Arrays can be large and consume a significant amount of memory. It can me worth choosing the most memory efficient array/collection.

Comparing array sizes

How much space does a `new int[1024]` take compared with an `new Integer[1024]`?

int[] ints = new int[1024];
for (int i = 0; i < ints.length; i++) ints[i] = i;

compared with

Integer[] ints = new Integer[1024];
for (int i = 0; i < ints.length; i++) ints[i] = i;

Note: 1/8th of Integer values will come from the auto-box cache and not use additional memory. All possible Boolean and Byte values are cached.

array	size in bytes 32-bit JVM	size in bytes 64-bit JVM
new BitSet(1024)	168	168
new boolean[1024]	1040	1040
new Boolean[1024]	4112	4112
new ArrayList<Boolean>(1024)	4136	4136
new LinkedList<Boolean>() with 1024	24624	24624
new byte[1024]	1040	1040
new Byte[1024]	4112	4112
new ArrayList<Byte>(1024)	4136	4136
new LinkedList<Byte>() with 1024	24624	24624
new char[1024]	2064	2064
new Character[1024]	18448	18448
new short[1024]	2064	2064
new Short[1024]	18448	18448
new ArrayList<Character/Short>(1024)	18472	18472
new LinkedList<Character/Short>() with 1024	38960	38960
new int[1024]	4112	4112
new Integer[1024]	18448	18448
new float[1024]	4112	4112
new Float[1024]	20496	20496
new ArrayList<Integer/Float>(1024)	18472	18472
new LinkedList<Integer/Float>() with 1024	38960	38960
new long[1024]	8208	8208
new Long[1024]	18448	25616
new double[1024]	8208	8208
new Double[1024]	20496	28688
new ArrayList<Long/Double>(1024)	18472	25640
new LinkedList<Long/Double>() with 1024	38960	46128
new String[1024]	52464	61456
new ArrayList<String>(1024)	52488	61480
new LinkedList<String>() with 1024	72976	81968

In both cases, Java 6 update 26 was used. For the 64-bit JVM of this version, -XX:+UseCompressedOops is the default option which uses 32-bit references. If you use more than 32 GB of memory, you would need to turn this option off, using more memory for the arrays of objects.

The full code

Is available here: MemoryUsageExamplesTest

From http://vanillajava.blogspot.com/2011/07/java-how-much-memory-do-different.html