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One issue about anonymous classes is that if the implementation of your anonymous class is very simple, such as an interface that contains only one method, the syntax of anonymous classes may seem too unwieldy and unclear. In these cases, you're usually trying to pass functionality as an argument to another method, such as what action should be taken when someone clicks a button. Lambda expressions enable you to do this, to treat functionality as method argument, or code as data.
The previous section, Anonymous Classes, shows you how to implement a base class without giving it a name. While this is often more concise than a named class, for classes with only one method, even an anonymous class seems a bit heavyweight. Lambda expressions let you express instances of single-method classes more compactly.
This section covers the following topics:
Suppose 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. The following table describes this use case in detail:
| Field | Description |
|---|---|
| Name | Perform action on selected members |
| Primary Actor | Administrator |
| Preconditions | Administrator is logged into the system. |
| Postconditions | Action is performed only on those members that fit the specified criteria. |
| Main Success Scenario |
|
| Extensions |
1a. Administrator has an option to preview those members who match the specified criteria before he or she specifies the action to be performed or before selecting the Submit button. |
| Frequency of Occurrence | Many times during the day. This is a powerful feature because this use case enables an administrator to perform any action on a group of members. |
Suppose that members of this social networking application are
represented by the following
Member class:
public class Member {
public enum Sex {
MALE, FEMALE
}
String name;
Date birthday;
Sex gender;
String emailAddress;
public int getAge() {
// ...
}
public void printMember() {
// ...
}
}
Suppose that the members of your social networking application
are stored in a List<Member> instance.
This section begins with a naive approach to this use case. It improves upon this approach with local and anonymous classes, then finishes with an efficient and concise approach using lambda expressions. Find the code excerpts described in this section in the example
RosterTest.
One simplistic approach is to create several methods; each method searches for members that match one characteristic such as gender or age. The following method prints members that are older than a specified age:
public static void printMembersOlderThan(List<member> roster, int age) {
for (member p : roster) {
if (p.getAge() >= age) {
p.printMember();
}
}
}
Note: A
List is an ordered
Collection. A collection is an object
that groups multiple elements into a single unit. Collections are
used to store, retrieve, manipulate, and communicate aggregate
data. For more information about collections, see the
Collections trail.
This approach can potentially make your application brittle: Suppose you upgraded your application and changed the structure of the Member class such that it contains different member variables; perhaps the class records and measures ages with a different data type or algorithm. You would have to rewrite a lot of your API to accommodate this change. In addition, this approach is unnecessarily restrictive; what if you
wanted to print members younger than a certain age, for example?
The following method is more generic than printMembersOlderThan; it prints members within a specified range of ages:
public static void printMembersWithinAgeRange(
List<Member> roster, int low, int high) {
for (Member p : roster) {
if (low <= p.getAge() && p.getAge() < high) {
p.printMember();
}
}
}
What if you want to print members of a specified sex, or a combination of a specified gender and age range? What if you decide to change the Member class and add other attributes such as relationship status or geographical location? Although this method is more generic than printMembersOlderThan, trying to create a separate method for each possible search query can still lead to brittle code. You can instead separate the code that specifies the criteria for which you want to search in a different class.
The following method does not contain any code that specifies search criteria:
public static void printMembers(
List<Member> roster, CheckMember tester) {
for (Member p : roster) {
if (tester.test(p)) {
p.printMember();
}
}
}
To specify the search criteria, you implement the
CheckMember interface:
interface CheckMember {
boolean test(Member p);
}
The following class filters members that are eligible for Selective Service in the United States: those who are male and between the ages of 18 and 25:
class CheckMemberEligibleForSelectiveService implements CheckMember {
public boolean test(Member p) {
return p.gender == Member.Sex.MALE &&
p.getAge() >= 18 &&
p.getAge() <= 25;
}
}
To use this class, you create a new
instance of it and invoke the printMembers method:
printMembers(
roster, new CheckMemberEligibleForSelectiveService());
Although this approach is less brittle—you don't have to rewrite methods if you change the structure of the Member—you still have additional code: a new interface and a local class for each search you plan to perform in your application. Because CheckMemberEligibleForSelectiveService
implements an interface, you can use an anonymous class
instead and bypass the need to declare a new class for each search.
