Java8的其中一个新特性，函数式接口。
- 什么是函数式接口？
有且仅有一个抽象方法的接口（不包括默认方法、静态方法以及对Object方法的重写） - 函数式接口有什么用呢？
函数式接口不同于以往的普通接口，它最大的作用其实是为了支持行为参数传递，比如传递Lambda、方法引用、函数式接口对应的实例对象等。 - 函数式接口在什么场景用呢？
函数中的某一段逻辑（几行code）想要自己定义，其他的部分都是公用的。那么可以将这一段逻辑抽取出来，通过函数式接口，定义不同的逻辑，作为参数传给目标函数。
1. 传统写法，定义一个接口，定义一个类，实现这个接口
package com.example.demo.functionalInterface;
interface Task {
void start();
}
class TaskImpl implements Task{
@Override
public void start() {
System.out.println("Start task");
}}
public class Test {
public static void main(String[] args) {
Task task = new TaskImpl();
task.start();
}
}
2. 定义一个接口，直接写一个匿名类来实例化
package com.example.demo.functionalInterface;
interface Task {
void start();
}
public class Test {
public static void main(String[] args) {
Task task = new Task() {
@Override
public void start() {
System.out.println("Start task!");
}};
task.start();
}
}
3. 用lambda函数来实现
package com.example.demo.functionalInterface;
interface Task {
void start();
}
public class Test {
public static void main(String[] args) {
Task task = () -> System.out.println("Start task!");
task.start();
}
}
4. 有返回值的函数，有（多个）参数的函数
4.1 0参数，有返回值
package com.example.demo.functionalInterface;
interface Task {
String start();
}
public class Test {
public static void main(String[] args) {
Task task = () -> "Start task!";
System.out.println(task.start());
}
}
4.2 有一个参数
package com.example.demo.functionalInterface;
interface Task {
String start(int id);
}
public class Test {
public static void main(String[] args) {
Task task = (e) -> "Start task " + e;
System.out.println(task.start(1));
}
}
4.3 多个参数
package com.example.demo.functionalInterface;
interface Task {
String start(int id, String user);
}
public class Test {
public static void main(String[] args) {
Task task = (e, u) -> "Start task " + e + " by " + u;
System.out.println(task.start(1, "hanmeimei"));
}
}
5. 接口中不能定义多个函数，可用@FunctionalInterface来限定
若接口中定义多个函数，不能使用lambda函数，会报错。
多个函数报错
这时lambda函数会报错，但是接口处并没有报错。可以用注解@FunctionalInterface进行限定。不过这个注解不是必须的，只是让编译器能够报错。
@FunctionalInterface
6. Java自带函数式接口, java.util.function里面定义了很多函数式接口，定义了不同的函数，举几个主要的例子
6.1 Consumer
Consumer定义了一个accept函数，接收一个参数，无返回值。主要用于只读的场景。
它还提供了andThen方法，可以串行地执行多个consumer。
Consumer定义：
**
* Represents an operation that accepts a single input argument and returns no
* result. Unlike most other functional interfaces, {@code Consumer} is expected
* to operate via side-effects.
*
* <p>This is a <a href="package-summary.html">functional interface</a>
* whose functional method is {@link #accept(Object)}.
*
* @param <T> the type of the input to the operation
*
* @since 1.8
*/
@FunctionalInterface
public interface Consumer<T> {
/**
* Performs this operation on the given argument.
*
* @param t the input argument
*/
void accept(T t);
/**
* Returns a composed {@code Consumer} that performs, in sequence, this
* operation followed by the {@code after} operation. If performing either
* operation throws an exception, it is relayed to the caller of the
* composed operation. If performing this operation throws an exception,
* the {@code after} operation will not be performed.
