HashMap Hashtable Resizing Mechanism

Both HashMap and Hashtable will undergo a resizing operation when the number of elements reaches a certain threshold. Their resizing mechanisms are similar, but there are also some differences.

The resizing mechanism of HashMap:

1. When initialized, a default-sized array is created, and when the number of elements reaches 75% of the array size, a resize operation is triggered.
2. The expansion operation will create a new array that is twice the size of the original array, then recalculate the hash value of each element in the original array and place them into the corresponding position in the new array.
3. The process of expanding capacity is quite time-consuming as it requires recalculating hash values and repositioning elements.

The resizing mechanism of Hashtable:

1. An array of default size is created during initialization, and a resizing operation is triggered once the number of elements reaches 75% of the array’s size.
2. The resizing operation will create a new array with twice the size, then recalculate the hash values of the elements in the original array and place them in the corresponding positions of the new array.
3. Because Hashtable is thread-safe, it uses a synchronization lock during resizing to ensure thread safety, which in turn results in lower performance during resizing.

In general, both HashMap and Hashtable perform resizing operations when the number of elements reaches a certain threshold, but the resizing performance of HashMap is better than Hashtable.