In Java, empty HashMap space allocation

Question

How can I find out how much space a pre-configured HashMap takes before adding any items? For example, how to determine how much memory the following takes:

HashMap<String, Object> map = new HashMap<String, Object>(1000000);

Answer 1

Basically you can:

• calculate it by theory:
  • look at the HashMap implementation to find out what this method does.
  • Look at the implementation of the virtual machine to find out how much space the individual created objects take.
• Measure it somehow.

Most of the other answers relate to the second method, so I will consider the first (in OpenJDK source, 1.6.0_20).

capacity, >= initialCapacity, , 1048576 = 2 ^ 20 . new Entry[capacity] table. ( ).

, HashMap ( 3 ints, float ) Entry []. ( ) (, ).

, , . - 32- 32 (= 4 ), 64- 64 (= 8 ).

, 32- 4 , 64- 8 , .

---

HashTable , Entry. int , 24 32- , , . , 1000000- HashMap ( > 1) ~ 28 32- ~ 56 64- .

, .

Answer 2

. :

long preMemUsage = Runtime.getRuntime().totalMemory() -
      Runtime.getRuntime().freeMemory();
HashMap<String> map = new HashMap<String>(1000000);
long postMemUsage = Runtime.getRuntime().totalMemory() -
      Runtime.getRuntime().freeMemory();

Answer 3

Java, JVM , .

~4 * 2^20, ~8 * 2^20 32- 64- jvm .

:

• HashMap Sun Java 1.6 table, -.

• () HashMap null, - , initialCapacity. (... .)

• 4 32- JVM 8 64- JVM. 64- JVM ( " oops" ), JVM, .

• 5 , table, .

• , .

, table , 2^20 ( 2 , 1,000,000), .

---

, , . , , , , . ( .)

Answer 4

, , VisualVM .

: http://www.velocityreviews.com/forums/t148009-java-hashmap-size.html

Answer 5

: http://www.javaworld.com/javaworld/javatips/jw-javatip130.html

, java C-style sizeof. , IMO, , .

, : java 40 . , , 40 ...

Answer 6

VisualVM ( JDK 6 ) .

Answer 7

, - . , 32- 64- JVM. - () , . , .

Answer 8

Java 1.7 ( 1.7.0_55) HashMap . put() - . "inflateTable()". , HashMap, - , , .