Start Using Java Lambda Expressions(转载)

摘要：原文

Lambda expressions are a new and important feature included in Java SE 8. A lambda expression provides a way to represent one method interface using an expression. A lambda expression is like a method, it provides a list of formal parameters and a body (which can be an expression or a block of code) expressed in terms of those parameters.
Lambda expressions also improve the Collection libraries. Java SE 8 added two packages related to bulk data operations for Collections, the java.util.function package, and the java.util.stream. A stream is like an iterator, but with a lot of extra functionality. Taken together, lambda expressions and streams are the biggest change to Java programming since the generics and annotations were added to the language. In this article, we will explore the power of lambda expressions and streams, from simple to complex examples.

If you haven’t installed Java 8, then this is the first thing you should do to be able to work with lambdas and streams. Tools and IDE’s like NetBeans and IntelliJ IDEA support Java 8 features, such as lambda expressions, repeatable annotations, compact profiles and others.
Below are the download links for Java SE 8 and NetBeans IDE 8:

Java Platform (JDK 8): Download Java 8 from Oracle, it can also be bundled with NetBeans IDE

NetBeans IDE 8: Download NetBeans IDE from the NetBeans official site

The basic syntax of a lambda is either:

(parameters) ->expression                or         (parameters) ->{ statements; }

The following are examples of Java lambda expressions:

1.  () -> 5    // takes no value and returns 5
2.  x -> 2 * x    // takes a number and returns the result of doubling it
3.  (x, y) -> x – y   // takes two numbers and returns their difference
4.  (int x, int y) -> x + y   // takes two integers and returns their sum
5.  (String s) -> System.out.print(s)   // takes a string and prints it to console without returning anything

Now that we have an idea of what lambdas are, let us start with some basic examples. In this section, we will see how lambda expressions affect the way we code. Having a list of players, the “for loop”, as programmers often refers to the for statement, can be translated in Java SE 8 as below:

String[] atp = {"Rafael Nadal", "Novak Djokovic", "Stanislas Wawrinka", "David Ferrer", "Roger Federer", "Andy Murray", "Tomas Berdych", "Juan Martin Del Potro"};
List players =  Arrays.asList(atp);
       
// Old looping
for (String player : players) {
     System.out.print(player + "; ");
}
       
// Using lambda expression and functional operations
players.forEach((player) -> System.out.print(player + "; "));
 
// Using double colon operator in Java 8
players.forEach(System.out::println);
As you can see, lambda expressions can reduced our code to one single line. Another example is in a graphical user interface application, when anonymous classes can be replaced with lambda expressions. The same thing happens when implementing the Runnable interface:
// Using anonymous innerclass
btn.setOnAction(new EventHandler() {
              @Override
                public void handle(ActionEvent event) {
                    System.out.println("Hello World!"); 
            }
        });
 
// Using lambda expression
btn.setOnAction(event -> System.out.println("Hello World!"));

Here is how we can write a Runnable using lambdas:

// Using anonymous innerclass 
new Thread(new Runnable() {
    @Override
    public void run() {
        System.out.println("Hello world !");
    }
}).start();
 
// Using lambda expression
new Thread(() -> System.out.println("Hello world !")).start();
 
// Using anonymous innerclass
Runnable race1 = new Runnable() {
    @Override
    public void run() {
        System.out.println("Hello world !");
    }
};
 
// Using lambda expression
Runnable race2 = () -> System.out.println("Hello world !");
 
// Run em!
race1.run();
race2.run();

The Runnable lambda expression, which uses the block format, converts five lines of code into one statement. Going further, in the next section we will use lambdas for sorting collections.

