Java8:Optional类的解析

Optional类的Javadoc描述如下：
Optional 类是一个可以为null的容器对象。如果值存在则isPresent()方法会返回true，调用get()方法会返回该对象。

Optional 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。
Optional 类的引入很好的解决空指针异常。

Java 8 Optional 优点：

• 不需要检查对象是否为空；
• 在运行时不再有NullPointerException；
• 我们可以开发干净整洁的API，减少代码量。

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Optional.of()
为非null的值创建一个Optional。

Optional.ofNullable() ofNullable与of方法相似，唯一的区别是可以接受参数为null的情况。

Optional.empty()
创建一个为空的Optional

public static void main(String[] args) {
    Optional<String> gender = Optional.of("MALE");
    String answer1 = "Yes";
    String answer2 = null;

    System.out.println("Non-Empty Optional:" + gender);
    // Non-Empty Optional:Optional[MALE]

    System.out.println("Non-Empty Optional: Gender value : " + gender.get());
    //Non-Empty Optional: Gender value : MALE
    System.out.println("Empty Optional: " + Optional.empty());
    //Empty Optional: Optional.empty

    System.out.println("ofNullable on Non-Empty Optional: " + Optional.ofNullable(answer1));
    //ofNullable on Non-Empty Optional: Optional[Yes]
    System.out.println("ofNullable on Empty Optional: " + Optional.ofNullable(answer2));
    //ofNullable on Empty Optional: Optional.empty

    // java.lang.NullPointerException
    System.out.println("ofNullable on Non-Empty Optional: " + Optional.of(answer2));
}

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Optional.map()
如果有值，则对其执行调用mapping函数得到返回值。如果返回值不为null，则创建包含mapping返回值的Optional作为map方法返回值，否则返回空Optional。
Optional.flatMap()
如果有值，为其执行mapping函数返回Optional类型返回值，否则返回空Optional。flatMap与map（Funtion）方法类似，区别在于flatMap中的mapper返回值必须是Optional。调用结束时，flatMap不会对结果用Optional封装。

public static void main(String[] args) {
    Optional<String> oGender = Optional.of("male");
    Optional<String> emptyGender = Optional.empty();

    System.out.println("Non-Empty Optional::" + oGender.map(String::toUpperCase));
    //Non-Empty Optional::Optional[MALE]
    System.out.println("Empty Optional::" + emptyGender.map(String::toUpperCase));
    //Empty Optional::Optional.empty

    Optional<Optional<String>> ooGender = Optional.of(Optional.of("male"));
    System.out.println("Optional value::" + ooGender);
    //Optional value::Optional[Optional[male]]
    System.out.println("Optional.map:: " + ooGender.map(gender -> gender.map(String::toUpperCase)));
    //Optional.map:: Optional[Optional[MALE]]
    System.out.println("Optional.flatMap::" + ooGender.flatMap(gender -> gender.map(String::toUpperCase)));
    //Optional.flatMap::Optional[MALE]
}

---

Optional.filter()
如果有值并且满足断言条件返回包含该值的Optional，否则返回空Optional。

public static void main(String[] args) {
    Optional<String> gender = Optional.of("male");
    Optional<String> emptyGender = Optional.empty();

    //Filter on Optional
    System.out.println(gender.filter(g -> g.equals("MALE")));
    //Optional.empty

    System.out.println(gender.filter(g -> g.equalsIgnoreCase("MALE")));
    //Optional[male]

    System.out.println(emptyGender.filter(g -> g.equalsIgnoreCase("MALE")));
    //Optional.empty

    // filter方法检查给定的Option值是否满足某些条件。
    // 如果满足则返回同一个Option实例，否则返回空Optional。
    Optional<String> longName = Optional.of("123456789").filter((value) -> value.length() > 6);
    System.out.println(longName.orElse("The name is less than 6 characters"));//输出Sanaulla
    //123456789

    //另一个例子是Optional值不满足filter指定的条件。
    Optional<String> anotherName = Optional.of("1234");
    Optional<String> shortName = anotherName.filter((value) -> value.length() > 6);
    System.out.println(shortName.orElse("The name is less than 6 characters"));
    //The name is less than 6 characters
}

