterator is not used properly.

Java 8 has introduced forEach method in java.lang.Iterable interface so that while writing code we focus on business logic only. forEach method takes java.util.function.Consumer object as argument, so it helps in having our business logic at a separate location that we can reuse. Let’s see forEach usage with simple example

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.function.Consumer;
import java.lang.Integer;

public class Java8ForEachExample {

    public static void main(String[] args) {

        //creating sample Collection
        List<Integer> myList = new ArrayList<Integer>();
        for(int i=0; i<10; i++) myList.add(i);

        //traversing using Iterator
        Iterator<Integer> it = myList.iterator();
        while(it.hasNext()){
            Integer i = it.next();
            System.out.println("Iterator Value::"+i);
        }

        //traversing through forEach method of Iterable with anonymous class
        myList.forEach(new Consumer<Integer>() {

            public void accept(Integer t) {
                System.out.println("forEach anonymous class Value::"+t);
            }

        });

        //traversing with Consumer interface implementation
        MyConsumer action = new MyConsumer();
        myList.forEach(action);

    }

}

//Consumer implementation that can be reused
class MyConsumer implements Consumer<Integer>{

    public void accept(Integer t) {
        System.out.println("Consumer impl Value::"+t);
    }


}

java.util.ConcurrentModificationException

java.util.ConcurrentModificationException is a very common exception when working with java collection classes. Java Collection classes are fail-fast, which means if the Collection will be changed while some thread is traversing over it using iterator, the iterator.next() will throw ConcurrentModificationException.

Let’s see the concurrent modification exception scenario with an example.

import java.util.*;

public class IteratorExample {

    public static void main(String args[]){
        List<String> myList = new ArrayList<String>();

        myList.add("1");
        myList.add("2");
        myList.add("3");
        myList.add("4");
        myList.add("5");

        Iterator<String> it = myList.iterator();
        while(it.hasNext()){
            String value = it.next();
            System.out.println("List Value:"+value);
            if(value.equals("3")) myList.remove(value);
        }

        Map<String,String> myMap = new HashMap<String,String>();
        myMap.put("1", "1");
        myMap.put("2", "2");
        myMap.put("3", "3");

        Iterator<String> it1 = myMap.keySet().iterator();
        while(it1.hasNext()){
            String key = it1.next();
            System.out.println("Map Value:"+myMap.get(key));
            if(key.equals("2")){
                myMap.put("1","4");
                //myMap.put("4", "4");
            }
        }

    }
}

Above program will throw java.util.ConcurrentModificationException when execute List Value:1 List Value:2 List Value:3 Exception in thread "main" java.util.ConcurrentModificationException at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372) at java.util.AbstractList$Itr.next(AbstractList.java:343) at com.journaldev.java.IteratorExample.main(IteratorExample.java:27)

From the output stack trace, its clear that the concurrent modification exception is coming when we call iterator next() function. If you are wondering how Iterator checks for the modification, its implementation is present in AbstractList class where an int variable modCount is defined. modCount provides the number of times list size has been changed. modCount value is used in every next() call to check for any modifications in a function checkForComodification().

Let us run an example using Concurrent Collection classes:

import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CopyOnWriteArrayList;

public class ThreadSafeIteratorExample {

    public static void main(String[] args) {

        List<String> myList = new CopyOnWriteArrayList<String>();

        myList.add("1");
        myList.add("2");
        myList.add("3");
        myList.add("4");
        myList.add("5");

        Iterator<String> it = myList.iterator();
        while(it.hasNext()){
            String value = it.next();
            System.out.println("List Value:"+value);
            if(value.equals("3")){
                myList.remove("4");
                myList.add("6");
                myList.add("7");
            }
        }
        System.out.println("List Size:"+myList.size());

        Map<String,String> myMap = new ConcurrentHashMap<String,String>();
        myMap.put("1", "1");
        myMap.put("2", "2");
        myMap.put("3", "3");

