JDK8 Optional解決判斷null的問題

1. Optional函數：

　　1.1 Optional.of(T value)

　　　　當value為空時，會報nullpointexception

　　1.2 Optional.ofNullable(T value)　

　　　　當value為空時創建空實例，不為空時創建optional實例

　　1.3 isPresent()

　　　　判斷是否包含值

　　　Optional<T> opt=Optional.ofNullable(T value); if(opt.isPresent){...}else{...}

　　1.4 orElse(T value)

　　　　如果調用對象包含值則返回值，否則返回orElse()中value　　

public class OptionalTest {
    public static void main(String[] args) {
        Apple apple1=new Apple();
        Apple apple2=new Apple();
        apple1.setColor("red");
        apple1.setWeight(3.0);
        List<Apple> appleList= Arrays.asList(apple1,apple2);
        orElseTest(appleList);
    }

    public static void orElseTest(List<Apple> appleList){
        Apple ap=new Apple();
        ap.setWeight(0.0);
        ap.setColor("none");
        for(Apple apple:appleList){
            String color=Optional.ofNullable(apple.getColor()).orElse(ap.getColor());
            System.out.println("color is:"+color);
        }
    }
}

result:
color is:red
color is:none

 

　　1.5 orElseGet(Supplier s)

　　　　如果調用對象包含值則返回值，否則返回執行s后獲取的值

public class OptionalTest {
    public static void main(String[] args) {
        Apple apple1=new Apple();
        Apple apple2=new Apple();
        apple1.setColor("red");
        apple1.setWeight(3.0);
        List<Apple> appleList= Arrays.asList(apple1,apple2);
        orElseGetTest(appleList);
    }

    public static void orElseGetTest(List<Apple> appleList){

        for(Apple apple:appleList){
            String color=Optional.ofNullable(apple.getColor()).orElseGet(()->{
                Apple apple1=new Apple();
                apple1.setColor("none");
                return "a haha no color";
            });
            System.out.println("color is:"+color);
        }
    }
}
result:
color is:red
color is:a haha no color

　　1.6 orElseThrow();

　　　　如果調用對象包含值則返回，否則拋出異常；orElseThrow(()->new IllegalStateException("This is an exception"));

　　1.7 map(Function f)

　　　　如果有值調用f進行處理，否則返回Optional.empty();這個和java8中map()一樣，對其中參數進行函數操作

public class OptionalTest {
    public static void main(String[] args) {
        Apple apple1=new Apple();
        Apple apple2=new Apple();
        apple1.setColor("red");
        apple1.setWeight(3.0);
        List<Apple> appleList= Arrays.asList(apple1,apple2);
        mapTest(appleList);
    }

    public static void mapTest(List<Apple> appleList){
        Apple ap=new Apple();
        ap.setWeight(0.0);
        ap.setColor("none");
        for(Apple apple:appleList){
            Optional<String> color=Optional.ofNullable(apple.getColor());
            System.out.println("optional result:"+color);
            color.map(c->{
                c=c+" is test for map";
                System.out.println("======"+c);
                return c;
            });
        }
    }
}
result:
optional result:Optional[red]
======red is test for map

optional result:Optional.empty

 

　　1.8 flatMap(Function f)

　　　　如果有值調用f進行處理，否則返回Optional.empty(),但要求返回值必須時Optional

　　1.9 empty()

　　　　判斷為null時返回empty對象,相當於Optional.ofNullable()后沒有orElse()或者orElseget()

public class OptionalTest {
    public static void main(String[] args) {
        Apple apple1=new Apple();
        Apple apple2=new Apple();
        apple1.setColor("red");
        apple1.setWeight(3.0);
        List<Apple> appleList= Arrays.asList(apple1,apple2);
        emptyTest(appleList);
    }

    public static void emptyTest(List<Apple> appleList){
        for(Apple apple:appleList){
            Optional<String> color=Optional.ofNullable(apple.getColor());
            System.out.println(color);
        }
    }
}
result:
Optional[red]
Optional.empty