Success

For linear probing we are guaranteed to find an empty place, provided the table is not completely full. If we use quadratic probing carelessly, this may not be true. A sequence of successive probes may only explore a fraction of the table, and this fraction may be quite small. In that case, we may fail to find an empty location to store a new item, even though the table is by no means full. It can be shown, however, that successful insertion is guaranteed provided

1. table.length is a prime number
2. loadFactor is less than 0.5

see Section 20.4 of Weiss, Data Structures & Problem Solving Using Java. Neither of these requirements is difficult to satisfy.