*
* @param after the operation to perform after this operation
* @return a composed {@code Consumer} that performs in sequence this
* operation followed by the {@code after} operation
* @throws NullPointerException if {@code after} is null
*/
default Consumer<T> andThen(Consumer<? super T> after) {
Objects.requireNonNull(after);
return (T t) -> { accept(t); after.accept(t); };
}
}
示例1：consumer的用法
package com.example.demo.functionalInterface;
import java.util.function.Consumer;
public class Test {
public static void main(String[] args) {
testConsumer("myName", t -> {
if (t.length() >3)
System.out.println("The name is " + t);
else
System.out.println("Unexpected name!");
});
testConsumer("China", t -> System.out.println("The country is " + t));
}
private static void testConsumer(String str, Consumer<String> con) {
System.out.println("Doing something");
con.accept(str);
System.out.println("Doing other things");
}
}
示例2: consumer andthen的用法
package com.example.demo.functionalInterface;
import java.util.function.Consumer;
public class Test {
public static void main(String[] args) {
testConsumer("myName", t -> System.out.println("The name is " + t), t -> System.out.println("Complete!"));
testConsumer("China", t -> System.out.println("The country is " + t), t -> System.out.println("Done!"));
}
private static void testConsumer(String str, Consumer<String> con1, Consumer<String> con2) {
System.out.println("Doing something");
con1.andThen(con2).accept(str);
System.out.println("Doing other things");
}
}
6.2 Supplier
Supplier定义了一个get函数，不接收参数，提供一个返回值。
Supplier定义：
/**
* Represents a supplier of results.
*
* <p>There is no requirement that a new or distinct result be returned each
* time the supplier is invoked.
*
* <p>This is a <a href="package-summary.html">functional interface</a>
* whose functional method is {@link #get()}.
*
* @param <T> the type of results supplied by this supplier
*
* @since 1.8
*/
@FunctionalInterface
public interface Supplier<T> {
/**
* Gets a result.
*
* @return a result
*/
T get();
}
Supplier用法示例：
package com.example.demo.functionalInterface;
import java.util.function.Supplier;
public class Test {
public static void main(String[] args) {
testSupplier(() -> "Test!");
testSupplier(() -> "Test again!");
}
private static void testSupplier(Supplier<String> sup) {
System.out.println("Doing something");
System.out.println(sup.get());
System.out.println("Doing other things");
}
}
6.3 Predicate
Predicate提供了一个test方法，接收一个参数，并返回boolean。
另外，它还提供了一些操作方法，例如and，or，negate.
Predicate定义：
/**
* Represents a predicate (boolean-valued function) of one argument.
*
* <p>This is a <a href="package-summary.html">functional interface</a>
* whose functional method is {@link #test(Object)}.
*
* @param <T> the type of the input to the predicate
*
* @since 1.8
*/
@FunctionalInterface
public interface Predicate<T> {
/**
* Evaluates this predicate on the given argument.
*
* @param t the input argument
* @return {@code true} if the input argument matches the predicate,
* otherwise {@code false}
*/
boolean test(T t);
/**
* Returns a composed predicate that represents a short-circuiting logical
* AND of this predicate and another. When evaluating the composed
* predicate, if this predicate is {@code false}, then the {@code other}
* predicate is not evaluated.
*
* <p>Any exceptions thrown during evaluation of either predicate are relayed
* to the caller; if evaluation of this predicate throws an exception, the
* {@code other} predicate will not be evaluated.
*
* @param other a predicate that will be logically-ANDed with this
* predicate
* @return a composed predicate that represents the short-circuiting logical
* AND of this predicate and the {@code other} predicate
* @throws NullPointerException if other is null
*/
default Predicate<T> and(Predicate<? super T> other) {
Objects.requireNonNull(other);
return (t) -> test(t) && other.test(t);
}
/**
* Returns a predicate that represents the logical negation of this
* predicate.
*
* @return a predicate that represents the logical negation of this
* predicate
*/
default Predicate<T> negate() {
return (t) -> !test(t);
}
/**
* Returns a composed predicate that represents a short-circuiting logical
* OR of this predicate and another. When evaluating the composed
* predicate, if this predicate is {@code true}, then the {@code other}
* predicate is not evaluated.
*
* <p>Any exceptions thrown during evaluation of either predicate are relayed
* to the caller; if evaluation of this predicate throws an exception, the
* {@code other} predicate will not be evaluated.
*
* @param other a predicate that will be logically-ORed with this
* predicate
* @return a composed predicate that represents the short-circuiting logical
* OR of this predicate and the {@code other} predicate
* @throws NullPointerException if other is null
*/
default Predicate<T> or(Predicate<? super T> other) {
Objects.requireNonNull(other);
return (t) -> test(t) || other.test(t);
}
/**
* Returns a predicate that tests if two arguments are equal according
* to {@link Objects#equals(Object, Object)}.
