JDK1.8中的雙冒號::是什么語法？

• 靜態(tài)方法引用，通過類名::靜態(tài)方法名， 如 Integer::parseInt
• 實例方法引用，通過實例對象::實例方法，如 str::substring
• 構(gòu)造方法引用，通過類名::new， 如 User::new

public?final?class?Integer?{
????public?static?int?parseInt(String s)?throws?NumberFormatException {
????????return?parseInt(s,10);
????}}

public?class?User?{
????private?String username;
????private?Integer age;
????public?User()?{
????} public?User(String username, Integer age)?{
????????this.username = username;
????????this.age = age;
????} @Override
????public?String toString()?{
????????return?"User{"?+
????????????????"username='"?+ username + '\''?+
????????????????", age="?+ age +
????????????????'}';
????} // Getter&Setter
}

public?static?void?main(String[] args) {
????// 使用雙冒號::來構(gòu)造靜態(tài)函數(shù)引用
????Function<String, Integer> fun = Integer::parseInt; Integer value = fun.apply("123");
????System.out.println(value); // 使用雙冒號::來構(gòu)造非靜態(tài)函數(shù)引用
????String?content = "Hello JDK8";
????FunctionString> func = content::substring; String?result = func.apply(1);
????System.out.println(result); // 構(gòu)造函數(shù)引用
????BiFunction<String, Integer, User> biFunction = User::new; User user = biFunction.apply("mengday", 28);
????System.out.println(user.toString()); // 函數(shù)引用也是一種函數(shù)式接口，所以也可以將函數(shù)引用作為方法的參數(shù)
????sayHello(String::toUpperCase, "hello");
}// 方法有兩個參數(shù)，一個是
private?static?void?sayHello(Function<String, String> func, String?parameter){
????String?result = func.apply(parameter);
????System.out.println(result);
}

package?java.util;
import?java.util.function.Consumer;
import?java.util.function.Function;
import?java.util.function.Predicate;
import?java.util.function.Supplier;
/**
?* @since?1.8
?*/
public?final?class?Optional<T> {
????private?static?final?Optional EMPTY = new?Optional<>();
????private?final?T value;
????private?Optional()?{
????????this.value = null;
????} // 返回一個空的 Optional實例
????public?static Optional empty()?{
????????@SuppressWarnings("unchecked")
????????Optional t = (Optional) EMPTY;
????????return?t;
????}
????private?Optional(T value)?{
????????this.value = Objects.requireNonNull(value);
????}
????// 返回具有 Optional的當(dāng)前非空值的Optional
????public?static? Optional of(T value)?{
????????return?new?Optional<>(value);
????}
????// 返回一個 Optional指定值的Optional，如果非空，則返回一個空的 Optional
????public?static? Optional ofNullable(T value)?{
????????return?value == null?? empty() : of(value);
????}
????// 如果Optional中有一個值，返回值，否則拋出 NoSuchElementException 。
????public?T get()?{
????????if?(value == null) {
????????????throw?new?NoSuchElementException("No value present");
????????}
????????return?value;
????}
????// 返回true如果存在值，否則為 false
????public?boolean?isPresent()?{
????????return?value != null;
????}
????// 如果存在值，則使用該值調(diào)用指定的消費者，否則不執(zhí)行任何操作。
????public?void?ifPresent(Consumersuper?T> consumer)?{
????????if?(value != null)
????????????consumer.accept(value);
????}
????// 如果一個值存在，并且該值給定的謂詞相匹配時，返回一個 Optional描述的值，否則返回一個空的 Optional
????public?Optional filter(Predicatesuper?T> predicate)?{
????????Objects.requireNonNull(predicate);
????????if?(!isPresent())
????????????return?this;
????????else
????????????return?predicate.test(value) ? this?: empty();
????}
????// 如果存在一個值，則應(yīng)用提供的映射函數(shù)，如果結(jié)果不為空，則返回一個 Optional結(jié)果的 Optional 。
????public Optional map(Functionsuper?T, ? extends U> mapper)?{
????????Objects.requireNonNull(mapper);
????????if?(!isPresent())
????????????return?empty();
????????else?{
????????????return?Optional.ofNullable(mapper.apply(value));
????????}
????}
????// 如果一個值存在，應(yīng)用提供的 Optional映射函數(shù)給它，返回該結(jié)果，否則返回一個空的 Optional 。
????public Optional flatMap(Functionsuper?T, Optional> mapper)?{
????????Objects.requireNonNull(mapper);
????????if?(!isPresent())
????????????return?empty();
????????else?{
????????????return?Objects.requireNonNull(mapper.apply(value));
????????}
????}
????// 如果值存在，就返回值，不存在就返回指定的其他值
????public?T orElse(T other)?{
????????return?value != null?? value : other;
????}
????public?T orElseGet(Supplier other)?{
????????return?value != null?? value : other.get();
????}
????public? T orElseThrow(Supplier exceptionSupplier)?throws?X {
????????if?(value != null) {
????????????return?value;
????????} else?{
????????????throw?exceptionSupplier.get();
????????}
????}
}

public?class?User?{
????private?String username;
????private?Integer age;
????private?User()?{
????} public?static?User of()?{
????????return?new?User();
????} private?User(String username, Integer age)?{
????????this.username = username;
????????this.age = age;
????} public?static?User of(String username, Integer age)?{
????????return?new?User(username, age);
????}}

public?static?void?main(String[] args) {
????// Optional類已經(jīng)成為Java 8類庫的一部分，在Guava中早就有了，可能Oracle是直接拿來使用了
????// Optional用來解決空指針異常，使代碼更加嚴(yán)謹(jǐn)，防止因為空指針NullPointerException對代碼造成影響
????String?msg = "hello";
????Optional<String> optional = Optional.of(msg);
????// 判斷是否有值，不為空
????boolean?present = optional.isPresent();
????// 如果有值，則返回值，如果等于空則拋異常
????String?value = optional.get();
????// 如果為空，返回else指定的值
????String?hi = optional.orElse("hi");
????// 如果值不為空，就執(zhí)行Lambda表達(dá)式
????optional.ifPresent(opt -> System.out.println(opt));
}