About Question com.enthuware.ets.scjp.v6.2.488 :

[ETS User]

Question: Which of these statements concerning maps are true?
Answer: It is permissible for a map to contain itself as a key.
Example code that generates StackOverflowError at runtime...

Soo, what does this question mean???

Code: Select all

import java.util.HashMap;

public class Main {
	public static void main(String[] args) {
		HashMap map = new HashMap();
		map.put(map, "hello"); // 1
		map.put(map, "world"); // 2
		//System.out.println(map.get(map));
		
	}
}

[admin]

Your code is correct. It is not permissible for a map to contain itself as a key. That is why it is an incorrect option.

HTH,
Paul.

[Guest]

Your code is correct. It is not permissible for a map to contain itself as a key. That is why it is an incorrect option.

HTH,
Paul.

Hmm, on Standard Test 2, I saw the above answer was picked as a correct option.

[Guest]

Your code is correct. It is not permissible for a map to contain itself as a key. That is why it is an incorrect option.

HTH,
Paul.

Hmm, on Standard Test 2, I saw the above answer was picked as a correct option.

Never mind, my bad.

[cosmindumy]

Is still unclear for me.
Map<Map, String> m=new HashMap<Map, String>();
m.put(m, "this map");
This code compiles fine. Is the question referring to something else?

[admin]

It is not just the compilation that you need to consider here. It fails at run time.

[ago123]

but this codes compiles and run well

Code: Select all

        
        Map<Map, String> m = new HashMap<>();
        m.put(m, "");
        System.out.println(m);
        

Map key.PNG (23.74 KiB) Viewed 1948 times

[admin]

Yes, but even if it compiles and runs, it is not allowed as per the JavaDoc API documentation of Map

Note: great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is a key in the map. A special case of this prohibition is that it is not permissible for a map to contain itself as a key. While it is permissible for a map to contain itself as a value, extreme caution is advised: the equals and hashCode methods are no longer well defined on such a map.

The run time behavior has changed in the recent versions of Java but the restriction is still there in the API.