        Iterator<String> it1 = myMap.keySet().iterator();
        while(it1.hasNext()){
            String key = it1.next();
            System.out.println("Map Value:"+myMap.get(key));
            if(key.equals("1")){
                myMap.remove("3");
                myMap.put("4", "4");
                myMap.put("5", "5");
            }
        }

        System.out.println("Map Size:"+myMap.size());
    }

}

**How to use forEach() method in Java 8 **

List<String> alphabets = new ArrayList<>(Arrays.asList("aa", "bbb", "cat", "dog"));
1. alphabets.forEach(s -> System.out.println(s));

2. alphabets.forEach(System.out::println); //replace lambda expression with method reference

3. alphabets.stream()
         .filter(s -> s.startsWith("a"))
         .forEach(System.out::println);   //using stream, filter and forEach() Example

4. alphabets.stream()
         .filter(s -> s.length() > 2)
         .forEach(System.out::println);
                  // it's much easier than using Iterator or any other ways to loop over List in Java.

2. default and static methods in Interfaces

Java 8 interface changes include static methods and default methods in interfaces. Prior to Java 8, we could have only method declarations in the interfaces. But from Java 8, we can have default methods and static methods in the interfaces.

Designing interfaces have always been a tough job because if we want to add additional methods in the interfaces, it will require change in all the implementing classes. As interface grows old, the number of classes implementing it might grow to an extent that it’s not possible to extend interfaces. That’s why when designing an application, most of the frameworks provide a base implementation class and then we extend it and override methods that are applicable for our application.

Java Interface Default Method:----

For creating a default method in java interface, we need to use “default” keyword with the method signature. For example,

public interface Interface1 {

    void method1(String str);

    default void log(String str){
        System.out.println("I1 logging::"+str);
    }
}

Notice that log(String str) is the default method in the Interface1. Now when a class will implement Interface1, it is not mandatory to provide implementation for default methods of interface. This feature will help us in extending interfaces with additional methods, all we need is to provide a default implementation.

public interface Interface2 {

    void method2();

    default void log(String str){
        System.out.println("I2 logging::"+str);
    }

}

We know that Java doesn’t allow us to extend multiple classes because it will result in the “Diamond Problem” where compiler can’t decide which superclass method to use. With the default methods, the diamond problem would arise for interfaces too. Because if a class is implementing both Interface1 and Interface2 and doesn’t implement the common default method, compiler can’t decide which one to chose.

A simple class that is implementing both Interface1 and Interface2 will be:

public class MyClass implements Interface1, Interface2 {

    @Override
    public void method2() {
    }

    @Override
    public void method1(String str) {
    }

    @Override
    public void log(String str){
        System.out.println("MyClass logging::"+str);
        Interface1.print("abc");
    }
}

Important points about java interface default methods:

1. Java interface default methods will help us in extending interfaces without having the fear of breaking implementation classes.
2. Java interface default methods has bridge down the differences between interfaces and abstract classes.
3. Java 8 interface default methods will help us in avoiding utility classes, such as all the Collections class method can be provided in the interfaces itself.
4. Java interface default methods will help us in removing base implementation classes, we can provide default implementation and the implementation classes can chose which one to override.
5. One of the major reason for introducing default methods in interfaces is to enhance the Collections API in Java 8 to support lambda expressions.

Java Interface Static Method:--

Java interface static method is similar to default method except that we can’t override them in the implementation classes. This feature helps us in avoiding undesired results incase of poor implementation in implementation classes. Let’s look into this with a simple example.

public interface MyData {

    default void print(String str) {
        if (!isNull(str))
            System.out.println("MyData Print::" + str);
    }

    static boolean isNull(String str) {
        System.out.println("Interface Null Check");

        return str == null ? true : "".equals(str) ? true : false;
    }
}

public class MyDataImpl implements MyData {

    public boolean isNull(String str) {
        System.out.println("Impl Null Check");

        return str == null ? true : false;
    }

    public static void main(String args[]){
        MyDataImpl obj = new MyDataImpl();
        obj.print("");
        obj.isNull("abc");
    }
}

Now when we will run the application, we get following output.

