So, usually you generate wallets, not keypairs directly, to avoid exposing the ultrasensitive private key directly. But if you just want to get a keypair, here's an easy way with some of the tooling I'm working on.
Prerequisite: Java 8 installed on your machine.
Caveats: No warranties, if anybody gets or sees the private key any value associated with the keypair
may be stolen, the entropy behind the generation of these keys is the default implementation and seeding
java.security.SecureRandom, which may or may not be adequate. This is a SNAPSHOT version that references
lots of other SNAPSHOT libraries, which may not be stable and will be redownloaded with annoying frequency
until (hopefully soon) it is upgraded to a release version.