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本篇內容介紹了“JDK1.8有什么新特性”的有關知識，在實際案例的操作過程中，不少人都會遇到這樣的困境，接下來就讓小編帶領大家學習一下如何處理這些情況吧！希望大家仔細閱讀，能夠學有所成！
函數式接口（functional Interface），有且僅有一個抽象方法的接口，但可以有多個非抽象的方法。
適用于Lambda表達式使用的接口。如創建線程：
new Thread(() -> System.out.println(Thread.currentThread().getName())).start();
其中，Lambda表達式代替了new Runnable()，這里的Runable接口就屬于函數式接口，最直觀的體現是使用了 @FunctionalInterface注解，而且使用了一個抽象方法(有且僅有一個)，如下：
package java.lang;
/**
* The <code>Runnable</code> interface should be implemented by any
* class whose instances are intended to be executed by a thread. The
* class must define a method of no arguments called <code>run</code>.
* <p>
* This interface is designed to provide a common protocol for objects that
* wish to execute code while they are active. For example,
* <code>Runnable</code> is implemented by class <code>Thread</code>.
* Being active simply means that a thread has been started and has not
* yet been stopped.
* </p><p>
* In addition, <code>Runnable</code> provides the means for a class to be
* active while not subclassing <code>Thread</code>. A class that implements
* <code>Runnable</code> can run without subclassing <code>Thread</code>
* by instantiating a <code>Thread</code> instance and passing itself in
* as the target. In most cases, the <code>Runnable</code> interface should
* be used if you are only planning to override the <code>run()</code>
* method and no other <code>Thread</code> methods.
* This is important because classes should not be subclassed
* unless the programmer intends on modifying or enhancing the fundamental
* behavior of the class.
*
* @author Arthur van Hoff
* @see java.lang.Thread
* @see java.util.concurrent.Callable
* @since JDK1.0
*/
@FunctionalInterface
public interface Runnable {
/**
* When an object implementing interface <code>Runnable</code> is used
* to create a thread, starting the thread causes the object's
* <code>run</code> method to be called in that separately executing
* thread.
* </p><p>
* The general contract of the method <code>run</code> is that it may
* take any action whatsoever.
*
* @see java.lang.Thread#run()
*/
public abstract void run();
}>修飾符 interface 接口名 { > > public abstract 返回值類型 方法名 (可選參數列表)； > >}
注：public abstract可以省略（因為默認修飾為public abstract）
如：
public interface MyFunctionalInterface {
public abstract void method();
}@FunctionalInterface，是JDK1.8中新引入的一個注解，專門指代函數式接口，用于一個接口的定義上。
和@Override注解的作用類似，@FunctionalInterface注解可以用來檢測接口是否是函數式接口。如果是函數式接口，則編譯成功，否則編譯失敗（接口中沒有抽象方法或者抽象方法的個數多余1個）。
package com.xcbeyond.study.jdk8.functional;
/**
* 函數式接口
* @Auther: xcbeyond
* @Date: 2020/5/17 0017 0:26
*/
@FunctionalInterface
public interface MyFunctionalInterface {
public abstract void method();
// 如果存在多個抽象方法，則編譯失敗，即：@FunctionalInterface飄紅
// public abstract void method1();
}函數式接口：
package com.xcbeyond.study.jdk8.functional;
/**
* 函數式接口
* @Auther: xcbeyond
* @Date: 2020/5/17 0017 0:26
*/
@FunctionalInterface
public interface MyFunctionalInterface {
public abstract void method();
// 如果存在多個抽象方法，則編譯失敗，即：@FunctionalInterface飄紅
// public abstract void method1();
}測試：
package com.xcbeyond.study.jdk8.functional;
/**
* 測試函數式接口
* @Auther: xcbeyond
* @Date: 2020/5/17 0017 0:47
*/
public class MyFunctionalInterfaceTest {
public static void main(String[] args) {
// 調用show方法，參數中有函數式接口MyFunctionalInterface，所以可以使用Lambda表達式，來完成接口的實現
show("hello xcbeyond!", msg -> System.out.printf(msg));
}
/**
* 定義一個方法，參數使用函數式接口MyFunctionalInterface
* @param myFunctionalInterface
*/
public static void show(String message, MyFunctionalInterface myFunctionalInterface) {
