ArrayList maximum size

There is no specified maximum size of these structures.

The actual practical size limit is probably somewhere in the region of Integer.MAX_VALUE [i.e. 2147483647, roughly 2 billion elements], as that's the maximum size of an array in Java.

• A HashSet uses a HashMap internally, so it has the same maximum size as that
  • A HashMap uses an array which always has a size that is a power of two, so it can be at most 230 = 1073741824 elements big [since the next power of two is bigger than Integer.MAX_VALUE].
  • Normally the number of elements is at most the number of buckets multiplied by the load factor [0.75 by default]. However, when the HashMap stops resizing, then it will still allow you to add elements, exploiting the fact that each bucket is managed via a linked list. Therefore the only limit for elements in a HashMap/HashSet is memory.
• A Vector uses an array internally which has a maximum size of exactly Integer.MAX_VALUE, so it can't support more than that many elements
• A LinkedList doesn't use an array as the underlying storage, so that doesn't limit the size. It uses a classical doubly linked list structure with no inherent limit, so its size is only bounded by the available memory. Note that a LinkedList will report the size wrongly if it is bigger than Integer.MAX_VALUE, because it uses a int field to store the size and the return type of size[] is int as well.

Note that while the Collection API does define how a Collection with more than Integer.MAX_VALUE elements should behave. Most importantly it states this the size[] documentation:

If this collection contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.

Note that while HashMap, HashSet and LinkedList seem to support more than Integer.MAX_VALUE elements, none of those implement the size[] method in this way [i.e. they simply let the internal size field overflow].

This leads me to believe that other operations also aren't well-defined in this condition.

So I'd say it's safe to use those general-purpose collections with up to Integer.MAX_VLAUE elements. If you know that you'll need to store more than that, then you should switch to dedicated collection implementations that actually support this.

In all cases, you're likely to be limited by the JVM heap size rather than anything else. Eventually you'll always get down to arrays so I very much doubt that any of them will manage more than 231 - 1 elements, but you're very, very likely to run out of heap before then anyway.

It very much depends on the implementation details.

A HashSet uses an array as an underlying store which by default it attempt to grow when the collection is 75% full. This means it will fail if you try to add more than about 750,000,000 entries. [It cannot grow the array from 2^30 to 2^31 entries]

Increasing the load factor increases the maximum size of the collection. e.g. a load factor of 10 allows 10 billion elements. [It is worth noting that HashSet is relatively inefficient past 100 million elements as the distribution of the 32-bit hashcode starts to look less random, and the number of collisions increases]

A Vector doubles its capacity and starts at 10. This means it will fail to grow above approx 1.34 billion. Changing the initial size to 2^n-1 gives you slightly more head room.

BTW: Use ArrayList rather than Vector if you can.

A LinkedList has no inherent limit and can grow beyond 2.1 billion. At this point size[] could return Integer.MAX_VALUE, however some functions such as toArray will fail as it cannot put all objects into an array, in will instead give you the first Integer.MAX_VALUE rather than throw an exception.

As @Joachim Sauer notes, the current OpenJDK could return an incorrect result for sizes above Integer.MAX_VALUE. e.g. it could be a negative number.

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