JDK8新特性之Optional
Optional 類主要解決的問(wèn)題是臭名昭著的空指針異常(NullPointerException），Optional<T>類在java.util包下屋确，是一個(gè)容器類纳击，它可以保存類型T的值，代表這個(gè)值存在攻臀；或者僅僅保存null焕数，表示這個(gè)值不存在。原來(lái)用null表示一個(gè)值不存在刨啸，現(xiàn)在Optional可以更好的表達(dá)這個(gè)概念堡赔，并且可以避免空指針異常。
Optional類的javadoc描述如下：這是一個(gè)可以為null的容器對(duì)象设联，如果值存在則ispresent()會(huì)返回true善已，調(diào)用get()方法會(huì)返回該對(duì)象。
Optional類方法介紹
先創(chuàng)建一個(gè)People類仑荐，在接下來(lái)的測(cè)試中會(huì)用到該類
public class People {
private String name;
private int age;
private People friend;
public People() {
}
public People(String name, int age, People friend) {
this.name = name;
this.age = age;
this.friend = friend;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public People getFriend() {
return friend;
}
public void setFriend(People friend) {
this.friend = friend;
}
@Override
public String toString() {
return "People{" +
"name='" + name + '\'' +
", age=" + age +
", friend=" + friend +
'}';
}
}
創(chuàng)建Optional類對(duì)象的方法
- Optional.of(T t)：創(chuàng)建一個(gè)Optional實(shí)例雕拼，t必須非空
- Optional.empty()：創(chuàng)建一個(gè)空的Optional實(shí)例
- Optional.ofNullable(T t)：t可以為null
@Test
public void test01(){
// Optional.of(T t)：創(chuàng)建一個(gè)Optional實(shí)例，t必須非空
People people = new People();
people.setName("張三");
people.setAge(18);
// 如果這行代碼放開(kāi)運(yùn)行會(huì)報(bào)NullPointerException錯(cuò)吊输，因?yàn)閜eople為Null
// people = null;
Optional<People> peopleOptional = Optional.of(people);
System.out.println(peopleOptional);// 結(jié)果：Optional[People{name='張三', age=18, friend=null}]
}
@Test
public void test02(){
// Optional.ofNullable(T t)：t可以為null
People people = new People();
people.setName("李四");
people.setAge(22);
// 如果people=null剩拢，則
// people = null;
Optional<People> peopleOptional = Optional.ofNullable(people);
/**
* people不為空時(shí)結(jié)果：Optional[People{name='李四', age=22, friend=null}]
* people為空時(shí)的結(jié)果：Optional.empty
*/
System.out.println(peopleOptional);
}
@Test
public void test03(){
// Optional.empty()：創(chuàng)建一個(gè)空的Optional實(shí)例
Optional emptyPeople = Optional.empty();
System.out.println(emptyPeople);// 結(jié)果：Optional.empty
/**
* Optional.empty()相當(dāng)于將以前的null 包裝了一下
* 以前是people == null
* 我們結(jié)合Optional.ofNullable來(lái)使用一下
*/
People people = null;
Optional<People> peopleOptional = Optional.ofNullable(people);
Optional<People> empty = Optional.empty();
if(peopleOptional == empty){
System.out.println("true");// 輸出true
}
}
判斷Optional容器是否包含對(duì)象
- boolean isPresent()：判斷是否包含對(duì)象
- void ifPresent(Consumer<? super T> consumer)：如果有值，就執(zhí)行consumer接口的實(shí)現(xiàn)代碼，并且該值會(huì)作為參數(shù)傳遞給它
@Test
public void test04(){
// boolean isPresent()：判斷是否包含對(duì)象
People people = new People();
people.setName("李四");
people.setAge(22);
Optional<People> peopleOptional1 = Optional.ofNullable(people);
System.out.println(peopleOptional1.isPresent());// 結(jié)果：true
people = null;
Optional<People> peopleOptional2 = Optional.ofNullable(people);
System.out.println(peopleOptional2.isPresent());// 結(jié)果：false
// void ifPresent(Consumer<? super T> consumer)：如果有值父叙，就執(zhí)行consumer接口的實(shí)現(xiàn)代碼，并且該值會(huì)作為參數(shù)傳遞給它
// 加入People存在則取輸出它的朋友信息
People people2 = new People("王五",23,new People("小明",21,null));
Optional<People> optionalPeople2 = Optional.ofNullable(people2);
optionalPeople2.ifPresent(p -> System.out.println(p.getFriend()));// 結(jié)果：People{name='小明', age=21, friend=null}
}
獲取Optional容器的對(duì)象
- T get()：如果調(diào)用對(duì)象包含值练俐，返回該值注暗，否則拋異常
- T orElse(T other)：如果有值則將其返回，否則返回指定的other對(duì)象
- T orElseGet(Supplier<? extends T> other)：如果有值則將其返回猫牡，否則返回由Supplier接口實(shí)現(xiàn)提供的對(duì)象
- T orElseThrow(Supplier<? extends X> exceptionSupplier)：如果有值則將其返回胡诗，否則拋出由Supplier接口實(shí)現(xiàn)提供的異常
@Test
public void test05(){
// T get()：如果調(diào)用對(duì)象包含值，返回該值淌友，否則拋異常
People people = new People("詹姆斯",36,null);
// people = null;
Optional<People> optionalPeople = Optional.ofNullable(people);
// 當(dāng)people為null時(shí)拋異常：java.util.NoSuchElementException: No value present
People result = optionalPeople.get();
System.out.println(result);// 結(jié)果是:People{name='詹姆斯', age=36, friend=null}
}
@Test
public void test06(){
People people = new People("詹姆斯",36,null);;
// people = null;
Optional<People> optionalPeople = Optional.ofNullable(people);
// T orElse(T other)：如果有值則將其返回煌恢，否則返回指定的other對(duì)象
People result = optionalPeople.orElse(new People("韋德", 28, null));
/**
* people不為null時(shí)返回people：People{name='詹姆斯', age=36, friend=null}
* 為null時(shí)返回自己設(shè)置的默認(rèn)對(duì)象：People{name='韋德', age=28, friend=null}
*/
System.out.println(result);
// T orElseGet(Supplier<? extends T> other)：如果有值則將其返回，否則返回由Supplier接口實(shí)現(xiàn)提供的對(duì)象
People result2 = optionalPeople.orElseGet(() -> new People("保羅", 38, null));
/**
* people不為null時(shí)返回people：People{name='詹姆斯', age=36, friend=null}
* 為null時(shí)返回自己設(shè)置的默認(rèn)對(duì)象：People{name='保羅', age=38, friend=null}
*/
System.out.println(result2);
}
@Test
public void test07(){
// T orElseThrow(Supplier<? extends X> exceptionSupplier)：如果有值則將其返回震庭，否則拋出由Supplier接口實(shí)現(xiàn)提供的異常
// 這里其實(shí)和上面的orElseGet有點(diǎn)類似瑰抵，只不過(guò)這里是返回異常對(duì)象
// 這個(gè)主要是用于可以拋出指定異常
People people = new People("詹姆斯",36,null);;
people = null;
Optional<People> optionalPeople = Optional.ofNullable(people);
People result = optionalPeople.orElseThrow(() -> new RuntimeException("值不存在"));
System.out.println(result);
}
總結(jié)
Optional類主要的作用其實(shí)是為了消除空指針，雖然并不能完全消除但是給我們帶來(lái)一些更方便的操作器联，比如為空時(shí)設(shè)置默認(rèn)值二汛，為空時(shí)拋出指定異常等婿崭。
