Java8新特性——Optional容器类的简单应用

Posted 2022-01-21 张起灵-小哥

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

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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());