What is capacity in the context of Java?

Capacity is the maximum number of elements that a collection can hold before it needs to change its internal representation (for example, an array). When the capacity is reached, the collection typically creates a new internal storage of larger size and copies the old elements into it.

Consider ArrayList as an example:

// Creating an ArrayList with an initial capacity of 10 (default)
ArrayList<String> list = new ArrayList<>();

// Adding elements. As long as the capacity is sufficient.
list.add("Element 1");
list.add("Element 2");

// If more than 10 elements are added, ArrayList increases its capacity.
// Usually, this is done by creating a new array of larger size
// (for example, 1.5 times the current capacity) and copying the elements.

The concept of capacity is important for understanding performance:

• Insufficient initial capacity can lead to frequent resize and copy operations, reducing performance when adding many elements.
• Excessive initial capacity can lead to inefficient memory usage, especially if the collection ends up containing few elements.

Some collections, such as HashMap, also have a concept related to capacity (number of "buckets" or "bins") and load factor, but the main idea of capacity most often relates to array-based structures like ArrayList or Vector."}]}]}