One of the arguments of the following invocation of the method printMembers is an anonymous class that filters members that are eligible for Selective Service in the United States: those who are male and between the ages of 18 and 25:
printMembers(
roster,
new CheckMember() {
public boolean test(Member p) {
return p.getGender() == Member.Sex.MALE
&& p.getAge() >= 18
&& p.getAge() <= 25;
}
}
);
This approach reduces the amount of code required—you don't have to create a new class for each search you want to perform. However, the syntax of anonymous classes is bulky considering that the CheckMember interface contains only one method. In this
case, you can use a lambda expression of an anonymous class, as
described in the next section.
The CheckMember
interface is a functional interface. A functional
interface is any interface that contains only one method. Because
a functional interface contains only one method, you can
omit the name of that method
when you implement it. To do this, instead of using an anonymous
class expression, you use a lambda
expression, which is
highlighted in the following method invocation:
printMembers(
roster,
(Member p) -> p.getGender() == Member.Sex.MALE
&& p.getAge() >= 18
&& p.getAge() <= 25
);
See Syntax of Lambda Expressions for information on how to define lambda expressions.
We can use a standard functional interface in place of the interface CheckMember, which reduces even further the amount of code required.
Let's revisit
the CheckMember
interface:
interface CheckMember {
boolean test(Member p);
}
This is a very
simple interface. It's a functional interface because it contains
only one method. This method takes one parameter and returns a
boolean value. The method is so simple that it might not be worth
it to define one in your application. Consequently, the JDK
defines several standard functional interfaces, which you can
find in the package java.util.function.
For example, you can use the
Predicate<T>
interface in place of CheckMember. This
interface contains the method boolean
test(T t):
interface Predicate<T> {
boolean test(T t);
}
The interface Predicate<T> is an example of a generic interface. (For more information about generics, see the
Generics (Updated) lesson.) Generic types (such as generic interfaces) specify one or more type parameters within angle brackets (<>). This interface contains only one type parameter, T. When you declare or instantiate a generic type with actual type arguments, you have a parameterized type. For example, the parameterized type Predicate<Member> is the following:
interface Predicate<Member> { boolean test(Membert); }
This parameterized type contains a method that has the same return type and parameters as CheckMember.boolean test(Member p). Consequently, you can use Predicate<T> in place of CheckMember as the following method demonstrates:
public void printMembersWithPredicate(
List<Member> roster, Predicate<Member> tester) {
for (Member p : roster) {
if (tester.test(p)) {
p.printMember();
}
}
}
As a result,
the following method invocation is the same as when we invoked
printMembers to obtain members who are eligible for Selective Service:
printMembersWithPredicate(
roster,
p -> p.getGender() == Member.Sex.MALE
&& p.getAge() >= 18
&& p.getAge() <= 25
);
This is not the only place in this method we could use a lambda expression. The following approach looks for other ways to use lambda expressions.
Let's revisit the method printMembersWithPredicate
to see where else we could use
lambda expressions:
public void printMembersWithPredicate(
List<Member> roster, Predicate<Member> tester) {
for (Member p : roster) {
if (tester.test(p)) {
p.printMember();
}
}
}
This method invokes the method printMember on each Member instance. You can specify the action performed on each Member instance with a lambda expression. Remember, to use a lambda expression, you need to implement a functional interface. In this case, you need a functional interface that contains a method that takes one argument (an object of type Member) and returns void. The
Consumer<T>
interface contains the method
void accept(T t). The
following method replaces the invocation
p.printMember() with an
instance of Block<Member> that invokes the method accept:
public void processMembers(
List<Member> roster,
Predicate<Member> tester,
Consumer<Member> block) {
for (Member p : roster) {
if (tester.test(p)) {
block.accept(p);
}
}
}
As a
result, the following method invocation is the same as when we invoked printMembers to obtain members who are eligible for Selective Service. The lambda expression used to
print members is highlighted:
processMembers(
roster,
p -> p.getGender() == Member.Sex.MALE
&& p.getAge() >= 18
&& p.getAge() <= 25,
p -> p.printMember()
);
What if
you wanted to do more with your members' profiles instead of
printing them out, such as validate their profiles or retrieve
their contact information? In this case, you need a functional
interface that contains a method that returns a value.
