Please correct me if I'm wrong...
Q: What is the size ratio of Integer to int?
An int is 4 bytes, an Integer additionally has a 2 word object header (8 bytes) and a 4 byte reference pointing to it. This adds up to 12 bytes for the object, which is rounded up to 16 bytes, because Hotspot aligns objects to 8 byte boundaries. Including the reference, the Integer needs 20 bytes, or 5 times as much as an int.
Q: How many bytes in an 8-character String?
* object header: 8 bytes
* reference to char[] storing the value: 4 bytes
* int field to cache hashcode: 4 bytes
-> 16 bytes for "wrapper"
* char[] storing the value:
8 characters in UTF16: 16 bytes
object header: 8 bytes
length field: 4 bytes length
-> 28 bytes, rounded up to 32 bytes
total: 48 bytes
d. does less, bigger: HashSet is a wrapper around HashMap
Q: Put the following 2-element collections in size order: ArrayList, HashSet, LinkedList, HashMap
Excluding the size of the actual elements:
* ArrayList, created with initial capacity of 2 (vs. default of 10):
* object header: 8 bytes
* size int: 4 bytes
* reference to Object[] holding element data: 4 bytes
-> 16 bytes for wrapper
* element data with 2 pointers: 2*4 bytes + 12 bytes, rounded
up to 24 bytes
-> 40 bytes total
* LinkedList:
* object header: 8 bytes
* size int: 4 bytes
* references to first + last node: 8 bytes
-> 20 bytes, rounded up to 24 bytes
* nodes (2x):
* object header: 8 bytes
* references to previous and next item: 8 bytes
* reference to element data: 4 bytes
-> 20 bytes, rounded up to 24
-> 72 bytes total
* HashMap, created with initial capacity of 2 (vs. 16) and default
load factor of 0.75
* object header: 8 bytes
* modification count int: 4 bytes
* size int: 4 bytes
* load factor float: 4 bytes
* entry set: ?
* resizing threshold int: 4 bytes
* pointer to hash table: 4 bytes
-> 28 bytes for wrapper, rounded up to 32 bytes
* hash table (Node[]):
* object header: 8 bytes
* length field: 4 bytes
* pointers: 4 * 4 bytes
-> 28 bytes, rounded up to 32 bytes
* Node objects (2x):
* object header: 8 bytes
* hashcode int: 4 bytes
* pointer to next node in chain: 4 bytes
* pointer to element data: 4 bytes
-> 20 bytes, rounded up to 24 bytes
-> total of 120 bytes
* HashSet, created with initial capacity of 2 (vs. 16) and default
load factor of 0.75
* object header: 8 bytes
* pointer to HashMap: 4 bytes
-> 12 bytes for wrapper, rounded up to 16
* HashMap: 120 bytes
-> 132 bytes total