Interface Null Check
Impl Null Check

If we make the interface method from static to default, we will get following output.

Impl Null Check
MyData Print::
Impl Null Check

Java interface static method is visible to interface methods only, if we remove the isNull() method from the MyDataImpl class, we won’t be able to use it for the MyDataImpl object. However like other static methods, we can use interface static methods using class name. For example, a valid statement will be:

Important points about java interface static method:

1. Java interface static method is part of interface, we can’t use it for implementation class objects.
2. Java interface static methods are good for providing utility methods, for example null check, collection sorting etc.
3. Java interface static method helps us in providing security by not allowing implementation classes to override them. 4.We can use java interface static methods to remove utility classes such as Collections and move all of it’s static methods to the corresponding interface, that would be easy to find and use.

3. Functional Interfaces and Lambda Expressions

Functional interfaces are new concept introduced in Java 8. An interface with exactly one abstract method becomes Functional Interface. We don’t need to use @FunctionalInterface annotation to mark an interface as Functional Interface. @FunctionalInterface annotation is a facility to avoid accidental addition of abstract methods in the functional interfaces. You can think of it like @Override annotation and it’s best practice to use it. java.lang.Runnable with single abstract method run() is a great example of functional interface.

One of the major benefits of functional interface is the possibility to use lambda expressions to instantiate them. We can instantiate an interface with anonymous class but the code looks bulky.

 Runnable r = new Runnable(){
            @Override
            public void run() {
                System.out.println("My Runnable");
            }};

Since functional interfaces have only one method, lambda expressions can easily provide the method implementation. We just need to provide method arguments and business logic. For example, we can write above implementation using lambda expression as:

Runnable r1 = () -> {
            System.out.println("My Runnable");
        };

we can know more about lambda expressions ----Java 8 Lambda Expressions
So lambda expressions are means to create anonymous classes of functional interfaces easily. A new package java.util.function has been added with bunch of functional interfaces to provide target types for lambda expressions and method references.

4. Java Stream API for Bulk Data Operations on Collections

A new java.util.stream has been added in Java 8 to perform filter/map/reduce like operations with the collection. Stream API will allow sequential as well as parallel execution. This is one of the best feature for Collections and usually with Big Data, we need to filter out them based on some conditions.

Collection interface has been extended with stream() and parallelStream() default methods to get the Stream for sequential and parallel execution. Let’s see their usage with simple example.

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Stream;

public class StreamExample {

    public static void main(String[] args) {

        List<Integer> myList = new ArrayList<>();
        for(int i=0; i<100; i++) myList.add(i);

        //sequential stream
        Stream<Integer> sequentialStream = myList.stream();

        //parallel stream
        Stream<Integer> parallelStream = myList.parallelStream();

        //using lambda with Stream API, filter example
        Stream<Integer> highNums = parallelStream.filter(p -> p > 90);
        //using lambda in forEach
        highNums.forEach(p -> System.out.println("High Nums parallel="+p));

        Stream<Integer> highNumsSeq = sequentialStream.filter(p -> p > 90);
        highNumsSeq.forEach(p -> System.out.println("High Nums sequential="+p));

    }

}

If you will run above example code, you will get output like this:

High Nums parallel=91
High Nums parallel=96
High Nums parallel=93 ... 

High Nums sequential=91
High Nums sequential=92
High Nums sequential=93 ...

let’s see why it was required. Suppose we want to iterate over a list of integers and find out sum of all the integers greater than 10.

Prior to Java 8, the approach to do it would be:

private static int sumIterator(List<Integer> list) {
    Iterator<Integer> it = list.iterator();
    int sum = 0;
    while (it.hasNext()) {
        int num = it.next();
        if (num > 10) {
            sum += num;
        }
    }
    return sum;
}

There are three major problems with the above approach:

1.We just want to know the sum of integers but we would also have to provide how the iteration will take place, this is also called external iteration because client program is handling the algorithm to iterate over the list. 2.The program is sequential in nature, there is no way we can do this in parallel easily. 3.There is a lot of code to do even a simple task.