The
Function<T,R>
interface contains the
method R apply(T
t). The following method retrieves the data
specified by the parameter mapper, and
then performs an action on it specified by the
parameter block:
public static void processMembersWithFunction(
List<Member> roster,
Predicate<Member> tester,
Function<Member, String> mapper,
Consumer<String> block) {
for (Member p : roster) {
if (tester.test(p)) {
String data = mapper.apply(p);
block.accept(data);
}
}
}
The
following method retrieves the e-mail address from each member
contained in roster and
then prints it:
processMembersWithFunction(
roster,
p -> p.getGender() == Member.Sex.MALE
&& p.getAge() >= 18
&& p.getAge() <= 25,
p -> p.getEmailAddress(),
email -> System.out.println(email)
);
To process events in a GUI application, such as keyboard actions, mouse actions, and scroll actions, you typically create event handlers, which usually involves implementing a particular interface. Often, event handler interfaces are functional interfaces; they tend to have only one method.
In the JavaFX example
HelloWorld.java discussed in the previous section
Anonymous Classes, you can
replace the anonymous class with a lambda expression in this
statement:
btn.setOnAction(new EventHandler<ActionEvent>() {
@Override
public void handle(ActionEvent event) {
System.out.println("Hello World!");
}
});
The method invocation btn.setOnAction specifies what
happens when you select the button represented by the btn object. This method requires
an object of type EventHandler<ActionEvent>. The EventHandler<ActionEvent>
interface contains only one method, void handle(T event).
This interface is a functional interface, so you could use a
lambda expression:
btn.setOnAction(
event -> System.out.println("Hello World!")
);
A lambda expression consists of the following:
A
comma-separated list of formal parameters enclosed in
parentheses; the CheckMember.test method contains one parameter,
p, which represents an instance of the
Member class.
Note: You may omit the data type of the parameters in a lambda expression. In addition, you may omit the parentheses if there is only one parameter. For example, the following lambda expression is also valid:
p -> p.getGender() == Member.Sex.MALE
&& p.getAge() >= 18
&& p.getAge() <= 25The arrow token, ->
A body, which consists of a single expression or a statement block; this example uses this expression:
p.getGender() == Member.Sex.MALE
&& p.getAge() >= 18
&& p.getAge() <= 25
If you specify a single expression, the Java runtime evaluates the expression and then returns its value. Alternatively, you can use a return statement:
p -> {
return p.getGender() == Member.Sex.MALE
&& p.getAge() >= 18
&& p.getAge() <= 25;
}
Note that
a return statement is not an expression; in a lambda expression,
you must enclose statements in curly braces ({}). However, you do not have to enclose a void method invocation in curly braces. For example, the following is a valid lambda expression:
email -> System.out.println(email)
Note that a lambda expression looks a lot like a method declaration; you can consider lambda expressions as anonymous methods—methods without a name.
