java8 Optional用法

1、准备对象

package org.example.note.global.pojo;


import lombok.*;

import javax.validation.constraints.NotNull;

/**
 * java 8
 *
 * @author admin
 */
// @Data
// @EqualsAndHashCode
@Getter
@Setter
@ToString
@NoArgsConstructor
@AllArgsConstructor
public class User {
    // alibaba规约：POJO类，全部使用包装类型（Integer、Long等）
    // private long id;
    private Long id;
    @NotNull(message = "userName 不能为空")
    private String userName;
    private String password;
    private String realName;
    //  private int age;
    private Integer age;

    public User(String userName) {
        this.userName = userName;
    }
}

2、用法测试用例

package org.example.note;

import org.example.note.global.pojo.User;
import org.junit.Test;

import java.util.Collections;
import java.util.List;
import java.util.Optional;

/**
 * @Author xiaoming-mac
 * @Date 2021/6/24
 * @Version 1.0
 */
public class OptionalTest {

    User user = null;
    User user1 = new User();
    List<User> userList = null;

    /**
     * 错误示例
     */
    @Test(expected = NullPointerException.class)
    public void expectedTest() {
        String a = Optional.ofNullable(user.getUserName()).orElse(null);
    }

    /**
     * 避免对象空指针
     */
    @Test
    public void ofNullableObjectTest() {
        User res = Optional.ofNullable(user1).orElse(new User());
        System.out.println(res.getAge());
    }

    /**
     * 避免对象空指针，获取属性
     */
    @Test
    public void ofNullableAttributeTest() {
        String userName = Optional.ofNullable(user).map(User::getUserName).orElse("default name");
    }

    /**
     * 避免集合判空时空指针
     */
    @Test
    public void ListEmptyTest() {
        List<User> users = Optional.ofNullable(userList).orElse(Collections.emptyList());
        // 判断集合是否为空
        System.out.println(users.isEmpty());
    }

    /**
     * 测试效率
     */
    @Test
    public void orElseGetTest() {
        // 非空，会执行后续函数(影响效率)
        User res1 = Optional.ofNullable(user1).orElse(createUser());
        // 非空，不会执行后续函数
        User res2 = Optional.ofNullable(user1).orElseGet(this::createUser);
        User res3 = Optional.ofNullable(user1).orElseGet(User::new);
    }

    private User createUser() {
        User user = new User();
        user.setUserName("create user");
        System.out.println(user);
        return user;
    }
}