To overcome all the above shortcomings, Java 8 Stream API was introduced. We can use Java Stream API to implement internal iteration, that is better because java framework is in control of the iteration.

Internal iteration provides several features such as sequential and parallel execution, filtering based on the given criteria, mapping etc.

Most of the Java 8 Stream API method arguments are functional interfaces, so lambda expressions work very well with them. Let’s see how can we write above logic in a single line statement using Java Streams.

private static int sumStream(List<Integer> list) {
    return list.stream().filter(i -> i > 10).mapToInt(i -> i).sum();
}

To know more about Streams goTo --- Java 8 Stream API features

5. Java Time API

It has always been hard to work with Date, Time and Time Zones in java. There was no standard approach or API in java for date and time in Java. One of the nice addition in Java 8 is the java.time package that will streamline the process of working with time in java.

Just by looking at Java Time API packages, it will be very easy to use. It has some sub-packages java.time.format that provides classes to print and parse dates and times and java.time.zone provides support for time-zones and their rules.

The new Time API prefers enums over integer constants for months and days of the week. One of the useful class is DateTimeFormatter for converting datetime objects to strings

Java 8 Date Time API is JSR-310 implementation. It is designed to overcome all the flaws in the legacy date time implementations. Some of the design principles of new Date Time API are:

1. Immutability: All the classes in the new Date Time API are immutable and good for multithreaded environments.
2. Separation of Concerns: The new API separates clearly between human readable date time and machine time (unix timestamp). It defines separate classes for Date, Time, DateTime, Timestamp, Timezone etc.

3. Clarity: The methods are clearly defined and perform the same action in all the classes. For example, to get the current instance we have now() method. There are format() and parse() methods defined in all these classes rather than having a separate class for them. All the classes use Factory Pattern and Strategy Pattern for better handling. Once you have used the methods in one of the class, working with other classes won’t be hard.

4. Utility operations: All the new Date Time API classes comes with methods to perform common tasks, such as plus, minus, format, parsing, getting separate part in date/time etc.

5. Extendable: The new Date Time API works on ISO-8601 calendar system but we can use it with other non ISO calendars as well.

Java 8 Date Time API Packages

Java 8 Date Time API consists of following packages.

1. java.time Package: This is the base package of new Java Date Time API. All the major base classes are part of this package, such as LocalDate, LocalTime, LocalDateTime, Instant, Period, Duration etc. All of these classes are immutable and thread safe. Most of the times, these classes will be sufficient for handling common requirements.
2. java.time.chrono Package: This package defines generic APIs for non ISO calendar systems. We can extend AbstractChronology class to create our own calendar system.
3. java.time.format Package: This package contains classes used for formatting and parsing date time objects. Most of the times, we would not be directly using them because principle classes in java.time package provide formatting and parsing methods.
4. java.time.temporal Package: This package contains temporal objects and we can use it for find out specific date or time related to date/time object. For example, we can use these to find out the first or last day of the month. You can identify these methods easily because they always have format “withXXX”.
5. java.time.zone Package: This package contains classes for supporting different time zones and their rules.

Java 8 Date Time API Examples

We have looked into most of the important parts of Java Date Time API. It’s time now to look into most important classes of Date Time API with examples.

1. LocalDate

LocalDate is an immutable class that represents Date with default format of yyyy-MM-dd. We can use now() method to get the current date. We can also provide input arguments for year, month and date to create LocalDate instance. This class provides overloaded method for now() where we can pass ZoneId for getting date in specific time zone. This class provides the same functionality as java.sql.Date. Let’s look at a simple example for it’s usage.

import java.time.LocalDate;
import java.time.Month;
import java.time.ZoneId;

public class LocalDateExample {

    public static void main(String[] args) {

        //Current Date
        LocalDate today = LocalDate.now();
        System.out.println("Current Date="+today);

