遇到空指针(npe)切莫慌 ✨ jdk 8中有妙招 ✨ 每日积累

创建Optional对象的方法：
 Optional.of(T t):创建一个Optional实例，t必须非空
 Optional.empty():创建一个空Optional实例
 Optional.ofNullable(T t):创建一个Optional实例，t可以为null
判断Optional对象中是否有对象：
 boolean isPresent():判断Optional容器中是否有对象
 void ifPresent(Consumer<? super T> consumer):如果存在一个值，则应用提供的映射函数，如果结果不为空，则返回一个 Optional结果的 Optional。
获取Optional容器中的对象：
 T get():如果调用对象包含值，返回该值，否则抛异常
 T orElse(T t1):如果调用容器中有值，则返回容器中的值，否则返回t1对象
 T orElseGet(Supplier<? extends T> other):如果有值则将其返回，否则返回由Supplier接口实现提供的对象
 T orElseThrow(Supplier<? extends X> exceptionSupplier):如果有值则返回，否则抛出由Supplier接口实现提供的异常

import java.util.Optional;
import java.util.function.Supplier;
public class TestOptional {
    public static void main(String[] args) {
        UserTest userTest = new UserTest();
        userTest.setUserId("1");
        //isPresent()：判断Optional容器中是否有对象
        String userName = userTest.getUserName().isPresent() == true ? userTest.getUserName().get() : "defaultName";
        System.out.println(userName);
        //orElse(T t1)
        System.out.println(userTest.getUserName().orElse("MrLi"));
        //orElseGet(Supplier<? extends T> other)：如果有值则返回，没值则执行函数
        final String s = userTest.getUserName().orElseGet(new Supplier<String>() {
            @Override
            public String get() {
                return userTest.getUserName().orElse("Jhon") + "-test";
            }
        });
        System.out.println(s);
        //orElseThrow：在报错时可抛出异常供我们捕获进行处理
        try {
            userTest.getUserName().orElseThrow(() -> new NullPointerException());
        } catch (Throwable throwable) {
            //do something
            System.out.println("空指针异常");
        }
        //高级用法：嵌套类中的链式调用
        Employee employee = new Employee();
        employee.setUser(new Employee.User("MrWan"));
        String employeeName = Optional.ofNullable(employee)
                .flatMap(employeeTest -> employeeTest.getUser())
                .map(employeeTest -> {
                    employeeTest.setName("MrQing");
                    return employeeTest;
                }).get().getName().get();
        System.out.println(employeeName);
    }
}
class UserTest{
    private String userName;
    private String userId;
    //get()使用Optional<T>封装一下
    public Optional<String> getUserName() {
        return Optional.ofNullable(userName);
    }
    public Optional<String> getUserId() {
        return Optional.ofNullable(this.userId);
    }
    public void setUserName(String userName) {
        this.userName = userName;
    }
    public void setUserId(String userId) {
        this.userId = userId;
    }
    @Override
    public String toString() {
        return "UserTest{" +
                "userName='" + userName + '\'' +
                ", userId='" + userId + '\'' +
                '}';
    }
}
class Employee{
    private User user;
    public Optional<User> getUser() {
        return Optional.ofNullable(user);
    }
    public void setUser(User user) {
        this.user = user;
    }
    static class User{
        private String name;
        public User(String name) {
            this.name = name;
        }
        public Optional<String> getName() {
            return Optional.ofNullable(name);
        }
        public void setName(String name) {
            this.name = name;
        }
    }
}