Java chaining map methods and using optional-CodePudding

I am using Java 8 method1() -> returns a Map<String,Map<String,Set<String>>>.

If I chain the method calls this way -> method1().get("A").get("B") a NullPointerException could happen. So I am using the following strategy to avoid using the traditional if( != null) code: Optional.ofNullable(method1()).map(x -> x.get("A")).map(y -> y.get("B")) but the this code return an Optional and I need that returns the Set<String>.
How can I cast it, and in case of null (in the case of the get methods returns null) how returns null?

Thanks in advance.

CodePudding user response：

use orElse like

    Set<String> result = Optional.ofNullable(method1())
                    .map(x -> x.get("A"))
                    .map(y -> y.get("B"))
                    .orElse(null);

CodePudding user response：

Polishing the code by gamgoon a little bit:

    Set<String> result = Optional.of(method1())
            .map(x -> x.get("A"))
            .map(y -> y.get("B"))
            .orElse(Collections.emptySet());
    System.out.println(result);

Output in case either key is not in a map where looked up:

[]

You may recognize the result of printing an empty collection. In places where you want a collection, you should not accept a null. Use an empty collection to signify that there are no elements. In the same vein I have assumed that your method1() may return an empty map but not null. So we don’t need ofNullable() when converting into an Optional — the simple of() is fine.