Performance Checklist

Most of these suggestions apply only after a bottleneck has been identified:

• Establish whether you have a memory problem.
• Reduce the number of temporary objects being used, especially in loops.
  • Avoid creating temporary objects within frequently called methods.
  • Presize collection objects.
  • Reuse objects where possible.
  • Empty collection objects before reusing them. (Do not shrink them unless they are very large.)
  • Use custom conversion methods for converting between data types (especially strings and streams) to reduce the number of temporary objects.
  • Define methods that accept reusable objects to be filled in with data, rather than methods that return objects holding that data. (Or you can return immutable objects.)
  • Canonicalize objects wherever possible. Compare canonicalized objects by identity.
  • Create only the number of objects a class logically needs (if that is a small number of objects).
  • Replace strings and other objects with integer constants. Compare these integers by identity.
  • Use primitive data types instead of objects as instance variables.
  • Avoid creating an object that is only for accessing a method.
  • Flatten objects to reduce the number of nested objects.
  • Preallocate storage for large collections of objects by mapping the instance variables into multiple arrays.
  • Use StringBuffer rather than the string concatenation operator (+).
  • Use methods that alter objects directly without making copies.
  • Create or use specific classes that handle primitive data types rather than wrapping the primitive data types.
• Consider using a ThreadLocal to provide threaded access to singletons with state.
• Use the final modifier on instance-variable definitions to create immutable internally accessible objects.
• Use WeakReferences to hold elements in large canonical lookup tables. (Use SoftReferences for cache elements.)
• Reduce object-creation bottlenecks by targeting the object-creation process.
  • Keep constructors simple and inheritance hierarchies shallow.
  • Avoid initializing instance variables more than once.
  • Use the clone( ) method to avoid calling any constructors.
  • Clone arrays if that makes their creation faster.
  • Create copies of simple arrays faster by initializing them; create copies of complex arrays faster by cloning them.
• Eliminate object-creation bottlenecks by moving object creation to an alternative time.
  • Create objects early, when there is spare time in the app, and hold those objects until required.
  • Use lazy initialization when there are objects or variables that may never be used, or when you need to distribute the load of creating objects.
  • Use lazy initialization only when there is a defined merit in the design, or when identifying a bottleneck that is alleviated using lazy initialization.