trimToSize

trimToSize is a method found in certain Java collection classes, most notably java.util.ArrayList and java.util.Vector. Its purpose is to reduce the internal storage capacity of the list so that it matches the list’s current size, thereby potentially lowering memory usage by discarding unused capacity.

In ArrayList, the list stores elements in an internal array (often referred to as elementData). The size field tracks how many elements are logically present, while the array length represents capacity. When trimToSize is called and there is unused capacity (size < elementData.length), the method replaces the internal array with a new array whose length equals the current size and copies the existing elements into it. If the capacity already equals the size, nothing changes. The modification is recorded as a structural change.

Performance and considerations: trimToSize is typically an O(n) operation because it copies elements to a new array. It reduces the memory footprint when a list has shrunk significantly, but it may incur a cost if the list grows again later, since a larger array will need to be allocated and elements re-copied. For ArrayList, when more elements are added after trimming, the internal array will grow according to the list’s growth policy (often around a 1.5x expansion factor).

When to use: consider trimToSize after bulk removals or when a long-lived list is known to hold a small portion of its previously allocated capacity. It is not required for normal usage and is not inherently thread-safe; external synchronization may be needed for concurrent code.