Java elegant empty judgment

Initialization

Optional provides three initialization methods:

@SpringBootTest
public class OptionalTest {<!-- -->

    @Test
    public void testOptional() {<!-- -->
Optional.empty();
        Optional.ofNullable(null);
        Optional.of(null);
    }
}

empty returns an empty Optional object.
of will report an error when encountering null, but the purpose of using Optional is to prevent null pointers, so ofNullable is generally used more often.

ifPresent

ifPresent–If exists then xxx

public void testOptional() {<!-- -->
        Student student = new Student();
        student.setName("feng");
        student.setAge(18);
        Optional.of(student);

        // Common practice
        if(student != null){<!-- -->
            System.out.println(student.getName());
        }
        
        // Optional common usage
        Optional<Student> optional = Optional.ofNullable(student);
        if(optional.isPresent()){<!-- -->
            Student student1 = optional.get();
            System.out.println(student1.getName());
        }

        // Correct usage
        optional.map(new Function<Student, String>() {<!-- -->
            @Override
            public String apply(Student student) {<!-- -->
                return student.getName();
            }
        }).ifPresent(System.out::println);

// Simplify
optional.map(Student::getName).ifPresent(System.out::println);
    }

orElse and orElseGet

If there is no object, a method that creates the object is called.
orElse is similar to Hungry style. If the current object is not empty, the current object is used. If it is empty, the method is called to create a new object.
orElseGet is similar to the lazy style. It will be called only when the current object is empty. OrElseGet is usually used more often.

The current object is not empty:

 public void testOptional() {<!-- -->
        Student student = new Student();
        student.setName("feng");
        student.setAge(18);
        Optional<Student> optional = Optional.ofNullable(student);
        Student student1 = optional.orElse(createStudent());
        // orElseGet receives a Suppilier, an anonymous inner class, and calls the get method to process the logic
        Student student2 = optional.orElseGet(new Supplier<Student>() {<!-- -->
            @Override
            public Student get() {<!-- -->
                return createStudent();
            }
        });
    }

    private Student createStudent(){<!-- -->
        System.out.println("create student");
        return new Student();
    }

The current object is empty:

 public void testOptional() {<!-- -->
        Student student = null;
        Optional<Student> optional = Optional.ofNullable(student);
        Student student1 = optional.orElse(createStudent());
        // orElseGet receives a Suppilier, an anonymous inner class, and calls the get method to process the logic
        Student student2 = optional.orElseGet(new Supplier<Student>() {<!-- -->
            @Override
            public Student get() {<!-- -->
                return createStudent();
            }
        });
    }

    private Student createStudent(){<!-- -->
        System.out.println("create student");
        return new Student();
    }

Filter

 public void testOptional() {<!-- -->
        Student student = new Student();
        student.setName("feng");
        student.setAge(18);
        Optional<Student> optional = Optional.ofNullable(student);
        optional.filter(student1 -> student1.getAge() > 1).ifPresent(student1 -> System.out.println(student1.getAge()));
    }

map and flatMap

Optional is a container class in Java used to handle values that may or may not contain values. Optional provides several methods to operate on these values, including map and flatMap.

1. Optional.map：
   • The map method is used to convert or operate on the Optional if it contains a value.
   • If the Optional contains a value, map will pass the value to the provided function and return a new Optional containing the result of the conversion.
   • If Optional is empty, map does not perform any operation and directly returns an empty Optional.
   • The signature of map is as follows: Optional map(Function mapper)

Optional<String> optional = Optional.of("Hello");
Optional<Integer> lengthOptional = optional.map(s -> s.length());

Optional<String> optional = Optional.of("Hello");
Optional<Integer> lengthOptional = optional.flatMap(s -> Optional.of(s.length()));

//Normal
public void before8(List<Student> Students) {<!-- -->
    for (Student student:Students) {<!-- -->
        if (student != null){<!-- -->
            if (student.getAge() > 18){<!-- -->
                Integer score = student.getScore();
                if (score != null & amp; & amp; score > 80){<!-- -->
                    System.out.println(student.getName());
                }
            }
        }
    }
}



//1 
public void after8(List<Student> students) {<!-- -->
   students.stream()
           .filter(Objects::nonNull) // Filter out empty student objects
           .filter(student -> student.getAge() > 18) // Filter by age
           .map(Student::getScore) //Map to score
           .filter(score -> score != null & amp; & amp; score > 80) // Filter by score
           .map(Student::getName) //Map to name
           .ifPresent(System.out::println); //Print if name exists
}

public void after8(List<Student> Students) {<!-- -->
        for (Student student:Students) {<!-- -->
            Optional<Student> studentOptional = Optional.of(student);
\t\t\t
// 2
         Integer sorce = studentOptional
                 .filter(s -> s.getAge() > 18)
                 .map(Student::getScore)
                 .orElse(0);//.orElse(0) returns a default value to prevent null pointer control
         if (sorce > 80){<!-- -->
             System.out.println(student.getName());
         }


// 3
         String name = studentOptional
                 .filter(s -> s.getAge() >= 18)
                 .filter(s -> s.getScore() > 80)
                 .map(Student::getName)
                 .orElse("wu");//.orElse(0) returns a default value to prevent null pointer control
         System.out.println(name);
     }
    }