How do you make your Java application memory efficient?

How do you optimize the heap size usage of an application that has a lot (millions) of long-lived objects? (big cache, loading lots of records from a db)

• Use the right data type
  • Avoid java.lang.String to represent other data types
• Avoid duplicated objects
  • Use enums if the values are known in advance
  • Use object pools
  • String.intern() (good idea?)
• Load/keep only the objects you need

I am looking for general programming or Java specific answers. No funky compiler switch.

Edit:

Optimize the memory representation of a POJO that can appear millions of times in the heap.

Use cases

• Load a huge csv file in memory (converted into POJOs)
• Use hibernate to retrieve million of records from a database

Resume of answers:

• Use flyweight pattern
• Copy on write
• Instead of loading 10M objects with 3 properties, is it more efficient to have 3 arrays (or other data structure) of size 10M? (Could be a pain to manipulate data but if you are really short on memory...)

You don't say what sort of objects you're looking to store, so it's a little difficult to offer detailed advice. However some (not exclusive) approaches, in no particular order, are:

• Use a flyweight pattern wherever possible.
• Caching to disc. There are numerous cache solutions for Java.
• There is some debate as to whether String.intern is a good idea. See here for a question re. String.intern(), and the amount of debate around its suitability.
• Make use of soft or weak references to store data that you can recreate/reload on demand. See here for how to use soft references with caching techniques.

Knowing more about the internals and lifetime of the objects you're storing would result in a more detailed answer.