In Java, the Comparator class is used for sorting collections. In the following examples, we will sort a list of players based on name, surname, name length and last name letter. We will first sort them as we did before, using anonymous inner classes, and then reduce our code using lambda expressions.
In the first example, we will sort our list by name. Using the old way, this looks like this:

String[] players = {"Rafael Nadal", "Novak Djokovic", "Stanislas Wawrinka", "David Ferrer", "Roger Federer", "Andy Murray", "Tomas Berdych", "Juan Martin Del Potro", "Richard Gasquet", "John Isner"};
 
// Sort players by name using anonymous innerclass
Arrays.sort(players, new Comparator() {
    @Override
    public int compare(String s1, String s2) {
        return (s1.compareTo(s2));
    }
});

With lambdas, the same thing can be achieved like this:

// Sort players by name using lambda expression
Comparator sortByName = (String s1, String s2) -> (s1.compareTo(s2));
 
Arrays.sort(players, sortByName);
// or this
Arrays.sort(players, (String s1, String s2) -> (s1.compareTo(s2)));

The remaining sorts are listed below. The same as the above example, the code applies a Comparator by using an anonymous inner class and a couple of lambda expressions:

// Sort players by surname using anonymous innerclass
Arrays.sort(players, new Comparator() {
    @Override
    public int compare(String s1, String s2) {
        return (s1.substring(s1.indexOf(" ")).compareTo(s2.substring(s2.indexOf(" "))));
    }
});
 
// Sort players by surname using lambda expression
Comparator sortBySurname = (String s1, String s2) -> (s1.substring(s1.indexOf(" ")).compareTo(s2.substring(s2.indexOf(" "))));
 
Arrays.sort(players, sortBySurname);
// or this
Arrays.sort(players, (String s1, String s2) -> (s1.substring(s1.indexOf(" ")).compareTo(s2.substring(s2.indexOf(" ")))));
 
// Sort players by name lenght using anonymous innerclass
Arrays.sort(players, new Comparator() {
    @Override
    public int compare(String s1, String s2) {
        return (s1.length() - s2.length());
    }
});
 
// Sort players by name lenght using lambda expression
Comparator sortByNameLenght = (String s1, String s2) -> (s1.length() - s2.length());
 
Arrays.sort(players, sortByNameLenght);
// or this
Arrays.sort(players, (String s1, String s2) -> (s1.length() - s2.length()));
 
// Sort players by last letter using anonymous innerclass
Arrays.sort(players, new Comparator() {
    @Override
    public int compare(String s1, String s2) {
        return (s1.charAt(s1.length() - 1) - s2.charAt(s2.length() - 1));
    }
});
 
// Sort players by last letter using lambda expression
Comparator sortByLastLetter = (String s1, String s2) -> (s1.charAt(s1.length() - 1) - s2.charAt(s2.length() - 1));
Arrays.sort(players, sortByLastLetter);
// or this
Arrays.sort(players, (String s1, String s2) -> (s1.charAt(s1.length() - 1) - s2.charAt(s2.length() - 1)));

That’s all, pretty straightforward. We will explore more lambdas capabilities in the next section, where we will combine them with streams.

Streams are wrappers around collections that use lambdas pervasively. They support many operations that use lambdas, like map, filter, limit, sorted, count, min, max, sum, collect and others. Also, streams use lazy evaluation and they are not actually reading all the data and methods like getFirst() can end the stream. In the next examples, we will explore what lambdas and streams can do. We created a Person class and use this class to add some data to a list that will be used in further streams operations. The Person class is just a simple POJO:

public class Person {
 
private String firstName, lastName, job, gender;
private int salary, age;
 
public Person(String firstName, String lastName, String job, String gender, int age, int salary)       {
          this.firstName = firstName;
          this.lastName = lastName;
          this.gender = gender;
          this.age = age;
          this.job = job;
          this.salary = salary;
}
// Getter and Setter 
. . . . .   
}          

Going further, we will create two lists, both containing Person objects:

