 Nyorai mberi kweiyi ndima - Johnson akafunga kupinda mu2009 NBA draft, asi akaita chipikirwa chomuromo chokudzokera nokuda kwegore rokutanga. Akatanga mwaka wake wejunior semwaka wekutanga-timu yekutanga-Big Ten sarudzo neCBS Sports. Mumwaka, akarekodha mapfumbamwe maviri-maviri, achitungamira Purdue kutanga kwayo kwakanakisa mumakore gumi nematanhatu 140. Johnson akatumidzwa zita reBig Ten Player yeVhiki mukupera kwaNdira, avhareji ye22.5 mapoinzi uye 9.5 rebounds mumitambo miviri 20. Musi waFebruary 9, 2010, Johnson akarekodha pfungwa yake yechi1000 yebasa mumutambo wemugwagwa kukunda 10 yakatarwa Michigan State. Achipedzisa ne 144 rekodhi mumusangano wekutamba, Johnson akatungamira Purdue kunzvimbo yayo yekutanga yeBig Ten mumakore gumi nemana, achigumisa nekukunda munzira vachipesana nePenn State, kwaaive nemapoinzi makumi maviri nerimwe, gumi rebounds, matatu mabhuroko, uye akapfura 1114 pamutsetse. Mune avhareji yemaminitsi 31.3 pamutambo, Johnson avhareji 15.4 mapoinzi 3rd pachikwata, 7.3 rebounds 1st, uye 1 kuba pamutambo. Johnson akapfura 72 muzana pamutsetse, apo akanga achitungamirira boka rePurdue rakanga rakarongwa pamusoro pegumi ne 2 mabhuroko mumutambo uye kupfura 51 muzana kubva pasi. Johnson akatumidzwa Second-Team All-Big Ten uye akagamuchira akateedzana Big Ten All-Defensive Team kukudzwa pakupera kwemwaka wenguva dzose. With Purdue kugamuchira 4 mbeu iri NCAA Tournament, Johnson akatungamirira Boilermakers muna First Round kukunda pamusoro Siena, apo akanga 23 pfungwa, 15 rebounds, uye 3 mabhuroko, zvichiita kuti anotevedzana Sweet gumi nenhanhatu kuoneka pashure kurova Texas A M mu nguva yokuwedzera. Mumutambo wake wekupedzisira wemwaka wake wejunior, Johnson aive nemapoka akakwirira aine mapoinzi makumi maviri nematatu, achiita gumi nematanhatu ekuyedza, achirova 5 rebounds uye 4 mabhuroko mukurasikirwa neDuke. Johnson akapedza mwaka uyu wejunior aine yechipiri yakawanda yebasa mabhuroko munhoroondo yechikoro ne184, mushure meJoe Barry Carrolls 349 kubva 1977 kusvika 1980. Johnson akabata wechitatu wepamusoro wepakati munhoroondo yechikoro aine 1.7 mabhuroko pamutambo, kumashure kwaCarroll naRussell Cross. Akapedza zhizha rakatevera achidzidzira neTeam USA. 