Java8::flatMap可选[重复]

Question

I am new to Java8. In recent project I am trying Optional with map and flatmap to prevent NPE.

public class Project {
    private ApplicationType applciationType;
}


@Getter
@Setter
public class ApplicationType {

    public String getDirectory() {      
        return "Inside my directory";
    }
}


public class Driver {

    public static void main(String[] args) {
        ApplicationType appType = new ApplicationType();
        Project project = new Project(appType);

         //When using map its running fine.
        Optional.ofNullable(project)
        .map(prjct-> prjct.getApplciationType())
        .map(appType1 -> appType1.getDirectory())
        .ifPresent(e -> System.out.println(e));

        //When using flatMap.. comiplation error.
        Optional.ofNullable(project)
        .map(prjct-> prjct.getApplciationType())
        .flatMap(appType1 -> appType1.getDirectory())
        .ifPresent(e -> System.out.println(e));

    }

}

编译错误->[无法推断flatMap（函数>）的类型参数]

Answer 1

查看flatmap的签名：

public<U> Optional<U> flatMap(Function<? super T, Optional<U>> mapper)

函数应该返回一个可选，因此为了使用它，您需要编写：

Optional.ofNullable(project)
    .map(prjct-> prjct.getApplciationType())
    .flatMap(appType1 -> Optional.ofNullable(appType1.getDirectory()))
    .ifPresent(e -> System.out.println(e));

当然，在本例中使用map更有意义（就像在第一个代码段中一样，它传递编译）。