Performance Impact of Data Structure Resizing in Java

Java applications tend to make very frequent use of StringBuilder or StringBuffer for assempling of Strings. Both StringBuilder and StringBuffer use char[] internally for their data storage. As elements are added to StringBuffer or StringBuilder , the underlying char[] may be subject to resizing. As a result a new char[] of larger size (2x size) is allocated and elements from old char[] are copied into new char[]. Old char[] are discarded then and it becomes available for Garbage collection.This whole process results in consumption of extra CPU cycles for new array allocation, copying of elements from old array to new array and at some future point cost of garbage collection cycle.

Above facts also true for other java collections that uses array for its internal data storage such as ArrayList, Vector, ConcurrentHashMap and HashMap. Other Collections such as LinkedList or TreeMap often use one or more object references between elements stored to chain together the elements managed by the Collections. Collections that use array for data storage , their relevant classes also provides constructors that provide optional size arguments , but these constructors are often not used or size provided in application program is not optimal for Collection's use.

For HashMaps , to actually view whether resizing is happening or not, you need to perform memory profiling either run time profiling or offline using dumps. In either case, if you are observing java.util.HashMap.resize(int) method call, then your HashMap is getting resized during application execution.