6. An apparatus for generating recommended items for a current user in an unfamiliar domain, comprising: a reference user determining module configured to: select a plurality of candidate reference users sharing a user type with the current user in a reference domain, wherein the reference domain differs from the unfamiliar domain; calculate a plurality of user similarity indices for the plurality of candidate reference users, comprising calculating a user similarity index for a candidate reference user with respect to a reference domain and the current user, wherein the user similarity index is based at least in part on common behaviors between the current user and the candidate reference user in the reference domain, and is calculated according to history behavior data of a plurality of demarcated users in the unfamiliar domain and the reference domain, wherein the plurality of demarcated users are similar users in two or more domains that are different than the current user; and select the candidate reference user as a reference user in the reference domain, responsive to the user similarity index of the candidate reference user satisfying a user similarity index condition; a domain similarity determining module configured to determine a domain similarity index between the unfamiliar domain and the reference domain, wherein the domain similarity index is distinct from the user similarity index, and wherein the domain similarity index indicates a degree of similarity between the unfamiliar domain and the reference domain; and one or more computer processors configured to: calculate a user weight for the reference user with respect to the current user, wherein the user weight is calculated by multiplying the user similarity index by the domain similarity index; generate two or more candidate items in the unfamiliar domain for the current user according to history behavior data of other users, comprising at least the reference user, in the unfamiliar domain; assign an item weight to each of the two or more candidate items, wherein for each of the two or more candidate items, the item weight assigned is calculated by adding the user weights of the other users who are associated with the candidate item; and select, as the recommended items, a subset of the candidate items whose item weights meet an item weight condition.
6. An apparatus for generating recommended items for a current user in an unfamiliar domain, comprising: a reference user determining module configured to: select a plurality of candidate reference users sharing a user type with the current user in a reference domain, wherein the reference domain differs from the unfamiliar domain; calculate a plurality of user similarity indices for the plurality of candidate reference users, comprising calculating a user similarity index for a candidate reference user with respect to a reference domain and the current user, wherein the user similarity index is based at least in part on common behaviors between the current user and the candidate reference user in the reference domain, and is calculated according to history behavior data of a plurality of demarcated users in the unfamiliar domain and the reference domain, wherein the plurality of demarcated users are similar users in two or more domains that are different than the current user; and select the candidate reference user as a reference user in the reference domain, responsive to the user similarity index of the candidate reference user satisfying a user similarity index condition; a domain similarity determining module configured to determine a domain similarity index between the unfamiliar domain and the reference domain, wherein the domain similarity index is distinct from the user similarity index, and wherein the domain similarity index indicates a degree of similarity between the unfamiliar domain and the reference domain; and one or more computer processors configured to: calculate a user weight for the reference user with respect to the current user, wherein the user weight is calculated by multiplying the user similarity index by the domain similarity index; generate two or more candidate items in the unfamiliar domain for the current user according to history behavior data of other users, comprising at least the reference user, in the unfamiliar domain; assign an item weight to each of the two or more candidate items, wherein for each of the two or more candidate items, the item weight assigned is calculated by adding the user weights of the other users who are associated with the candidate item; and select, as the recommended items, a subset of the candidate items whose item weights meet an item weight condition. 10. The apparatus according to claim 6 , wherein the reference user determining module is further configured to: determine a current purchase vector for the current user, wherein each element of the current purchase vector corresponds to a purchasable item, and wherein the value of each element in the current purchase vector indicates whether the current user has purchased the purchasable item corresponding to the element; determine a reference purchase vector for the reference user, wherein each element of the reference purchase vector corresponds to a purchasable item, and wherein the value of each element in the reference purchase vector indicates whether the reference user has purchased the purchasable item corresponding to the element; and calculate an irrelevancy index based on a sum of the squares of the differences between the elements of the current purchase vector and the reference purchase vector; wherein the user similarity index is based at least in part on the irrelevancy index.