Java and JavaScript - Utilizing a Map

Question

Java and JavaScript - Utilizing a Map

I'm currently using Rhino to run JavaScript code within my Java program, but I've encountered a problem with iterating over the map in JavaScript.

On the Java side:

private final Map<Long, ClassEmployees > employees ;
...
employees.put (numEmployees, new ClassEmployees());
...

On the JavaScript side:

keys = employees.keySet();
for (var i in keys) {
  print ("++ " + i);   // ===> why is this printing methods?
}

Output :

++ getClass
++ iterator
++ toArray
++ addAll
++ remove
++ equals
++ containsAll
++ class
++ hashCode
++ contains
++ wait
++ add
++ size
++ clear
++ isEmpty
++ notify
++ empty
++ retainAll
++ toString
++ notifyAll
++ removeAll

javascript java rhino

Answer 1

Answer №1

In Java, the variable keys is pointing to a Set object. However, when using JavaScript's for in loop, it doesn't know how to handle iterating over this type of object and instead treats it as a regular object.

The for...in loop in JavaScript loops through the enumerable properties of an object.

Since the methods of a Set object are considered enumerable properties, that's what you'll end up iterating over.

If you want to iterate through the actual values stored in your Set, you should use an explicit iterator like in Java or convert it to an array first:

for each (var i in keys.toArray()) {
     //...
}

Answer 2

In Java, the variable keys is pointing to a Set object. However, when using JavaScript's for in loop, it doesn't know how to handle iterating over this type of object and instead treats it as a regular object.

The for...in loop in JavaScript loops through the enumerable properties of an object.

Since the methods of a Set object are considered enumerable properties, that's what you'll end up iterating over.

If you want to iterate through the actual values stored in your Set, you should use an explicit iterator like in Java or convert it to an array first:

for each (var i in keys.toArray()) {
     //...
}

Java and JavaScript - Utilizing a Map

On the Java side:

On the JavaScript side:

Output :

Answer №1

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