java8中使用Optional容器类来尽可能的避免或快速定位空指针异常
Optional类的基本使用
public class Person { private Integer age; private Double salary; public Person() { } public Person(Integer age, double salary) { this.age = age; this.salary = salary; } public Integer getAge() { return age; } public void setAge(Integer age) { this.age = age; } public double getSalary() { return salary; } public void setSalary(double salary) { this.salary = salary; } @Override public String toString() { return "Person{" + "age=" + age + ", salary=" + salary + '}'; } } //of方法 创建一个Optional对象 Optional<Person> of = Optional.of(new Person()); Optional<Person> of1 = Optional.of(null); System.out.println(of.get()); System.out.println(of1.get());//java.lang.NullPointerException //empty方法 创建一个空的Optional实例 Optional<Person> empty = Optional.empty(); System.out.println(empty.get()); //java.util.NoSuchElementException: No value present //ofNullable(T t) 如果T不为空，创建Optional实例，如果为空，创建空实例，下面就是这个方法的实现，综合了上面两个方法 // public static <T> Optional<T> ofNullable(T value) { // return value == null ? empty() : of(value); // } Optional<Person> ofNullable = Optional.ofNullable(new Person()); //isPersent方法 判断是否包含空值 Optional<Person> isPer= Optional.ofNullable(new Person()); //如果不为空 输出 if(isPer.isPresent()){ System.out.println(isPer.get()); } //orElse(T t) 如果调用对象包含值 返回该值，否则返回t Optional<Person> orEl = Optional.ofNullable(null); System.out.println(orEl.orElse(new Person(12, 222))); //输出 Person{age=12, salary=222.0} //orElseGet(Supplier s) 如果调用对象包含值 返回该值，否则返回s获取的值 Optional<Object> orEG = Optional.ofNullable(null); System.out.println(orEG.orElseGet(() -> new Integer(33))); //输出33 //map(Function f) 如果有值 对其处理，并返回处理后的Optional，否则返回Optional.empty() Optional<Person> oMap = Optional.ofNullable(new Person(12, 2222)); Optional<Integer> oAge = oMap.map(e -> e.getAge()); System.out.println(oAge.get()); //输出12 //flatMap(Function mapper) 与map类似，就是返回值必须是Optional 进一步避免空指针异常 Optional<Person> oFlatMap = Optional.ofNullable(new Person(12, 2222)); Optional<Integer> oFlatAge = oMap.flatMap(e -> Optional.of(e.getAge())); System.out.println(oAge.get()); //输出12
这里放一个例子
有一个Teacher类，Teacher中有一个Student属性，
public class Teacher { private Optional<Student> student = Optional.empty(); public void setStudent(Optional<Student> student) { this.student = student; } public Optional<Student> getStudent() { return student; } }
Student类有一个name属性
public class Student { private String name; public Student(String name) { this.name = name; } public String getName() { return name; } public void setName(String name) { this.name = name; } }
使用Optional容器来避免空指针异常
public static void main(String[] args) { Optional<Teacher> op = Optional.ofNullable(null); String studentName = getStudentName(op); System.out.println(studentName); } public static String getStudentName(Optional<Teacher> teacher){ return teacher.orElse(new Teacher()) .getStudent() .orElse(new Student("小明")) .getName(); }
