This article brings you an introduction to the usage of Java8’s new feature Optional (with code). It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you. helped.

Optional class (java.util.Optional) is a container class that represents the presence or absence of a value. Originally, null was used to represent the absence of a value. Now Optional can better express this concept. And can avoid null pointer exception.

1, of

Create an Optional for a non-null value. If the value is null, a NullPointerException will be thrown

Optional<Integer> optional = Optional.of(13);
System.out.println(optional)

2, ofNullable

Creates an Optional for the specified value, or returns an empty Optional if the specified value is null.

Optional<String> optional2 = Optional.ofNullable(null);
System.out.println(optional2);
optional2 = Optional.ofNullable("aaa");
System.out.println(optional2);

3. isPresent

Returns true if the value exists, otherwise returns false.

Optional<String> optional2 = Optional.ofNullable(null);
System.out.println(optional2);
System.out.println(optional2.isPresent());

optional2 = Optional.ofNullable("aaa");
System.out.println(optional2);
System.out.println(optional2.isPresent());

4, get

If Optional has a value, return it, otherwise throw NoSuchElementException.

optional2 = Optional.ofNullable("aaa");
System.out.println(optional2);
System.out.println(optional2.isPresent());
System.out.println(optional2.get());

5. ifPresent

If the Optional instance has a value, call the consumer for it, otherwise no processing will be done. Hermit made a null judgment among them.

public static void main(String[] args) {
    String str1 = "hello";
    String str2 = null;
    Optional<String> optional1 = Optional.ofNullable(str1);
    Optional<String> optional2 = Optional.ofNullable(str2);

    optional1.ifPresent(str->System.out.println(str.length()));
    optional2.ifPresent(str-> System.out.println(str.length()));
}

6, orElse

If there is a value, return it, otherwise return the specified other value.

public static void main(String[] args) {
    String str1 = "hello";
    String str2 = null;
    Optional<String> optional1 = Optional.ofNullable(str1);
    Optional<String> optional2 = Optional.ofNullable(str2);
    System.out.println(optional1.orElse("默认字符串"));
    System.out.println(optional2.orElse("默认字符串"));
}

7, orElseGet

orElseGet is similar to the orElse method, the difference lies in the default value obtained. The orElse method takes an incoming value as the default value. The orElseGet method can accept the implementation of the Supplier interface to generate the default value

public static void main(String[] args) {
    Integer i = 20;
    Optional<Integer> op = Optional.ofNullable(i);
    System.out.println(op.orElseGet(()->0));
    System.out.println(Optional.ofNullable(null).orElseGet(()->0));
}

8, orElseThrow

If there is a value, it will be returned, otherwise Throws an exception created by the supplier interface.

public static void main(String[] args) {
    // 自己指定异常
    System.out.println((Optional.ofNullable("a").orElseThrow(()->new IndexOutOfBoundsException())));
    System.out.println((Optional.ofNullable(null).orElseThrow(()->new IndexOutOfBoundsException())));
}

9, map

map handles the situation where there is a value. If there is a value, execute the function in the map parameter to get the return value, otherwise it returns an empty Optional.

public static void main(String[] args) {
   List<String> list = new ArrayList<String>(){
       {
            add("aaa");
            add("bbb");
            add(null);
       }
   };

   list.stream().forEach(temp ->{
        Optional<String> opt = Optional.ofNullable(temp);
        System.out.println(opt.map(n -> {
           return n.toUpperCase();
        }).orElse("Nothing"));
   });

   // 简写为如下形式
    list.stream().forEach(temp->{
        System.out.println(Optional.ofNullable(temp).map(n->n.toUpperCase()).orElse("Nothing"));
    });
}

10. flatMap

If there is a value, execute the mapping function to return an Optional type return value, otherwise return an empty Optional. flatMap is similar to the map(Funtion) method, the difference is that the mapper return value in flatMap must be Optional. At the end of the call, flatMap will not encapsulate the result with Optional. You can add content

public static void main(String[] args) {
    Optional<String> opt1 = Optional.ofNullable("aaa");
    Optional<String> opt2 = Optional.ofNullable(null);
    System.out.println(opt1.flatMap(n->{
        return Optional.ofNullable(n.toUpperCase());
    }).orElse("还会没有吗"));

    System.out.println( opt2.flatMap(n->{
        return Optional.ofNullable(n.toUpperCase());
    }).orElse("还会没有吗"));
}

11, filter

If there is a value and the assertion condition is met, an Optional containing the value is returned, otherwise an empty Optional is returned.

public static void main(String[] args) {
    Optional<String> opt1 = Optional.ofNullable("aabbbbaa");
    Optional<String> opt2 = Optional.ofNullable(null);

    System.out.println(opt1.filter(value->value.length()>6));
    System.out.println(opt2.filter(value->value.length()>6));
}

Okay, this is where the new feature of Java8, Optional, is introduced.

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