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

//测试类1
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;
    }

    public Optional<Godness> getGodness() {
        return godness;
    }

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

    @Override
    public String toString() {
        return "NewMan [godness=" + godness + "]";
    }

}
//测试类2
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;
    }

    public Optional<Godness> getGodness() {
        return godness;
    }

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

    @Override
    public String toString() {
        return "NewMan [godness=" + godness + "]";
    }

}
//测试3
public class Godness {

    private String name;

    public Godness() {
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Godness [name=" + name + "]";
    }

}

//Optional示例
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()) {//判断是否有值
            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(new Employee());//of 和 empty的综合
        System.out.println(op.get());

        Optional<Employee> op2 = Optional.empty();
        System.out.println(op2.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：获取一个名字
    public String getGodnessName(Man man) {
        if (man != null) {
            Godness g = man.getGod();

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

        return "赵小二";
    }

    //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("赵小二"))
                .getName();
    }
}