深入理解Java8新特性之Optional容器类的应用

Z时代
2024-01-10
分类：IT

1.Optional容器类

Optional<T> 类(java.util.Optional) 是一个容器类，代表一个值存在或不存在，原来用 null 表示一个值不存在，现在 Optional 可以更好的表达这个概念。并且可以避免空指针异常。

常用方法：

Optional.of(T t) : 创建一个 Optional 实例

Optional.empty() : 创建一个空的 Optional 实例

Optional.ofNullable(T t) : 若 t 不为 null，创建 Optional 实例；否则创建空实例

isPresent() : 判断是否包含值

orElse(T t) : 如果调用对象包含值，返回该值，否则返回t

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

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()

flatMap(Function mapper) : 与 map 类似，要求返回值必须是Optional

2.应用举例

下面的代码案例按照上面所列方法顺序给出。首先需要一个Employee类用作测试。


package com.szh.java8;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
/**
 *
 */
@Data
@NoArgsConstructor
@AllArgsConstructor
public class Employee {
    private Integer id;
    private String name;
    private Integer age;
    private Double salary;
}


    @Test
    public void test1() {
        Optional<Employee> op = Optional.of(new Employee());
        Employee employee = op.get();
        System.out.println(employee);
    }


    @Test
    public void test2() {
        Optional<Employee> op = Optional.empty();
        System.out.println(op.get());
    }


    @Test
    public void test3() {
        Optional<Employee> op = Optional.ofNullable(new Employee(1001,"张起灵",18,6666.66));
        System.out.println(op.get());
    }


    @Test
    public void test4() {
        Optional<Employee> op = Optional.of(new Employee());
        if (op.isPresent()) {
            System.out.println(op.get());
        }
    }


    @Test
    public void test5() {
        Optional<Employee> op1 = Optional.ofNullable(null);
        Employee employee1 = op1.orElse(new Employee(1001,"张起灵",18,6666.66));
        System.out.println(employee1);
        Optional<Employee> op2 = Optional.ofNullable(new Employee());
        Employee employee2 = op2.orElse(new Employee(1001,"张起灵",18,6666.66));
        System.out.println(employee2);
    }


    @Test
    public void test6() {
        Optional<Employee> op1 = Optional.ofNullable(null);
        Employee employee1 = op1.orElseGet(() -> new Employee(1001,"张起灵",18,6666.66));
        System.out.println(employee1);
        Optional<Employee> op2 = Optional.ofNullable(new Employee());
        Employee employee2 = op2.orElseGet(() -> new Employee(1001,"张起灵",18,6666.66));
        System.out.println(employee2);
    }


    @Test
    public void test7() {
        Optional<Employee> op1 = Optional.of(new Employee(1001,"张起灵",18,6666.66));
        Optional<String> op2 = op1.map(Employee::getName);
        System.out.println(op2.get());
        Optional<String> op3 = op1.flatMap((e) -> Optional.of(e.getName()));
        System.out.println(op3.get());
    }