Map.entry(...) 对于类型 Map 未定义

Question

I was messing around with Java and hashmaps on a Computer at School.

I was able to instantiate an Entry of type String, Integer on the fly. Then add it to an ArrayList of Entrie s. I emailed this code to myself for later.

However on my home computer and my laptop, it wont let me. Same exact code says

The method entry(string,int) is undefined for type Map

The code is:

Map.Entry<String, Integer> entry = Map.entry(largest, maxCount);

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I think this has something to do with the fact that Map.Entry is an interface?

As google results seem to say you have to make your own Entry class to do this? But did that change in newer versions of Java? Why was I able to execute this code just fine earlier?

Answer 1

Explanation

Lookup the Javadoc of this method. It says:

Since: 9

Which means that this method was added in Java 9.

So your machine at home likely did not have Java 9 or newer.

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Upgrade

Run java -version and javac -version in your CMD to verify your version. Then install a more recent Java (for example from AdoptOpenJDK ).

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Pre Java 9

In case you are searching for a solution that works without Java 9, you can use something like:

Map.Entry<String, Integer> entry = new AbstractMap.SimpleEntry<String, Integer>(largest, maxCount);

As explained in more detail here: Java - How to create new Entry (key, value)

You said:

As google results seem to say you have to make your own Entry class to do this?

That is true for versions before Java 6. The mentioned class AbstractMap.SimpleEntry was added in Java 6 and can be (ab)used for this purpose. Before that, you would have to create your own implementation.

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OOP

Another option, which might arguably be better, could be to create your own little class to hold that data and then give it a proper name.

For example, instead of having a Map.Entry<String, Integer> that stores a person by his String name and int age , why not create a Person class directly:

public class Person {
    private final String name;
    private final int age;

    // Constructor and getters, ...
}

Is much more readable, easier to maintain and extend.