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()); }
