Java 8 新增的 Optional

Posted 2021-05-16 认真对待世界的小白

TestOptional类

import java.util.Optional;

import org.junit.Test;

/*
 * 一、Optional 容器类：用于尽量避免空指针异常
 *     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
 */
public class TestOptional {

    @Test
    public void test4() {
        Optional<Employee> op = Optional.of(new Employee(101, "张三", 18, 9999.99));

        Optional<String> op2 = op.map(Employee::getName);
        System.out.println(op2.get());

        Optional<String> op3 = op.flatMap((e) -> Optional.of(e.getName()));
        System.out.println(op3.get());
    }

    @Test
    public void test3() {
        Optional<Employee> op = Optional.ofNullable(new Employee());

        if (op.isPresent()) {
            System.out.println(op.get());
        }

        Employee emp = op.orElse(new Employee("张三"));
        System.out.println(emp);

        Employee emp2 = op.orElseGet(() -> new Employee());
        System.out.println(emp2);
    }

    @Test
    public void test2() {
        /*Optional<Employee> op = Optional.ofNullable(null);
        System.out.println(op.get());*/

//        Optional<Employee> op = Optional.empty();
//        System.out.println(op.get());
    }

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

    @Test
    public void test5() {
        Man man = new Man();

        String name = getGodnessName(man);
        System.out.println(name);
    }

    //需求：获取一个男人心中女神的名字
    public String getGodnessName(Man man) {
        if (man != null) {
            Godness g = man.getGod();

            if (g != null) {
                return g.getName();
            }
        }

        return "苍老师";
    }

    //运用 Optional 的实体类
    @Test
    public void test6() {
        Optional<Godness> godness = Optional.ofNullable(new Godness("林志玲"));

        Optional<NewMan> op = Optional.ofNullable(new NewMan(godness));
        String name = getGodnessName2(op);
        System.out.println(name);
    }

    public String getGodnessName2(Optional<NewMan> man) {
        return man.orElse(new NewMan())
                .getGodness()
                .orElse(new Godness("苍老师"))
                .getName();
    }
}

Man类

@Data
public class Man {
    private Godness god;

    public Man() {}

    public Man(Godness god) {
        this.god = god;
    }
}

Godness类

@Data
public class Godness {
    private String name;

    public Godness() {}

    public Godness(String name) {
        this.name = name;
    }
}

NewMan类

import java.util.Optional;

//注意：Optional 不能被序列化
public class NewMan {
    private Optional<Godness> godness = Optional.empty();
    
    private Godness god;
    
    public Optional<Godness> getGod(){
        return Optional.of(god);
    }

    public NewMan() {}

    public NewMan(Optional<Godness> godness) {
        this.godness = godness;
    }
}