011—JAVA新特性Option类

Optional实际上是个容器：它可以保存类型T的值，或者仅仅保存null。Optional提供很多有用的方法，这样我们就不用显式进行空值检测。

（1）static Optional empty() ：用来创建一个空的Optional

（2）static Optional of(T value) ：用来创建一个非空的Optional

（3）static Optional ofNullable(T value) ：用来创建一个可能是空，也可能非空的Optional

public class OptionTest {
    @Test
    public void test03(){
        String s = "hello";
//        String s = null;


        Optional s1 = Optional.ofNullable(s);
        //非null显示原有值   null的时候自定义异常   
        String s2 = s1.orElseThrow(ArrayIndexOutOfBoundsException::new);
        System.out.println("s2 = " + s2);

        


    }

    @Test
    public void test02(){
        String s = "hello";
        Optional s1 = Optional.ofNullable(s);
        //如果数据是null 采用备用值，否则使用原有的值
        String orElse = s1.orElse("世界");
        System.out.println("s = " + s);

    }

    @Test
    public void test01(){
        String s = null;
        System.out.println("s.length() = " + s.length());
    }

}

public class Girl {
    String name;

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

    @Override
    public String toString() {
        return "Girl{" +
                "name='" + name + ''' +
                '}';
    }
}
class Boy{
    String name;
    Girl girlFriend;

    public Boy(String name, Girl girlFriend) {
        this.name = name;
        this.girlFriend = girlFriend;
    }

    public String getName() {
        return name;
    }

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

    public Girl getGirlFriend() {
        return girlFriend;
    }

    public void setGirlFriend(Girl girlFriend) {
        this.girlFriend = girlFriend;
    }

    @Override
    public String toString() {
        return "Boy{" +
                "name='" + name + ''' +
                ", girlFriend=" + girlFriend +
                '}';
    }
}
class TestBoy{
    public static void main(String[] args) {
        Boy zhangsan = new Boy("张三", null);
        Boy lihua = new Boy("李华", new Girl("小花"));


        Optional girlFriend = Optional.ofNullable(zhangsan.getGirlFriend());

        //正常
//        Optional girlFriend1 = Optional.ofNullable(lihua.getGirlFriend());
//
//        Optional.ofNullable(girlFriend1.orElse(new Girl("嫦娥"))).ifPresent(System.out::println);

        //异常
        Optional.ofNullable(girlFriend.orElse(new Girl("嫦娥"))).ifPresent(System.out::println);


    }

}

1）声明学生类，包含姓名和年龄

（2）添加几个学生对象到一个ArrayList集合中

（3）对集合中的学生进行操作，找出年龄大于30岁的，并取出第一个学生，如果没有这样的学生，用无参构造new一个学生对象，打印学生信息

public class Student {
    private String name;
    private int age;
    public Student(String name, int age) {
        super();
        this.name = name;
        this.age = age;
    }
    public Student() {
        super();
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    @Override
    public String toString() {
        return "Student [name=" + name + ", age=" + age + "]";
    }
}
class Test{
    public static void main(String[] args) {
        ArrayList list = new ArrayList<>();
        list.add(new Student("张三", 23));
        list.add(new Student("李四", 22));
        list.add(new Student("王五", 25));

        Student student = list.stream()
                .filter(s -> s.getAge() > 30) //找出年龄大于30的
                .findFirst().orElse(new Student()); //取出第一个  如果没有年龄大于30的，此时取出操作肯定会报空指针异常
        System.out.println("学生的年龄" + student.getAge());


    }
}