Object cannot be converted to string when using Optional class in Java

问题

I have the following code trying to use the Optional class:

import java.util.Optional;

// one class needs to have a main() method
public class HelloWorld
{
  public String orelesMethod() {
    return "hello";
  }

  public void test() {
    String value;
    value = Optional.ofNullable(null).orElse(orelesMethod());
    System.out.println(value); 
  }

  // arguments are passed using the text field below this editor
  public static void main(String[] args)
  {
    HelloWorld hello = new HelloWorld();

    hello.test();
  }
}

when compiling it, it says:

incompatible types: Object cannot be converted to String
    value = Optional.ofNullable(null).orElse(orelesMethod());

I can not find it out what is the problem, can anyone help on it?

Thank you!

回答1:

You have defined value to be of type String, but Optional.ofNullable(null) returns Optional<Object> because any object type can be null and you did not specify the actual type. Then you are calling orElse on the object of type Optional<Object> and because T is Object, the orElse method returns an object of type Object which cannot be converted to String.

Therefore you need to specify the type while calling ofNullable:

value = Optional.<String>ofNullable(null).orElse(orelesMethod());

You might also want to use orElseGet method in order not to call orelesMethod when the optional contains value.

回答2:

A String is an Object, but an Object is not necessarily a String and thus this error. Try this :

public String orelesMethod() {
        return "hello";
    }
    public void test() {
        Object value;
        value = Optional.ofNullable(null).orElse(orelesMethod());
        System.out.println(value);
    }

回答3:

Optional<T> is the new (since Java 8) way of handling values which might be null.

You get the value by calling .get().

Note that calling .get() without the value being set throws an NoSuchElementException.

See the reference: Class Optional

Edit:

Also, the return value of your orElse() will be of the same time you initialized you class with. Since that's not a String but null you don't get a String as return.

来源：https://stackoverflow.com/questions/51931835/object-cannot-be-converted-to-string-when-using-optional-class-in-java

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java

optional