For what do you use collections? You don't simply store objects in a collection and leave them there. In most cases, you use collections to retrieve取回检索 items stored in them.
Consider again the scenario场景 described in the section Lambda Expressions. Suppose that you are creating a social networking application. You want to create a feature that enables an administrator to perform any kind of action, such as sending a message, on members of the social networking application that satisfy certain criteria标准.
As before, suppose that members of this social networking application are represented by the following Person class:
public class Person {
public enum Sex {
MALE, FEMALE
}
String name;
LocalDate birthday;
Sex gender;
String emailAddress;
// ...
public int getAge() {
// ...
}
public String getName() {
// ...
}
}
The following example prints the name of all members contained in the collection roster with a for-each loop:
for (Person p : roster) {
System.out.println(p.getName());
}
The following example prints all members contained in the collection roster but with the aggregate operation forEach:
roster
.stream()
.forEach(e -> System.out.println(e.getName());
Although, in this example, the version that uses aggregate operations is longer than the one that uses a for-each loop, you will see that versions that use bulk大量的-data operations will be more concise简介 for more complex复杂 tasks.
The following topics are covered:
Find the code excerpts described in this section in the example BulkDataOperationsExamples.
public class BulkDataOperationsExamples {
public static void main(String[] args) {
// Create sample data
List<Person> roster = Person.createRoster();
// 1. Print names of members, for-each loop
System.out.println("Members of the collection (for-each loop):");
for (Person p : roster) {
System.out.println(p.getName());
}
// 2. Print names of members, forEach operation
System.out.println("Members of the collection (bulk data operations):");
roster.stream().forEach(e -> System.out.println(e.getName()));
// 3. Print names of male members, forEach operation
System.out.println("Male members of the collection (bulk data operations):");
roster.stream().filter(e -> e.getGender() == Person.Sex.MALE)
.forEach(e -> System.out.println(e.getName()));
// 4. Print names of male members, for-each loop
System.out.println("Male members of the collection (for-each loop):");
for (Person p : roster) {
if (p.getGender() == Person.Sex.MALE) {
System.out.println(p.getName());
}
}
// 5. Get average age of male members of the collection:
double average = roster.stream()
.filter(p -> p.getGender() == Person.Sex.MALE)
.mapToInt(Person::getAge)
.average()
.getAsDouble();
System.out.println(
"Average age of male members (bulk data operations): " +
average);
}
}A pipeline is a sequence of aggregate operations. The following example prints the male members contained in the collection roster with a pipeline that consists of the aggregate operations filter andforEach:
roster
.stream()
.filter(e -> e.getGender() == Person.Sex.MALE)
.forEach(e -> System.out.println(e.getName()));
Compare this example to the following that prints the male members contained in the collection roster with a for-each loop:
for (Person p : roster) {
if (p.getGender() == Person.Sex.MALE) {
System.out.println(p.getName());
}
}
A pipeline contains the following components:
-
A source: This could be a collection, an array, a generator function, or an I/O channel. In this example, the source is the collection
roster. -
Zero or more intermediate operations. An intermediate operation, such as
filter, produces a new stream.A stream is a sequence of elements. Unlike a collection, it is not a data structure that stores elements. Instead, a stream carries values from a source through a pipeline. This example creates a stream from the collection
rosterby invoking the methodstream.The
filteroperation returns a new stream that contains elements that match its predicate (this operation's parameter). In this example, the predicate is the lambda expressione -> e.getGender() == Person.Sex.MALE. It returns the boolean valuetrueif thegenderfield of objectehas the valuePerson.Sex.MALE. Consequently, thefilteroperation in this example returns a stream that contains all male members in the collectionroster. -
A terminal终点 operation. A terminal operation, such as
forEach, produces a non-stream result, such as a primitive value (like a double value), a collection, or in the case offorEach, no value at all. In this example, the parameter of theforEachoperation is the lambda expressione -> System.out.println(e.getName()), which invokes the methodgetNameon the objecte. (The Java runtime and compiler infer that the type of the objecteisPerson.)
The following example calculates the average age of all male members contained in the collection roster with a pipeline that consists of the aggregate operations filter, mapToInt, and average:
double average = roster
.stream()
.filter(p -> p.getGender() == Person.Sex.MALE)
.mapToInt(Person::getAge)
.average()
.getAsDouble();
The mapToInt operation returns a new stream of type IntStream (which is a stream that contains only integer values). The operation applies the function specified in its parameter to each element in a particular stream. In this example, the function is Person::getAge, which is a method reference that returns the age of the member. (Alternatively, you could use the lambda expression e -> e.getAge().) Consequently, the mapToInt operation in this example returns a stream that contains the ages of all male members in the collection roster.
The average operation calculates the average value of the elements contained in a stream of type IntStream. It returns an object of type OptionalDouble. If the stream contains no elements, then theaverage operation returns an empty instance of OptionalDouble, and invoking the method getAsDouble throws a NoSuchElementException. The JDK contains many terminal operations such asaverage that return one value by combining the contents of a stream. These operations are called reduction operations; see the section Reduction for more information.
Differences Between Aggregate Operations and Iterators
Aggregate operations, like forEach, appear to be like iterators. However, they have several fundamental基本 differences:
Aggregate operations, like forEach, appear to be like iterators. However, they have several fundamental differences:
-
They use internal iteration: Aggregate operations do not contain a method like
nextto instruct委托吩咐 them to process the next element of the collection. With internal delegation, your application determines what collection it iterates, but the JDK determines how to iterate the collection. With external iteration, your application determines both what collection it iterates and how it iterates it. However, external iteration can only iterate over the elements of a collection sequentially. Internal iteration does not have this limitation. It can more easily take advantage of parallel并行 computing, which involves dividing a problem into subproblems, solving those problems simultaneously同时, and then combining the results of the solutions to the subproblems. See the section Parallelism for more information. -
They process elements from a stream: Aggregate operations process elements from a stream, not directly from a collection. Consequently, they are also called stream operations.
-
They support behavior as parameters: You can specify lambda expressions as parameters for most aggregate operations. This enables you to customize the behavior of a particular aggregate operation.