---

Optional.isPresent()
如果值存在返回true，否则返回false。

Optional.get()
如果Optional有值则将其返回，否则抛出NoSuchElementException。

Optional.ifPresent()
如果Optional实例有值则为其调用consumer，否则不做处理。

Optional.orElse()
如果有值则将其返回，否则返回指定的其它值。

Optional.orElseGet()
orElseGet与orElse方法类似，区别在于得到的默认值。orElse方法将传入的字符串作为默认值，orElseGet方法可以接受Supplier接口的实现用来生成默认值。

Optional.orElseThrow()
如果有值则将其返回，否则抛出supplier接口创建的异常。

public static void main(String[] args) {
    Optional<String> gender = Optional.of("MALE");
    Optional<String> emptyGender = Optional.empty();

    if (gender.isPresent()) {
      System.out.println("Value available.");
    } else {
      System.out.println("Value not available.");
    }

    try {
      //在空的Optional实例上调用get()，抛出NoSuchElementException
      System.out.println(emptyGender.get());
    } catch (NoSuchElementException ex) {
      System.out.println(ex.getMessage());
      //No value present
    }

    gender.ifPresent(g -> System.out.println("In gender Option, value is " + g));
    //In gender Option, value is MALE

    //condition failed, no output print
    emptyGender.ifPresent(g -> System.out.println("In emptyGender Option, value available."));

    System.out.println(gender.orElse("<N/A>"));
    //MALE
    System.out.println(emptyGender.orElse("<N/A>"));
    //<N/A>

    System.out.println(gender.orElseGet(() -> "<N/A>"));
    //MALE
    System.out.println(emptyGender.orElseGet(() -> "<N/A>"));
    //<N/A>
    try {
      //orElseThrow与orElse方法类似。与返回默认值不同，
      //orElseThrow会抛出lambda表达式或方法生成的异常
      emptyGender.orElseThrow(ValueAbsentException::new);
    } catch (Throwable ex) {
      System.out.println(ex.getMessage());
      //No value present in the Optional instance
    }
}

public static class ValueAbsentException extends Throwable {
    public ValueAbsentException() {
      super();
    }

    public ValueAbsentException(String msg) {
      super(msg);
    }

    @Override
    public String getMessage() {
      return "No value present in the Optional instance";
    }
}

例：Without Java 8 Optional

//分辨率
public class ScreenResolution {
  private int width;
  private int height;
  //...
}
//显示屏
public class DisplayFeatures {
  private String size; // In inches
  private ScreenResolution resolution;
  //...
}
//手机
public class Mobile {
  private long id;
  private String brand;
  private String name;
  private DisplayFeatures displayFeatures;
  //...
}
public static void main(String[] args) {
    ScreenResolution resolution = new ScreenResolution(750, 1334);
    DisplayFeatures dfeatures = new DisplayFeatures("4.7", resolution);
    Mobile mobile = new Mobile(2015001, "Apple", "iPhone 6s", dfeatures);

    int mobileWidth = 0;
    if (mobile != null) {
      DisplayFeatures df = mobile.getDisplayFeatures();
      if (df != null) {
        ScreenResolution sr = df.getResolution();
        if (sr != null) {
          return sr.getWidth();
        }
      }
    }
    System.out.println("Apple iPhone 6s Screen Width = " + mobileWidth);
    //Apple iPhone 6s Screen Width = 750
}

例：With Java 8 Optional

//分辨率
public class ScreenResolution {
  private int width;
  private int height;
  //...
}
//显示屏
public class DisplayFeaturesOptional {
  private String size; // In inches
  private Optional<ScreenResolution> resolution;
  //...
}
//手机
public class MobileOptional {
  private long id;
  private String brand;
  private String name;
  private Optional<DisplayFeaturesOptional> displayFeatures;
  //...
}
public static void main(String[] args) {
    ScreenResolution resolution = new ScreenResolution(750, 1334);
    DisplayFeaturesOptional dfeatures = new DisplayFeaturesOptional("4.7", Optional.of(resolution));
    MobileOptional mobile = new MobileOptional(2015001, "Apple", "iPhone 6s", Optional.of(dfeatures));

    int width = mobile.flatMap(MobileOptional::getDisplayFeatures)
        .flatMap(DisplayFeaturesOptional::getResolution)
        .map(ScreenResolution::getWidth)
        .orElse(0);
    System.out.println("Apple iPhone 6s Screen Width = " + width);
    //Apple iPhone 6s Screen Width = 750
}