        //Creating LocalDate by providing input arguments
        LocalDate firstDay_2014 = LocalDate.of(2014, Month.JANUARY, 1);
        System.out.println("Specific Date="+firstDay_2014);


        //Try creating date by providing invalid inputs
        //LocalDate feb29_2014 = LocalDate.of(2014, Month.FEBRUARY, 29);
        //Exception in thread "main" java.time.DateTimeException: 
        //Invalid date 'February 29' as '2014' is not a leap year

        //Current date in "Asia/Kolkata", you can get it from ZoneId javadoc
        LocalDate todayKolkata = LocalDate.now(ZoneId.of("Asia/Kolkata"));
        System.out.println("Current Date in IST="+todayKolkata);

        //java.time.zone.ZoneRulesException: Unknown time-zone ID: IST
        //LocalDate todayIST = LocalDate.now(ZoneId.of("IST"));

        //Getting date from the base date i.e 01/01/1970
        LocalDate dateFromBase = LocalDate.ofEpochDay(365);
        System.out.println("365th day from base date= "+dateFromBase);

        LocalDate hundredDay2014 = LocalDate.ofYearDay(2014, 100);
        System.out.println("100th day of 2014="+hundredDay2014);
    }

}
LocalDate methods explanation is provided in comments, when we run this program, we get following output.

Current Date=2014-04-28
Specific Date=2014-01-01
Current Date in IST=2014-04-29
365th day from base date= 1971-01-01
100th day of 2014=2014-04-10

1. LocalTime

LocalTime is an immutable class whose instance represents a time in the human readable format. It’s default format is hh:mm:ss.zzz. Just like LocalDate, this class provides time zone support and creating instance by passing hour, minute and second as input arguments. Let’s look at it’s usage with a simple program.

import java.time.LocalTime;
import java.time.ZoneId;

public class LocalTimeExample {

    public static void main(String[] args) {

        //Current Time
        LocalTime time = LocalTime.now();
        System.out.println("Current Time="+time);

        //Creating LocalTime by providing input arguments
        LocalTime specificTime = LocalTime.of(12,20,25,40);
        System.out.println("Specific Time of Day="+specificTime);


        //Try creating time by providing invalid inputs
        //LocalTime invalidTime = LocalTime.of(25,20);
        //Exception in thread "main" java.time.DateTimeException: 
        //Invalid value for HourOfDay (valid values 0 - 23): 25

        //Current date in "Asia/Kolkata", you can get it from ZoneId javadoc
        LocalTime timeKolkata = LocalTime.now(ZoneId.of("Asia/Kolkata"));
        System.out.println("Current Time in IST="+timeKolkata);

        //java.time.zone.ZoneRulesException: Unknown time-zone ID: IST
        //LocalTime todayIST = LocalTime.now(ZoneId.of("IST"));

        //Getting date from the base date i.e 01/01/1970
        LocalTime specificSecondTime = LocalTime.ofSecondOfDay(10000);
        System.out.println("10000th second time= "+specificSecondTime);

    }

}
When we run above program for LocalTime examples, we get following output.

Current Time=15:51:45.240
Specific Time of Day=12:20:25.000000040
Current Time in IST=04:21:45.276
10000th second time= 02:46:40

1. LocalDateTime

LocalDateTime is an immutable date-time object that represents a date-time, with default format as yyyy-MM-dd-HH-mm-ss.zzz. It provides a factory method that takes LocalDate and LocalTime input arguments to create LocalDateTime instance. Let’s look it’s usage with a simple example.