The following example,
Calculator is an example of lambda expressions that take
more than one formal parameter:
public class Calculator {
interface IntegerMath {
int operation(int a, int b);
}
public int operateBinary(int a, int b, IntegerMath op) {
return op.operation(a, b);
}
public static void main(String... args) {
Calculator myApp = new Calculator();
IntegerMath addition = (a, b) -> a + b;
IntegerMath subtraction = (a, b) -> a - b;
System.out.println("40 + 2 = " +
myApp.operateBinary(40, 2, addition));
System.out.println("20 - 10 = " +
myApp.operateBinary(20, 10, subtraction));
}
}
The method operateBinary performs a
mathematical operation on two integer operands. The operation
itself is specified by an instance of IntegerMath. The example defines two operations with lambda expressions, addition and subtraction. The example prints
the following:
40 + 2 = 42 20 - 10 = 10
Lambda expressions have the same
access to local variables of the enclosing scope as local and
anonymous classes. However, unlike local and anonymous classes, lambda expressions do not have any shadowing issues (see
Shadowing for more information). Lambda expressions are lexically scoped. This means that they do not inherit any names from a supertype nor introduce a new level of scoping. Declarations in a lambda expression are interpreted just as they are in the enclosing environment. The following example,
LambdaScopeTest demonstrates this:
import java.util.functions.Block;
public class LambdaScopeTest {
public int x = 0;
class FirstLevel {
public int x = 1;
void methodInFirstLevel(int x) {
// The following statement causes the compiler to generate
// the error "local variables referenced from a lambda expression
// must be final or effectively final" in statement A:
//
// x = 99;
Block<Integer> myBlock = (y) ->
{
System.out.println("x = " + x); // Statement A
System.out.println("y = " + y);
System.out.println("this.x = " + this.x);
System.out.println("LambdaScopeTest.this.x = " +
LambdaScopeTest.this.x);
};
myBlock.apply(x);
}
}
public static void main(String... args) {
LambdaScopeTest st = new LambdaScopeTest();
LambdaScopeTest.FirstLevel fl = st.new FirstLevel();
fl.methodInFirstLevel(23);
}
}
This example generates the following output:
x = 23 y = 23 this.x = 1 LambdaScopeTest.this.x = 0
If you substitute the parameter x in place of y in the declaration of the lambda expression myBlock, the compiler generates an error:
Block<Integer> myBlock = (y) -> {
// ...
}
The compiler generates the error, "variable x is already defined in method methodInFirstLevel(int)" because the lambda expression does not introduce a new level of scoping. Consequently, you can directly access fields, methods, and local variables of the enclosing scope. For example, the lambda expression directly accesses the parameter x of the method methodInFirstLevel. To access variables in the enclosing class, use the keyword this. In this example, this.x refers to the member variable FirstLevel.x.
However, like local and anonymous classes, a lambda expression can only access local variables and parameters of the enclosing block that are final or
effectively final. For example, suppose you add
the following assignment statement immediately after the methodInFirstLevel definition statement as follows:
void methodInFirstLevel(int x) {
x = 99;
// ...
}
Because of this assignment
statement, the variable FirstLevel.x is not effectively final anymore. As a result, the Java compiler generates an error message similar to
"local variables referenced from a lambda expression must be final or effectively final" where the lambda expression myBlock tries to access the FirstLevel.x variable:
System.out.println("x = " + x);
How do you determine the type of a lambda expression? Recall the lambda expression that selected members who are male and between the ages 18 and 25 years:
p -> p.getGender() == Member.Sex.MALE
&& p.getAge() >= 18
&& p.getAge() <= 25
Recall that we used the same lambda expression in the following two methods:
public void printMembers(List<Member> roster, CheckMember tester)
public void printMembersWithPredicate(List<Member> roster, Predicate<Member> tester)
When the Java runtime invokes the method printMembers, it's expecting a data type of CheckMember, so the
lambda expression is of this type. However,
when the Java runtime invokes the method printMembersWithPredicate,
it's expecting a data type of Predicate<Member>,
so the lambda expression is of this type. The data type that
these methods expect is called the target type. To determine the type of a lambda
expression, the Java compiler uses the target type of the context
or situation in which the lambda expression was found. It follows
that you can only use lambda expressions in situations in which
the Java compiler can determine a target type:
Variable declarations
Assignments
Return statements
Array initializers
Method or constructor arguments
Lambda expression bodies
Conditional expressions, ?:
Cast expressions
For method arguments, the Java compiler determines the target type with two other language features: overload resolution and type argument inference.
Consider the following two functional interfaces,
java.lang.Runnable, and
java.util.concurrent.Callable<V>:
public interface Runnable {
void run();
}
public interface Callable<V> {
V call();
}
The method Runnable.run does not return a value while Callable<V>.call does.
Suppose you have overloaded the method invoke as follows.
(Overloaded methods are those that have the same name, belong to
the same class, but have different parameter lists or return
values. See
Defining Methods for more information):
void invoke(Runnable r) {
r.run();
}
<T> T invoke(Callable<T> c) {
return c.call();
}
Which method will be invoked in the following statement?
String s = invoke(() -> "done");
The method invoke(Callable<T>) will be
invoked because that method returns a value; the method
invoke(Runnable) does not. In this case, the type of the lambda expression () -> "done" is Callable<T>.
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