Given the following map:
☕🎓JAVA CERTIFICATION QUESTION: Unmodifiable maps
Given the following map:
var m = Map.of(1,2,3,4);
Which code fragment is guaranteed to print 37 to the console? Choose one.
A. m.forEach((x, y) -> System.out.print(x + y));
B. for (var v : m) {
System.out.print(v.getKey() + v.getValue());
}
C. for (var v : m.keySet()) {
System.out.print(v + m.get(v));
}
D. for (var t : m.entrySet()) {
System.out.print(t.getKey() + t.getValue());
}
E. None of the above
#java #certificationquestion #ocp
Answer: None of the above
Look at the effect of the Map.of(1,2,3,4) method call.
It creates a Map〈Integer, Integer〉 with two key-value pairs.
The keys for these pairs are 1 and 3, and the corresponding values are 2 and 4.
Note that the Map.of methods that take arguments in this form treat those arguments as a key, a value, a key, a value, and so on.
The keys and values, of course, will be autoboxed to provide Integer objects.
Since you have two pairs in this Map—1-2 and 3-4—i
t’s a simple observation of arithmetic that adding the key-value pairs will give the results 3 and 7,
which seems at least promising for producing the desired output.
Now, look at the code of each option in turn.
m.forEach((x, y) -> System.out.print(x + y));
The map.forEach method in this case accepts a BiConsumer〈Integer, Integer〉 as its argument,
and the accept method of that argument will be called with a key and value pair from each key-value pair.
But the iteration of a Map in Java does not provide a predictable order.
for (var v : m) {
System.out.print(v.getKey() + v.getValue());
}
will not compile because java.util.Map itself does not implement the Iterable interface required for use in an enhanced for loop.
for (var v : m.keySet()) {
System.out.print(v + m.get(v));
}
extracts a keySet from the Map, which gives you a set of the keys.
But the iteration of a Map in Java does not provide a predictable order.
iterates over a Set of Map.Entry objects.
But the iteration of a Map in Java does not provide a predictable order.
So none of the answers are correct.
https://docs.oracle.com/en/java/javase/19/docs/api/java.base/java/util/Map.html#unmodifiable -> “The iteration order of mappings is unspecified and is subject to change.”