Question

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.)

Answer 1

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.