Optional Class

OptionalClass is a container object may be a null. If the value exists isPresent()method returns true, call the get()method returns the object.

OptionalA container: it can hold the value of type T, or just to save null. OptionalOffers many useful ways, so we do not explicitly be null detection.

OptionalThe introduction of class a good solution to a null pointer exception.

① test

public class OptionalTest {
    
    public static void main(String[] args) {
        OptionalTest test = new OptionalTest();
        Integer val1 = null;
        Integer val2 = new Integer(10);
        //Optional.ofNullable - 允许传递为 null 参数
        Optional<Integer> op1 = Optional.ofNullable(val1);
        //Optional.of - 如果传递的参数是 null，抛出异常 NullPointerException
        Optional<Integer> op2 = Optional.of(val2);
        System.out.println(test.sum(op1,op2));
    }
    
    public Integer sum(Optional<Integer> a, Optional<Integer> b) {
        //Optional.isPresent()：判断值是否为null
        System.out.println("参数1的值是否为null：" + a.isPresent());
        System.out.println("参数2的值是否为null：" + b.isPresent());
        //Optional.orElse - 如果值存在，返回它，否则返回默认值
        Integer val1 = a.orElse(0);
        //Optional.get - 获取值，值需要存在
        Integer val2 = b.get();
        return val1+val2;
    }
}

② class method

No.	Method and Description
1	static <T> Optional<T> empty()Optional return empty instance.
2	boolean equals(Object obj)Determining whether another object is equal Optional.
3	Optional<T> filter(Predicate<? super <T> predicate)If the value is present, and this value matches the given predicate, returns a value to describe Optional otherwise an empty Optional.
4	<U> Optional<U> flatMap(Function<? super T,Optional<U>> mapper)If the value is present, the return value mapping method comprising Optional based, otherwise an empty Optional
5	T get()If you include the value in this Optional, the return value, otherwise an exception is thrown: NoSuchElementException
6	int hashCode()Returns the hash code value exists, returns 0 if the value does not exist.
7	void ifPresent(Consumer<? super T> consumer)If the value exists then use that value to call consumer, or do nothing.
8	boolean isPresent()Returns true if the value of the method exists, otherwise false.
9	<U>Optional<U> map(Function<? super T,? extends U> mapper)If there is value, then calling the mapping function to get its return value. If the return value is not null, then create a map that contains the return value of Optionala mapmethod returns a value, otherwise return empty Optional.
10	static <T> Optional<T> of(T value)Optional returns a non-null value specified.
11	static <T> Optional<T> ofNullable(T value)If non-empty, return Optional The described specified value, otherwise empty Optional.
12	T orElse(T other)If the value is present, the return value, otherwise other.
13	T orElseGet(Supplier<? extends T> other)If the value is present, return values, or trigger other, and returns the result other call.
14	<X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier)If the value is present, the value contained, otherwise an exception is thrown inherited by Supplier
15	String toString()Optional returns a non-null string, used to debug