In Jeffrey Rosen's profile of Justice Stevens, there is a discussion about the latter's views on affirmative action:

Consider affirmative action, an area in which many people believe Stevens became more liberal. In 1980, he dissented from the court�s decision upholding racial preferences in federal contracting, explosively comparing them with the Nazi laws excluding Jews from citizenship. More recently, however, he has voted enthusiastically to uphold affirmative action in universities and public-school enrollment plans, comparing them to welcome mats rather than no-trespassing signs. To Stevens, however, his views have been consistent. "There's a tremendous difference in using affirmative action when you get a group to build a highway and affirmative action in the educational context," he told me. "I think my rhetoric was probably a little strong," he continued, but the federal law authorizing racial preferences for highway contracts was a "slapdash statute" that was based on pork-barrel politics, benefiting one group of contractors rather than citizens as a whole. In schools and universities, by contrast, "the whole student body profits from having diversity in the classes. So I really don't think I've changed my views about this."

I'm looking at this superficially, without reading his opinions in the relevant cases, but frankly the above distinction between Stevens-on-federal-contracting (affirmative action BAD) and Stevens-on-education (affirmative action GOOD) does not seem accurate. Look at the almost neverending story of Adarand Constructors, which is exactly about racial preferences for highway contracts, and in which Stevens has supported the government's policy of giving the primary contractor additional money if he hires minority subcontractors.

When the majority in Adarand v. Pena held against such a preference, Stevens's dissent cited his dissenting opinion in Fullilove v. Klutznick, the 1980 case he mentioned, but never reconciles with it. Instead, he holds up the Fullilove plurality opinion and other pro-affirmative action decisions as precedents the Court is bound to follow. If Stevens wants to argue that he has been consistent on the procedure the Court should use in its decision-making (e.g., adherence to a 15-year-old line of precedent), that's fine, but as I understand the cases, he has not been consistent on the substantive law the Court should apply in its decision-making.