Java ArrayList.ensureCapacity() is a method in the Java ArrayList class that ensures the list has at least the specified capacity. It is used to minimize the number of reallocation operations as the list grows by pre-allocating space. Introduced in Java 1995 with Java 1.2, ensureCapacity() is commonly used for optimizing performance when the size of the list is known in advance, especially in situations where the list will be frequently modified.

https://docs.oracle.com/javase/8/docs/api/java/util/ArrayList.html