In practice Java will be about 20% slower than C, assuming a pure C implementation (no SIMD assembly) and you're not doing a ridiculous amounts of array ops (though, the JVM is only getting better at eliding bounds checks these days).
In all honestly, unless you're performing shitloads of matrix operations or using copious amounts of S?SSE[1-4] assembly, you probably aren't making an application that'll see a speed difference between the two.
Now memory usage and start up times; Yea, Java sucks shit at that.