(1) 這個Optional.of方法還是不接收null作為輸入參數：

而Optional.ofNullable可以接收null參數：

isPresent判斷有無數據，這個沒啥可以說的：

如果Optional容器包含的值不是null，執行action.

orElse: 如果Optional對象存在，返回原始對象，否則返回orElse里構造的新對象：

上圖84行的getName其實是一個映射器，將Person對象映射成String.

在debugger里看到這個mapper實際上是個Lambda Function：

結果是包裹類型為映射函數的第二個參數：

ifPresent的參數是一個action：

我的測試代碼：

package optionalTest;import java.util.Optional; import java.util.function.Function;public class OptionalTest {class Person {private String mName;private Skill mSkill;public Person(String name) {mName = name;}public Skill getSkill(){return mSkill;}public Optional<Skill> getSkillOptional(){return Optional.ofNullable(mSkill);}public void setSkill(Skill skill){mSkill = skill;}public String getName() {return mName;}public void greet() {System.out.println("I am: " + mName);}}class Skill{private String mName;private int mLevel;public Skill(String name, int level) {mName = name;mLevel = level;}public void display(){System.out.println("Skill: " + mName + " level: " + mLevel);}}@SuppressWarnings("unused")public void scenario1(){Person person = null;try {Optional<Person> p1 = Optional.of(person);}catch ( Exception e) {System.out.println("Error: " + e.getClass().getName() + " - " + e.getLocalizedMessage());}Optional<Person> p2 = Optional.ofNullable(person);System.out.println(p2.isPresent());Person jerry = new Person("Jerry");Optional<Person> p3 = Optional.ofNullable(jerry);System.out.println(p3.isPresent());System.out.println("p2 test");p2.ifPresent((p) -> p.greet());System.out.println("p3 test");p3.ifPresent((p) -> p.greet());System.out.println("Example 3");// before Jave8Person oldImplementation = ( person != null? person:new Person("Ji")); Person newPerson = p2.orElse(new Person("Ji"));Person existPerson = p3.orElse(new Person("Mr Unknown"));newPerson.greet();existPerson.greet();if( jerry != null && jerry.getName().equals("Jerry")) {System.out.println("old style-> Jerry found: " + jerry.getName());}p3.filter( personHolder -> "Jerry".equals(personHolder.getName())).ifPresent((personH2) -> System.out.println("new style-> Jerry found: " + personH2.getName()));Function<Person,String> mapper = Person::getName;Optional<String> mapResult = p3.map(mapper);p3.map(Person::getName).filter( Name -> "Jerry".equals(Name)).ifPresent( foundName -> System.out.println("new style 2-> Jerry found: " + foundName));/*jerry.setSkill("JavaScript");Optional<String> jerrySkill = p3.map(Person::getSkill);System.out.println(jerrySkill.isPresent());p3.map(Person::getSkill).filter(Skill -> "JavaScript".equals(Skill)).ifPresent( Skill2 -> System.out.println("Jerry's skill is: " + Skill2 ) );*/Skill jsSkill = new Skill("JavaScript", 80);jerry.setSkill(jsSkill);Optional<Skill> temp = p3.map(Person::getSkill); Optional<Skill> temp2 = p3.flatMap(Person::getSkillOptional);Skill s1 = temp.get();Skill s2 = temp2.get();s1.display();s2.display();}public static void main(String[] args) {OptionalTest test = new OptionalTest();test.scenario1();}}

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