myFunctionalInterface.method(message);
}
}函數式接口，用起來是不是更加的靈活，可以在具體調用處進行接口的實現。
函數式接口，可以很友好地支持Lambda表達式。
在JDK1.8之前已經有了大量的函數式接口，最熟悉的就是java.lang.Runnable接口了。
JDK 1.8 之前已有的函數式接口:
java.lang.Runnable
java.util.concurrent.Callable
java.security.PrivilegedAction
java.util.Comparator
java.io.FileFilter
java.nio.file.PathMatcher
java.lang.reflect.InvocationHandler
java.beans.PropertyChangeListener
java.awt.event.ActionListener
javax.swing.event.ChangeListener
而在JDK1.8新增了java.util.function包下的很多函數式接口，用來支持Java的函數式編程，從而豐富了Lambda表達式的使用場景。
這里主要介紹四大核心函數式接口：
java.util.function.Consumer：消費型接口
java.util.function.Supplier：供給型接口
java.util.function.Predicate：斷定型接口
java.util.function.Function：函數型接口
java.util.function.Consumer接口，是一個消費型的接口，消費數據類型由泛型決定。
package java.util.function;
import java.util.Objects;
/**
* 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><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); };
}
}Consumer接口中的抽象方法void accept(T t)，用于消費一個指定泛型T的數據。
舉例如下：
/**
* 測試void accept(T t)
*/
@Test
public void acceptMethodTest() {
acceptMethod("xcbeyond", message -> {
// 完成字符串的處理，即：通過Consumer接口的accept方法進行對應數據類型(泛型)的消費
String reverse = new StringBuffer(message).reverse().toString();
System.out.printf(reverse);
});
}
/**
* 定義一個方法，用于消費message字符串
* @param message
* @param consumer
*/
public void acceptMethod(String message, Consumer<string> consumer) {
consumer.accept(message);
}方法andThen，可以用來將多個Consumer接口連接到一起，完成數據消費。
/**
* 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); };
}舉例如下：
/**
* 測試Consumer<t> andThen(Consumer<!--? super T--> after)
* 輸出結果：
* XCBEYOND
* xcbeyond
*/
@Test
public void andThenMethodTest() {
andThenMethod("XCbeyond", t -> {
// 轉換為大小輸出
System.out.println(t.toUpperCase());
}, t -> {
// 轉換為小寫輸出
System.out.println(t.toLowerCase());
});
}
/**
* 定義一個方法，將兩個Consumer接口連接到一起，進行消費
* @param message
* @param consumer1
* @param consumer2
*/
public void andThenMethod(String message, Consumer<string> consumer1, Consumer<string> consumer2) {
consumer1.andThen(consumer2).accept(message);
}java.util.function.Supplier接口，是一個供給型接口，即：生產型接口。只包含一個無參方法：T get()，用來獲取一個泛型參數指定類型的數據。
package java.util.function;
/**
* Represents a supplier of results.
*
* </string></string></t></t></string></t></t></t></p><p>There is no requirement that a new or distinct result be returned each
* time the supplier is invoked.
*
* </p><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();
}舉例如下：
@Test
public void test() {
String str = getMethod(() -> "hello world!");
System.out.println(str);
}
public String getMethod(Supplier<string> supplier) {
return supplier.get();
}java.util.function.Predicate接口，是一個斷定型接口，用于對指定類型的數據進行判斷，從而得到一個判斷結果（boolean類型的值）。
package java.util.function;
import java.util.Objects;
/**
* Represents a predicate (boolean-valued function) of one argument.
*
* </string></t></t></p><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.
*
* </t></t></p><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.
*
* </t></t></p><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);
}
}抽象方法boolean test(T t),用于條件判斷。
/**
* 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);舉例如下：
/**
* 測試boolean test(T t);
*/
@Test
public void testMethodTest() {
String str = "xcbey0nd";
boolean result = testMethod(str, s -> s.equals("xcbeyond"));
System.out.println(result);
}
/**
* 定義一個方法，用于字符串的判斷。
* @param str
* @param predicate
* @return
*/
public boolean testMethod(String str, Predicate predicate) {
return predicate.test(str);
}方法Predicate<t> and(Predicate<!--? super T--> other)，用于將兩個Predicate進行邏輯”與“判斷。
/**
* 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.
