Shulou(Shulou.com)05/31 Report--

This article mainly introduces "how to use java.util.Optional". In daily operation, I believe many people have doubts about how to use java.util.Optional. The editor consulted all kinds of materials and sorted out simple and easy-to-use operation methods. I hope it will be helpful to answer the doubts about "how to use java.util.Optional". Next, please follow the editor to study!

First kind。 Optional.of (Object object): the input parameter object cannot be empty, otherwise a null pointer exception will be thrown. If you look at the Optional source code, you will find that the Objects.requireNonNull method will be called.

Public static T requireNonNull (T obj) {if (obj = = null) throw new NullPointerException (); return obj;}

The second kind. Optional.ofNullable (Object object): the input parameter object can be empty. If object is not empty, create an Optional instance. If object is empty, return an Option object of static fainal. Note that instead of creating a new Option instance, you will use an instance of static final EMPTY. What is interesting here is the problem of generics. For example, I need two Optional objects with the types String and Integer, and the code is as follows:

Optional optionalStr = Optional.ofNullable (null); Optional optionalInt = Optional.ofNullable (null)

How do you ensure that the same object is returned with different types? Looking directly at the source code of ofNullable, you can find that the empty method will be called:

Public static Optional empty () {@ SuppressWarnings ("unchecked") Optional t = (Optional) EMPTY; return t;}

It turned out to be implemented through a cast, and then look at the EMPTY object:

Private static final Optional EMPTY = new Optional ()

Is through "?" Declared

The third kind. Optional.empty (): this is the empty method used in the above analysis of Optional.ofNullable, which directly returns an instance of static final EMPTY

The use of the Optional.of () method is a bit like assertion. When the object is empty, it represents some business unacceptable exception that needs to be handled as soon as possible, and the business refuses to execute. In this scenario, Optional.of can be used.

Next, let's start the actual combat.

# object used in the example: Student### Student is a normal bean with three fields and corresponding get&set methods

Package com.bolingcavalry;/** * @ author willzhao * @ version V1.0 * @ Description: an ordinary bean * @ email zq2599@gmail.com * @ Date 11:23 on 2017-8-26 * / public class Student {private int id; private String name; private int age; public int getId () {return id;} public void setId (int id) {this.id = id } 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 Student (int id, String name, int age) {this.id = id; this.name = name This.age = age;}}

# usage of Optional.ofNullable # # the following example illustrates the most commonly used Optional.ofNullable. We intend to obtain the student object from other systems according to its name. If the object is empty, the default object is returned. First, let's see what we usually write when we don't use Optional. As shown in the following code, the standard if&else judgment:

Private Student queryById (int id) {/ / TODO here simulates querying return null;} public Student getStudent (int id) {Student student= queryById (id) from the database; / / returns the DEFAULT object return student==null if empty? DEFAULT: student;}

After using Optional, as shown below, there is no need to avoid null pointer exceptions by null:

Private Student queryById (int id) {/ / TODO here simulates querying return null;} public Student getStudent (int id) {Optional optional = Optional.ofNullable (queryById (id)) from the database; / / returns the DEFAULT object return optional.orElse (DEFAULT) if empty;}

The orElse method can specify a return object when value is empty. If the object needs to be obtained by calling the method (for example, if we cannot get the DEFAULT object, we can only get it through the getDefault () method), we need the orElseGet method to achieve the goal, as shown below:

Private Student queryById (int id) {/ / TODO here simulates querying return null;} private Student getDefault () {return DEFAULT;} public Student getStudent (int id) {Optional optional = Optional.ofNullable (queryById (id)) from the database; / / returns the DEFAULT object return optional.orElseGet (()-> getDefault ()) if empty;}

# map method of Optional # # if our requirement is that the student object is not empty, return the uppercase of name, and if the student object is empty, return "invalid". Write as follows when there is no Optional. In addition to checking whether the student variable is empty, check whether the name is empty:

Private Student queryById (int id) {/ / TODO here simulates querying return null;} public String getStudentUpperName (int id) {Student student = queryById (id) from the database; if (studentautomatically null & student.getName ()! = null) {return student.getName (). ToUpperCase ();} return "invalid";}

With Optional, you can write:

Private Student queryById (int id) {/ / TODO here simulates querying return null;} public String getStudentUpperName (int id) {Optional optional = Optional.ofNullable (queryById (id)); return optional.map (student-> student.getName ()) .map (name-> name.toUpperCase ()) .orElse ("invalid");}

As you can see from the above code, map can convert one Optional object to another, the first time it converts Optional to Optional, the second time it converts Optional into another Optional, but this time it converts the string to uppercase

The source code of this actual combat has been uploaded to git, the address is git@github.com:zq2599/blog_demos.git, and there are several projects in it. Optionaldemo is used this time, as shown in the red box below:

At this point, the study on "how to use java.util.Optional" is over. I hope to be able to solve your doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.