California Code, Elections Code - ELEC § 13108

(a) At the first elections for Representative in Congress, State Senator, Member of the Assembly, and Member of the Board of Equalization in each congressional, senatorial, Assembly, and Board of Equalization district following the adjustment of the boundary lines of the congressional, senatorial, Assembly, and Board of Equalization districts by the Citizens Redistricting Commission pursuant to Section 6 of Article IV, Section 17 of Article XIII, and Section 1 of Article XXI, of the California Constitution, that candidate who shall be deemed the incumbent in a given district for purposes of the election shall be that candidate who is running for the same office that he or she then holds, and who is running for reelection in a district that has the identical boundaries and number as the district from which he or she was last elected.

(b) In the event there is no candidate to whom subdivision (a) applies, the incumbent shall be that candidate who is running for the same office that he or she then holds, and who is running for reelection in a district that has the identical boundaries as the district from which he or she was last elected, but that has a different number.

(c) In the event there is no candidate to whom subdivision (a) or (b) apply, the incumbent shall be that candidate who is running for the same office that he or she then holds, and who is running for reelection in a district that contains some portion of the territory previously contained within the district from which he or she was last elected.  However, in a new district that contains portions of the territory of more than one former district, the incumbent shall be that candidate the greater portion of the population, as determined by the most recent federal decennial census, of whose former district is included within the new district.

(d) In the event there is no candidate to whom subdivision (a), (b), or (c) apply, the incumbent shall be that candidate who is running for the same office that he or she then holds, and who is running for reelection in a district that has the identical number as the district from which he or she was last elected.  However, a candidate for the office of Member of the Assembly shall be considered the incumbent in this case only if the district bearing the same number is located in the same county as the district that previously bore that number.

(e) If there is no candidate in a given district to which any of the above provisions apply, the incumbent shall be the person who is a candidate for the same office that he or she then holds who fulfills the residency requirements of law for candidacy within the district.