            List javaProgrammers = new ArrayList() {
            {
                add(new Person("Elsdon", "Jaycob", "Java programmer", "male", 43, 2000));
                add(new Person("Tamsen", "Brittany", "Java programmer", "female", 23, 1500));
                add(new Person("Floyd", "Donny", "Java programmer", "male", 33, 1800));
                add(new Person("Sindy", "Jonie", "Java programmer", "female", 32, 1600));
                add(new Person("Vere", "Hervey", "Java programmer", "male", 22, 1200));
                add(new Person("Maude", "Jaimie", "Java programmer", "female", 27, 1900));
                add(new Person("Shawn", "Randall", "Java programmer", "male", 30, 2300));
                add(new Person("Jayden", "Corrina", "Java programmer", "female", 35, 1700));
                add(new Person("Palmer", "Dene", "Java programmer", "male", 33, 2000));
                add(new Person("Addison", "Pam", "Java programmer", "female", 34, 1300));
            }
        };
 
        List phpProgrammers = new ArrayList() {
            {
                add(new Person("Jarrod", "Pace", "PHP programmer", "male", 34, 1550));
                add(new Person("Clarette", "Cicely", "PHP programmer", "female", 23, 1200));
                add(new Person("Victor", "Channing", "PHP programmer", "male", 32, 1600));
                add(new Person("Tori", "Sheryl", "PHP programmer", "female", 21, 1000));
                add(new Person("Osborne", "Shad", "PHP programmer", "male", 32, 1100));
                add(new Person("Rosalind", "Layla", "PHP programmer", "female", 25, 1300));
                add(new Person("Fraser", "Hewie", "PHP programmer", "male", 36, 1100));
                add(new Person("Quinn", "Tamara", "PHP programmer", "female", 21, 1000));
                add(new Person("Alvin", "Lance", "PHP programmer", "male", 38, 1600));
                add(new Person("Evonne", "Shari", "PHP programmer", "female", 40, 1800));
            }
        };

Let’s now use the forEach method to iterate the above lists:

System.out.println("Show programmers names:");
javaProgrammers.forEach((p) -> System.out.printf("%s %s; ", p.getFirstName(), p.getLastName()));
phpProgrammers.forEach((p) -> System.out.printf("%s %s; ", p.getFirstName(), p.getLastName()));
We will now use the same forEach method and try to increase programmer’s salary by 5%:
System.out.println("Increase salary by 5% to programmers:");
Consumer giveRaise = e -> e.setSalary(e.getSalary() / 100 * 5 + e.getSalary());
 
javaProgrammers.forEach(giveRaise);
phpProgrammers.forEach(giveRaise);
 
Another useful method is the filter method. To make use of that, let’s show PHP programmers that earn more than $1,400:
System.out.println("Show PHP programmers that earn more than $1,400:")
phpProgrammers.stream()
          .filter((p) -> (p.getSalary() > 1400))
          .forEach((p) -> System.out.printf("%s %s; ", p.getFirstName(), p.getLastName()));
We can also define filters and then reuse them in further operations:
// Define some filters
Predicate ageFilter = (p) -> (p.getAge() > 25);
Predicate salaryFilter = (p) -> (p.getSalary() > 1400);
Predicate genderFilter = (p) -> ("female".equals(p.getGender()));
 
System.out.println("Show female PHP programmers that earn more than $1,400 and are older than 24 years:");
phpProgrammers.stream()
          .filter(ageFilter)
          .filter(salaryFilter)
          .filter(genderFilter)
          .forEach((p) -> System.out.printf("%s %s; ", p.getFirstName(), p.getLastName()));
 
// Reuse filters
System.out.println("Show female Java programmers older than 24 years:");
javaProgrammers.stream()
          .filter(ageFilter)
          .filter(genderFilter)
          .forEach((p) -> System.out.printf("%s %s; ", p.getFirstName(), p.getLastName()));
            
Results can also be limited, if we use the limit method:
System.out.println("Show first 3 Java programmers:");
javaProgrammers.stream()
          .limit(3)
          .forEach((p) -> System.out.printf("%s %s; ", p.getFirstName(), p.getLastName()));
 
