Introduce

In this article, I will try to give some examples of the different types of null checks or NPE (NullPointerException) avoidance techniques that is used.

java.util.Optional

A container object which may or may not contain a non-null value. If a value is present, isPresent() will return true and get() will return the value.

Optional.ofNullable()

Returns an Optional describing the given value, if non-null, otherwise returns an empty Optional.¹

...
return Optional.ofNullable(mongoClient.getDatabase("dbName")
 .getCollection("collectionName", Student.class)
 .find(eq("id", id))
 .first());
...

The returned value from this method will never be null. If it is null, the returned value will be Optional.empty().

Optional.orElseThrow()

If a value is present, returns the value, otherwise throws NoSuchElementException.

If you want to throw an exception you could just use orElseThrow().

...
 return studentDao.findStudentsById(id).orElseThrow(()-> new CustomException(STUDENT_NOT_FOUND)); 
...

Optional.orElse()

If a value is present, returns the value, otherwise returns other.¹

...
final var student = Student._builder_()  
.id(1L)  
.name("ege")  
.build();  
  
return studentDao.findStudentsById(id).orElse(student);
...

Optional.get()

If a value is present, returns the value, otherwise throws NoSuchElementException.

I mostly use get()in unit tests when I am sure that there is data returned from the method I called.

But I would not use it without isPresent() in the actual code, even if I am sure that there will be no null.

The value passed torElse()is also returned when there is student but no name to it.

@Override  
public String getStudentName(Long id) {  
return studentDao.findStudentsById(id)  
.map(Student::getName)  
.orElse("Hayley");  
}

apache.commons

For Collection instances: CollectionUtils.isEmpty() or CollectionUtils.isEmpty()
For Map instances : MapUtils.isEmpty() or MapUtils.isNotEmpty()
For String : StringUtils.isEmpty()or StringUtils.isNotEmpty()

My suggestion is to try to use the latest version of the tool classes.

Objects::nonNull

Returns true if the provided reference is non-null otherwise returns false

A method commonly used by individuals.

 final var list = Arrays._asList_(1, 2, null, 3, null, 4);  
   
 list.stream()  
 .filter(Objects::_nonNull_)  
 .forEach(System._out_::print);

requireNonNull

requireNonNull()

Checks that the specified object reference is not null and throws a customized _NullPointerException_if it is. This method is designed primarily for doing parameter validation in methods and constructors with multiple parameters.

requireNonNull：“this field is required, so if it is null; throw a NPE with “id is required” message”

requireNonNullElse()

Returns the first argument if it is non-null and otherwise returns the non-null second argument.

requireNonNullElse："this field is required, so if it is null; don’t throw an exception but set a default value for it"

requireNonNullElseGet()

Returns the first argument if it is non-null and otherwise returns the non-null value of supplier.get().

requireNonNullElseGet："this field is required, so if it is null; don’t throw exception but set a default value for it“

The difference from requireNonNullElse is that, this method expects Supplier as a second parameter.

NotNull, NotEmpty, NotBlank Annotations

@NotNull : “The annotated element must not be null. Accepts any type.”

@NotEmpty : _“The annotated element must not be null or empty. Supported types are CharSequence, Collection, Map, Array.”

@NotBlank: “The annotated element must not be null and must contain at least one non-whitespace character. Accepts CharSequence”

Below are some examples

@NotNull  
private Long id;  
@NotEmpty  
private String name;  
@NotEmpty  
private List<SchoolClass> classes;  
private Map<Integer, Teacher> teacherMap;

// `@Validated` annotation for the request body of API as below
@PostMapping()  
public void saveStudent(@Validated @RequestBody Student student){  
 studentService.saveStudent(student);  
}

Lombok’s Builder.Default

Personally, I have reservations about Lombok, sometimes it's convenient, but sometimes it's annoying.

Below is example：

You can use put @Builder.Default before the related field and give it a default value.

 @Getter
 
 @Setter
 
 @Builder
 
 @NoArgsConstructor
 
 @AllArgsConstructor
 
 public class Student {
 
 private Long id;
 
 @Builder.Default
 
 private String name = "hayley";
 
 @Builder.Default
 
 private List<SchoolClass> classes = new ArrayList<>();

When an instance of this Student class is created, it will have “classes” as an empty list, not null.

final var student1 = Student._builder_().build();  
final var student2 = new Student();  
final var student3 = Student._builder_().id(1L).name("ege").build();  
  
System._out_.println(student1.getClasses());  
System._out_.println(student2.getClasses());  
System._out_.println(student3.getClasses());  
  
System._out_.println(student1.getName());  
System._out_.println(student2.getName());  
System._out_.println(student3.getName());

The result:

[]  
[]  
[]  
hayley  
hayley  
ege