*
* </t></t></t></t></t></p><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);
}方法Predicate<t> negate()，用于取反判斷。
/**
* 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);
}方法Predicate<t> or(Predicate<!--? super T--> other)，用于兩個Predicate的邏輯”或“判斷。
/**
* 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.
*
* </t></t></t></t></p><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);
}java.util.function.Function接口，是一個函數型接口，用來根據一個類型的數據得到另外一個類型的數據。
package java.util.function;
import java.util.Objects;
/**
* Represents a function that accepts one argument and produces a result.
*
* </t></p><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;
}
}抽象方法R apply(T t)，根據類型T的參數獲取類型R的結果。
/** * Applies this function to the given argument. * * @param t the function argument * @return the function result */ R apply(T t);
舉例如下：
/**
* 測試R apply(T t)，完成字符串整數的轉換
*/
@Test
public void applyMethodTest() {
// 字符串類型的整數
String numStr = "123456";
Integer num = applyMethod(numStr, n -> Integer.parseInt(n));
System.out.println(num);
}
public Integer applyMethod(String str, Function<string, integer> function) {
return function.apply(str);
}方法<v> Function<v, r> compose(Function<!--? super V, ? extends T--> before),獲取apply的function。
/**
* 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));
}方法<v> Function<t, v> andThen(Function<!--? super R, ? extends V--> after),用來進行組合操作，即：”先做什么，再做什么“的場景。
/**
* 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));
}函數式編程并不是Java提出的新概念，它將計算機運算看作是函數的計算。函數式編程最重要的基礎是λ演算，而且λ演算的函數是可以接受函數當作輸入（參數）和輸出（返回值）的。
和指令式編程相比，函數式編程強調函數的計算比指令的執行重要。
和過程化編程相比，函數式編程里函數的計算可隨時調用。
當然，Java大家都知道是面向對象的編程語言，一切都是基于對象的特性（抽象、封裝、繼承、多態）。在JDK1.8出現之前，我們關注的往往是某一對象應該具有什么樣的屬性，當然這也就是面向對象的核心——對數據進行抽象。但JDK1.8出現以后，這一點開始出現變化，似乎在某種場景下，更加關注某一類共有的行為（有點類似接口），這也就是JDK1.8提出函數式編程的目的。如下圖所示，展示了面向對象編程到函數式編程的變化。
Lambda表達式就是更好的體現了函數式編程，而為了支持Lambda表達式，才有了函數式接口。
另外，為了在面對大型數據集合時，為了能夠更加高效的開發，編寫的代碼更加易于維護，更加容易運行在多核CPU上，java在語言層面增加了Lambda表達式。在上一節中，我們已經知道Lambda表達式是多么的好用了 。
在JDK1.8中，函數式編程隨處可見，在你使用過程中簡直很爽，例如：Stream流。
函數式編程的優點，也很多，如下：
函數式編程大量使用函數，減少了代碼的重復，因此程序比較短，開發速度較快。
函數式編程的自由度很高，可以寫出很接近自然語言的代碼。
例如，兩數只差，可以寫成(x, y) -> x – y
函數式編程不依賴、也不會改變外界的狀態，只要給定輸入參數，返回的結果必定相同。因此，每一個函數都可以被看做獨立單元，很有利于進行單元測試（unit testing）和除錯（debugging），以及模塊化組合。
函數式編程不需要考慮"死鎖"，因為它不修改變量，所以根本不存在"鎖"線程的問題。不必擔心一個線程的數據，被另一個線程修改，所以可以很放心地把工作分攤到多個線程，部署"并發編程"。
函數式編程沒有副作用，只要保證接口不變，內部實現是外部無關的。所以，可以在運行狀態下直接升級代碼，不需要重啟，也不需要停機。
在JDK1.8中，函數式接口/編程將會隨處可見，也有有助于你更好的理解JDK1.8中的一些新特性。關于函數式接口，在接下來具體特性、用法中將會體現的淋漓盡致。
JDK1.8提出的函數式接口，你是否贊同呢？
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