System.out.println("Show first 3 female Java programmers:");
javaProgrammers.stream()
          .filter(genderFilter)
          .limit(3)
          .forEach((p) -> System.out.printf("%s %s; ", p.getFirstName(), p.getLastName()));

What about sorting? Can we do that with streams? The answer is yes, we can. In the examples below, we will sort Java programmers by name and salary, collect them into a list and then show the list:

System.out.println("Sort and show the first 5 Java programmers by name:");
List sortedJavaProgrammers = javaProgrammers
          .stream()
          .sorted((p, p2) -> (p.getFirstName().compareTo(p2.getFirstName())))
          .limit(5)
          .collect(toList());
 
sortedJavaProgrammers.forEach((p) -> System.out.printf("%s %s; %n", p.getFirstName(), p.getLastName()));
 
System.out.println("Sort and show Java programmers by salary:");
sortedJavaProgrammers = javaProgrammers
          .stream()
          .sorted((p, p2) -> (p.getSalary() - p2.getSalary()))
          .collect(toList());
 
sortedJavaProgrammers.forEach((p) -> System.out.printf("%s %s; %n", p.getFirstName(), p.getLastName()));

If we are interested only in the lowest and the highest salary, faster than sorting and choosing the first (or the last) are the min and max methods:

System.out.println("Get the lowest Java programmer salary:");
Person pers = javaProgrammers
          .stream()
          .min((p1, p2) -> (p1.getSalary() - p2.getSalary()))
          .get()
 
System.out.printf("Name: %s %s; Salary: $%,d.", pers.getFirstName(), pers.getLastName(), pers.getSalary())
 
System.out.println("Get the highest Java programmer salary:");
Person person = javaProgrammers
          .stream()
          .max((p, p2) -> (p.getSalary() - p2.getSalary()))
          .get()
 
System.out.printf("Name: %s %s; Salary: $%,d.", person.getFirstName(), person.getLastName(), person.getSalary())

We have already seen above an example on how the collect method works. Combined with the map method, we can use collect method to gather our results in a String, in a Set or in a TreeSet:

System.out.println("Get PHP programmers first name to String:");
String phpDevelopers = phpProgrammers
          .stream()
          .map(Person::getFirstName)
          .collect(joining(" ; "));    // this can be used as a token in further operations
 
System.out.println("Get Java programmers first name to Set:");
Set javaDevFirstName = javaProgrammers
          .stream()
          .map(Person::getFirstName)
          .collect(toSet());
 
System.out.println("Get Java programmers last name to TreeSet:");
TreeSet javaDevLastName = javaProgrammers
          .stream()
          .map(Person::getLastName)
          .collect(toCollection(TreeSet::new));
 
Streams can also be parallel. An example is below:
System.out.println("Calculate total money spent for paying Java programmers:");
int totalSalary = javaProgrammers
          .parallelStream()
          .mapToInt(p -> p.getSalary())
          .sum();
To obtain various summary data about the elements of a stream we can use the summaryStatistics method. Going further, we then have access to methods like getMax, getMin, getSum or getAverage:
//Get count, min, max, sum, and average for numbers
List numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
IntSummaryStatistics stats = numbers
          .stream()
          .mapToInt((x) -> x)
          .summaryStatistics();
 
System.out.println("Highest number in List : " + stats.getMax());
System.out.println("Lowest number in List : " + stats.getMin());
System.out.println("Sum of all numbers : " + stats.getSum());
System.out.println("Average of all numbers : " + stats.getAverage()); 

That’s all, hope you like it!

In this article, we have discovered different ways of using lambda expressions, starting from basic examples to complex ones, where we used lambdas with streams. Moreover, we have also learned how to use lambda expressions with the Comparator class for sorting Java collections.

原文：Start Using Java Lambda Expressions