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I am trying to insert about 50,000 objects (and therefore 50,000 keys) into a java.util.HashMap<java.awt.Point, Segment>. However, I keep getting an OutOfMemory exception. (Segment is my own class - very light weight - one String field, and 3 int fields).
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at java.util.HashMap.resize(HashMap.java:508)
at java.util.HashMap.addEntry(HashMap.java:799)
at java.util.HashMap.put(HashMap.java:431)
at bus.tools.UpdateMap.putSegment(UpdateMap.java:168)
This seems quite ridiculous since I see that there is plenty of memory available on the machine - both in free RAM and HD space for virtual memory.
Is it possible Java is running with some stringent memory requirements? Can I increase these?
Is there some weird limitation with HashMap? Am I going to have to implement my own? Are there any other classes worth looking at?
(I am running Java 5 under OS X 10.5 on an Intel machine with 2GB RAM.)
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In Sun's JVM, HashMap uses an array which is a power of 2. The largest power of two allowed for an array size is 2^30 . And the largest number of elements you can have before the HashMap will try to double its size to 2^31 (which it cannot do) is ( 2^30 * loadFactor ) or about 700 million for the default load factor.
A HashMap in Java can have a maximum of 2^30 buckets for storing entries - this is because the bucket-assignment technique used by java. util. HashMap requires the number of buckets to be a power of 2, and since ints are signed in Java, the maximum positive value is 2^31 - 1, so the maximum power of 2 is 2^30.
OutOfMemoryError exception. Usually, this error is thrown when there is insufficient space to allocate an object in the Java heap. In this case, The garbage collector cannot make space available to accommodate a new object, and the heap cannot be expanded further.
As explained in the above paragraph this OutOfMemory error in java comes when the Permanent generation of heap is filled up. To fix this OutOfMemoryError in Java, you need to increase the heap size of the Perm space by using the JVM option "-XX: MaxPermSize".
You can increase the maximum heap size by passing -Xmx128m (where 128 is the number of megabytes) to java. I can't remember the default size, but it strikes me that it was something rather small.
You can programmatically check how much memory is available by using the Runtime class.
// Get current size of heap in bytes
long heapSize = Runtime.getRuntime().totalMemory();
// Get maximum size of heap in bytes. The heap cannot grow beyond this size.
// Any attempt will result in an OutOfMemoryException.
long heapMaxSize = Runtime.getRuntime().maxMemory();
// Get amount of free memory within the heap in bytes. This size will increase
// after garbage collection and decrease as new objects are created.
long heapFreeSize = Runtime.getRuntime().freeMemory();
(Example from Java Developers Almanac)
This is also partially addressed in Frequently Asked Questions About the Java HotSpot VM, and in the Java 6 GC Tuning page.
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