Determine the number of objects created in Java

Question

Are objects only created using the new keyword? How many objects were created after this code reached the comments? I say 4 new objects because the constructor raises new Exception() every time Car initialized. How to check the number of created objects?

 class Car { Car() { try { throw new Exception(); } catch (Exception ex) { System.out.println("Do Nothing"); } } } class Test { public static void main(String[] args) { Car carOne = new Car(); Car carTwo = new Car(); Car carThree = carTwo; // how many objects have been created? 4? } } 

Answer 1

There is no (at least as far as I know) automated way to count the number of objects created in java. In your example, however, 4 objects, 2 exceptions, and two cars are created.

If you want to count how many objects are for a given class, you can have a static counter that increments every time an object is created.

Edit: you can count created objects using jmap as Andreas pointed out, although this is not a very practical solution

Answer 2

You can use jmap and jhat to get a JVM memory dump.

Adding a Thread.sleep(20000) comment to the comment will cause the program to wait 20 seconds to execute the following commands. If necessary, increase the waiting time.

On one command line, run your program:

 java -cp . Car 

On another command line, within 20 seconds of waiting, run

 jps jmap -dump:file=C:/temp/x.dmp 99999 jhat C:/temp/x.dmp 

Where 99999 is the process identifier specified in the jps output.

The jhat program starts the web server on port 7000 , so go to:

 http://localhost:7000/ 

Click "Show instance instances for all classes (excluding platform)" and you will see only 2 objects:

 2 instances of class Car 

This is because you excluded the Exception class from the mapping.

If you click the link "... (including the platform)", you will see many objects with something like this:

 Total of 6214 instances occupying 8071681 bytes. 

If you implement your own exception, eq CarException and throw it instead, you will see 4 "user" objects:

 2 instances of class Car 2 instances of class CarException 

So what is the correct answer? 2, 4 or 6214?
The expected answer is probably 4, as in the section "How many objects did your program create?"

---

Regarding the question of whether objects were created only when using the new keyword, the answer would be No. There are many other ways (for example, listed in this answer: fooobar.com/questions/44483 / ... ), but here are a few, with a comment on the number of objects created by the construct:

 // Using "new" new MyObject() // 1 + number of objects created by constructor new int[0] // 1 new int[10] // 1 new int[] {1,2,3,4} // 1 new int[10][] // 1 new int[10][20] // 11 (1 outer array + 10 inner arrays) // Not using "new" int[] x = {1,2,3,4} // 1 Integer x = 1 // 1 <-- autoboxing printf("", i, j) // 3 (autobox of i + autobox of j + varargs array) String x = "a" + i // 3 (StringBuilder + String + array backing String) Integer[][] x = {{1111},{2222},{3333,4444},{},{}} // 10 (1 outer array + 5 inner arrays + 4 Integers)