How's this for maturity?!? 

Casper Smart tells TMZ Sports there's ZERO jealousy or animosity or bitterness at Alex Rodriguez for dating his ex-girlfriend, Jennifer Lopez ... and actually praised A-Rod as a "legend."

Of course, Casper and J Lo dated for years before finally calling it quits in 2016. She's now dating A-Rod ... and it appears he has Casper's full blessing.

In fact, Casper says he's only heard great things about the ex-Yankees star and says when it comes to his feelings toward the baseball player -- "It's all respect and love."