JDK1.8 series articles

• New in JDK1.8 (1) : Lambda expressions
• New in JDK1.8 (2) : Optional
• JDK1.8 new feature (3) : Stream
• New in JDK1.8 (4) : Maps
• New JDK1.8 feature (5) : new date and time API

Nullpointerexceptions are ubiquitous in our development. We often need defensive checks to avoid nullpointerexceptions, so we see them in our code. = null). Fortunately, in JDK1.8, Java provides us with an Optional class, which allows us to avoid the complicated non-empty judgment. Let’s take a quick look at the methods provided in Optional:

The Optional class provides an Optional method. I’m not afraid of NullPointerException ** finding me in my code anymore.

Optional class

Let’s write a few examples to see what each method does:

One of them,

If the specified value is null, a NullPointerException is thrown

// Create a String ofOptional Optional<String> ofOptional = option.of (" span "); Optional<String> nullOptional = Optional. Of (null);Copy the code

Output:

java.lang.NullPointerException at java.util.Objects.requireNonNull(Objects.java:203) at Java. Util. Optional. < init > (96) Optional. Java: · · · · · ·Copy the code

// Create Optional<String> nullOptional1 = Optional. OfNullable (null); // Create Optional<String> nullOptional1 = Optional. Optional<String> nullOptional2 = Optional.ofNullable("list");Copy the code

Third, the empty

Create an empty Optional object

// Create an emptyOptional String object of type Optional<String> emptyOptional = option.empty ();Copy the code

Optional<String> stringOptional = Optional.of("Zhang");
System.out.println(stringOptional.get());
Copy the code

Zhang SANCopy the code

Optional<String> emptyOptional = Optional.empty();
System.out.println(emptyOptional.get());
Copy the code

java.util.NoSuchElementException: No value present at java.util.optional. Get (Optional. Java :135) at com.***.test.test4 (test.java :93)Copy the code

Fifth, orElse

Return this value if the Optional value exists, otherwise a default value is returned

Optional<String> stringOptional = Optional. Of (" zhang 3 "); System.out.println(stringOptional.orElse("zhangsan")); Optional<String> emptyOptional = Optional.empty(); System. The out. Println (emptyOptional. OrElse (" li si "));Copy the code

Output:

Tom, dick and harryCopy the code

Optional<String> stringOptional = Optional. Of (" zhang 3 "); System.out.println(stringOptional.orElseGet(() -> "zhangsan")); Optional<String> emptyOptional = Optional.empty(); System.out.println(emptyOptional.orElseGet(() -> "orElseGet"));Copy the code

Zhang SAN orElseGetCopy the code

Seven, orElseThrow

Return the value of Optional if it exists, otherwise raise an exception generated by the specified Supplier interface first by defining an exception class

class CustomException extends RuntimeException { private static final long serialVersionUID = -4399699891687593264L; Public CustomException() {super(" CustomException "); } public CustomException(String message) { super(message); }}Copy the code

The test code is as follows

Optional<String> stringOptional = Optional. Of (" zhang 3 "); System.out.println(stringOptional.orElseThrow(CustomException::new)); Optional<String> emptyOptional = Optional.empty(); System.out.println(emptyOptional.orElseThrow(CustomException::new));Copy the code

Output:

com.***.Test$CustomException: Custom exception at com. * * *. The Test. The lambda $test4 $1 (JiankeTest. Java: 96) at Java. Util. Optional. The orElseThrow ats (Optional. Java: 290) Com. * * *. Test. Test4 (96) JiankeTest. Java: · · · · · ·Copy the code

Optional<String> stringOptional = Optional.of("zhangsan"); System.out.println(stringOptional. Filter (e -> e.long () > 5).orelse ()); stringOptional = Optional.empty(); System.out.println(stringOptional.filter(e -> e.length() > 5).orElse("lisi"));Copy the code

zhangsan
lisi
Copy the code

Nine, the map

If the value in the Optional creation exists, the supplied Function call is made to that value

// The map method executes the lambda argument passed in and changes the value of the Optional instance. The returned value is still an Optional object Optional<String> stringOptional = Optional.of("zhangsan"); System.out.println(stringOptional. Map (e -> e.topperCase ()).orelse (" failed ")); stringOptional = Optional.empty(); System.out.println(stringOptional. Map (e -> e.topperCase ()).orelse (" failed "));Copy the code

Output:

ZHANGSAN failureCopy the code

// The return value of the lambda expression in the map method can be any type, and is wrapped as Optional before the map function returns. // But the lambda expression in the flatMap method must return an Optionl instance Optional<String> stringOptional = option. of("zhangsan"); System.out.println(stringOptional. FlatMap (e -> Optional. Of ("lisi")).orelse (" fail ")); stringOptional = Optional.empty(); System.out.println(stringoption.flatmap (e -> option.empty ()).orelse (" fail "));Copy the code

Lisi failureCopy the code

Eleven, isPresent

Return true if the value in Optional is present, false otherwise

Optional<String> stringOptional = Optional.of("zhangsan");
System.out.println(stringOptional.isPresent());

Optional<String> stringOptional2 = Optional.empty();
System.out.println(stringOptional2.isPresent());
Copy the code

Output:

true
false
Copy the code

// The ifPresent method takes an implementation class of the Consumer. The Consumer class contains an abstract method that handles the value passed in but does not return a value. Optional<String> stringOptional = Optional.of("zhangsan"); StringOptional. IfPresent (e -> system.out.println) stringOptional. IfPresent (e -> System. + e));Copy the code

I was dealt with... zhangsanCopy the code

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