*
* @param <T> the type of arguments to the predicate
* @param targetRef the object reference with which to compare for equality,
* which may be {@code null}
* @return a predicate that tests if two arguments are equal according
* to {@link Objects#equals(Object, Object)}
*/
static <T> Predicate<T> isEqual(Object targetRef) {
return (null == targetRef)
? Objects::isNull
: object -> targetRef.equals(object);
}
}
Supplier用法示例：
package com.example.demo.functionalInterface;
import java.util.Arrays;
import java.util.function.Predicate;
public class Test {
public static void main(String[] args) {
testPredicate(e -> {
if (Arrays.asList("Monday", "Tuesday").contains(e)) {
return true;
}
return false;
});
}
private static void testPredicate(Predicate<String> sup) {
System.out.println("Doing something");
if (sup.test("Monday")) {
System.out.println("It is a work day!");
}
System.out.println("Doing other things");
}
}
Supplier or:
private static void testPredicate(Predicate<String> pre1, Predicate<String> pre2) {
System.out.println("Doing something");
if (pre1.or(pre2).test("Saturday")) {
System.out.println("It is a work day!");
}
System.out.println("Doing other things");
}
negate取反：
private static void testPredicate(Predicate<String> pre1, Predicate<String> pre2) {
System.out.println("Doing something");
if (pre1.negate().test("Saturday")) {
System.out.println("It is a work day!");
}
System.out.println("Doing other things");
}
6.4 Function
Function接口提供了apply函数，接收某一个类型的参数，返回另一个类型的结果。
Function定义：
/**
* Represents a function that accepts one argument and produces a result.
*
* <p>This is a <a href="package-summary.html">functional interface</a>
* whose functional method is {@link #apply(Object)}.
*
* @param <T> the type of the input to the function
* @param <R> the type of the result of the function
*
* @since 1.8
*/
@FunctionalInterface
public interface Function<T, R> {
/**
* Applies this function to the given argument.
*
* @param t the function argument
* @return the function result
*/
R apply(T t);
/**
* Returns a composed function that first applies the {@code before}
* function to its input, and then applies this function to the result.
* If evaluation of either function throws an exception, it is relayed to
* the caller of the composed function.
*
* @param <V> the type of input to the {@code before} function, and to the
* composed function
* @param before the function to apply before this function is applied
* @return a composed function that first applies the {@code before}
* function and then applies this function
* @throws NullPointerException if before is null
*
* @see #andThen(Function)
*/
default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
Objects.requireNonNull(before);
return (V v) -> apply(before.apply(v));
}
/**
* Returns a composed function that first applies this function to
* its input, and then applies the {@code after} function to the result.
* If evaluation of either function throws an exception, it is relayed to
* the caller of the composed function.
*
* @param <V> the type of output of the {@code after} function, and of the
* composed function
* @param after the function to apply after this function is applied
* @return a composed function that first applies this function and then
* applies the {@code after} function
* @throws NullPointerException if after is null
*
* @see #compose(Function)
*/
default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) {
Objects.requireNonNull(after);
return (T t) -> after.apply(apply(t));
}
/**
* Returns a function that always returns its input argument.
*
* @param <T> the type of the input and output objects to the function
* @return a function that always returns its input argument
*/
static <T> Function<T, T> identity() {
return t -> t;
}
}
Function用法示例：
public class Test {
public static void main(String[] args) {
testPredicate(e -> {
if ("Monday".equals(e))
return 1;
return 0;
});
}
private static void testPredicate(Function<String, Integer> function) {
System.out.println("Doing something");
System.out.println(function.apply("Monday"));
System.out.println("Doing other things");
}
}
7. 自定义函数接口
定义一个函数式接口，类似于Function，抛出异常。
@FunctionalInterface
protected interface Function<ParamType, ResultType> {
public ResultType apply(ParamType param) throws Exception;
}
定义一个函数，其中一个参数为上面接口的实现
public Response handleMsg(String msg, Function<Context, Response> resFunction) {
...
Context context = new Context(msg);
...
Response response = resFunction.apply(context);
...
}
调用处，传一个lambda函数过去：
public UpdateResponse handleUpdateMsg(String msg) {
return handleMsg(msg, (ctx) -> {
UpdateResponse updateRes = new UpdateResponse();
updateRes.setValue(handleUpdateMsg(ctx));
return updateRes;
});
}
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