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.LocalTime;
import java.time.Month;
import java.time.ZoneId;
import java.time.ZoneOffset;

public class LocalDateTimeExample {

    public static void main(String[] args) {

        //Current Date
        LocalDateTime today = LocalDateTime.now();
        System.out.println("Current DateTime="+today);

        //Current Date using LocalDate and LocalTime
        today = LocalDateTime.of(LocalDate.now(), LocalTime.now());
        System.out.println("Current DateTime="+today);

        //Creating LocalDateTime by providing input arguments
        LocalDateTime specificDate = LocalDateTime.of(2014, Month.JANUARY, 1, 10, 10, 30);
        System.out.println("Specific Date="+specificDate);


        //Try creating date by providing invalid inputs
        //LocalDateTime feb29_2014 = LocalDateTime.of(2014, Month.FEBRUARY, 28, 25,1,1);
        //Exception in thread "main" java.time.DateTimeException: 
        //Invalid value for HourOfDay (valid values 0 - 23): 25


        //Current date in "Asia/Kolkata", you can get it from ZoneId javadoc
        LocalDateTime todayKolkata = LocalDateTime.now(ZoneId.of("Asia/Kolkata"));
        System.out.println("Current Date in IST="+todayKolkata);

        //java.time.zone.ZoneRulesException: Unknown time-zone ID: IST
        //LocalDateTime todayIST = LocalDateTime.now(ZoneId.of("IST"));

        //Getting date from the base date i.e 01/01/1970
        LocalDateTime dateFromBase = LocalDateTime.ofEpochSecond(10000, 0, ZoneOffset.UTC);
        System.out.println("10000th second time from 01/01/1970= "+dateFromBase);

    }

}
When we run above class, we get following output.

Current DateTime=2014-04-28T16:00:49.455
Current DateTime=2014-04-28T16:00:49.493
Specific Date=2014-01-01T10:10:30
Current Date in IST=2014-04-29T04:30:49.493
10000th second time from 01/01/1970= 1970-01-01T02:46:40

In all the three examples, we have seen that if we provide invalid arguments for creating Date/Time, then it throws java.time.DateTimeException that is a RuntimeException, so we don’t need to explicitly catch it.

We have also seen that we can get Date/Time data by passing ZoneId, you can get the list of supported ZoneId values from it’s javadoc.

To know other most important classes of Date Time API go through --- Java 8 Date Time API Features

6. Collection API improvements

We have already seen forEach() method and Stream API for collections. Some new methods added in Collection API are:

1. Iterator default method forEachRemaining(Consumer action) to perform the given action for each remaining element until all elements have been processed or the action throws an exception.
2. Collection default method removeIf(Predicate filter) to remove all of the elements of this collection that satisfy the given predicate.
3. Collection spliterator() method returning Spliterator instance that can be used to traverse elements sequentially or parallel.
4. Map replaceAll(), compute(), merge() methods.
5. Performance Improvement for HashMap class with Key Collisions

7. Concurrency API improvements

Some important concurrent API enhancements are:

1. ConcurrentHashMap compute(), forEach(), forEachEntry(), forEachKey(), forEachValue(), merge(), reduce() and search() methods.
2. CompletableFuture that may be explicitly completed (setting its value and status).
3. Executors newWorkStealingPool() method to create a work-stealing thread pool using all available processors as its target parallelism level.

8. Java IO improvements

Some IO improvements known to me are:

1. Files.list(Path dir) that returns a lazily populated Stream, the elements of which are the entries in the directory.
2. Files.lines(Path path) that reads all lines from a file as a Stream.
3. Files.find() that returns a Stream that is lazily populated with Path by searching for files in a file tree rooted at a given starting file.
4. BufferedReader.lines() that return a Stream, the elements of which are lines read from this BufferedReader.

9. Miscellaneous Core API improvements

Some misc API improvements that might come handy are:

1. ThreadLocal static method withInitial(Supplier supplier) to create instance easily.
2. Comparator interface has been extended with a lot of default and static methods for natural ordering, reverse order etc.
3. min(), max() and sum() methods in Integer, Long and Double wrapper classes.
4. logicalAnd(), logicalOr() and logicalXor() methods in Boolean class.
5. ZipFile.stream() method to get an ordered Stream over the ZIP file entries. Entries appear in the Stream in the order they appear in the central directory of the ZIP file.
6. Several utility methods in Math class.
7. jjs command is added to invoke Nashorn Engine.
8. jdeps command is added to analyze class files
9. JDBC-ODBC Bridge has been removed.
10. PermGen memory space has been removed