java小技巧（三）--Optional

1.Optional.of()或者Optional.ofNullable()
创建Optional对象，差别在于of不允许参数是null，而ofNullable则无限制。

// 参数不能是null
Optional<Integer> optional1 = Optional.of(1);
 
// 参数可以是null
Optional<Integer> optional2 = Optional.ofNullable(null);

2.Optional.empty()
所有null包装成的Optional对象：

Optional<Integer> optional1 = Optional.ofNullable(null);
Optional<Integer> optional2 = Optional.ofNullable(null);
System.out.println(optional1 == optional2);// true
System.out.println(optional1 == Optional.<Integer>empty());// true
 
Object o1 = Optional.<Integer>empty();
Object o2 = Optional.<String>empty();
System.out.println(o1 == o2);// true

3.isPresent()
判断值是否存在

Optional<Integer> optional2 = Optional.ofNullable(null);// isPresent判断值是否存在
System.out.println(optional1.isPresent() == true);
System.out.println(optional2.isPresent() == false);

4.ifPresent(Consumer consumer)
如果option对象保存的值不是null，则调用consumer对象，否则不调用

Optional<Integer> optional1 = Optional.ofNullable(1);
Optional<Integer> optional2 = Optional.ofNullable(null);
 
// 如果不是null,调用Consumer
optional1.ifPresent(new Consumer<Integer>() {
    @Override
    public void accept(Integer t) {
        System.out.println("value is " + t);
    }
});
 
// null,不调用Consumer
optional2.ifPresent(new Consumer<Integer>() {
    @Override
    public void accept(Integer t) {
        System.out.println("value is " + t);
    }
});

5.orElse(value)
如果optional对象保存的值不是null，则返回原来的值，否则返回value

Optional<Integer> optional1 = Optional.ofNullable(1);
Optional<Integer> optional2 = Optional.ofNullable(null);
 
// orElse
System.out.println(optional1.orElse(1000) == 1);// true
System.out.println(optional2.orElse(1000) == 1000);// true

6.orElseGet(Supplier supplier)
功能与orElse一样，只不过orElseGet参数是一个对象

Optional<Integer> optional1 = Optional.ofNullable(1);
Optional<Integer> optional2 = Optional.ofNullable(null);
 
System.out.println(optional1.orElseGet(() -> {
    return 1000;
}) == 1);//true
 
System.out.println(optional2.orElseGet(() -> {
    return 1000;
}) == 1000);//true

7.orElseThrow()

值不存在则抛出异常，存在则什么不做，有点类似Guava的Precoditions

Optional<Integer> optional1 = Optional.ofNullable(1);
Optional<Integer> optional2 = Optional.ofNullable(null);
 
optional1.orElseThrow(()->{throw new IllegalStateException();});
 
try
{
    // 抛出异常
    optional2.orElseThrow(()->{throw new IllegalStateException();});
}
catch(IllegalStateException e )
{
    e.printStackTrace();
}

8.filter(Predicate)
判断Optional对象中保存的值是否满足Predicate，并返回新的Optional。

Optional<Integer> optional1 = Optional.ofNullable(1);
Optional<Integer> optional2 = Optional.ofNullable(null);
 
Optional<Integer> filter1 = optional1.filter((a) -> a == null);
Optional<Integer> filter2 = optional1.filter((a) -> a == 1);
Optional<Integer> filter3 = optional2.filter((a) -> a == null);
System.out.println(filter1.isPresent());// false
System.out.println(filter2.isPresent());// true
System.out.println(filter2.get().intValue() == 1);// true
System.out.println(filter3.isPresent());// false

9.map(Function)
对Optional中保存的值进行函数运算，并返回新的Optional(可以是任何类型)

Optional<Integer> optional1 = Optional.ofNullable(1);
Optional<Integer> optional2 = Optional.ofNullable(null);
 
Optional<String> str1Optional = optional1.map((a) -> "key" + a);
Optional<String> str2Optional = optional2.map((a) -> "key" + a);
 
System.out.println(str1Optional.get());// key1
System.out.println(str2Optional.isPresent());// false

10.flatMap()
功能与map()相似，差别请看如下代码。flatMap方法与map方法类似，区别在于mapping函数的返回值不同。map方法的mapping函数返回值可以是任何类型T，而flatMap方法的mapping函数必须是Optional。

Optional<Integer> optional1 = Optional.ofNullable(1);
 
Optional<Optional<String>> str1Optional = optional1.map((a) -> {
    return Optional.<String>of("key" + a);
});
 
Optional<String> str2Optional = optional1.flatMap((a) -> {
    return Optional.<String>of("key" + a);
});
 
System.out.println(str1Optional.get().get());// key1
System.out.println(str2Optional.get());// key