System.currentTimeMillis - 現在時刻を数値で取得

public class Foo {

    public static void main(String[] args) throws Exception {
        long timeMillisStart = System.currentTimeMillis();
        
        long sum = 0;
        for (int i=0; i<1000000000; i++) {
            sum += i;
        }
        
        long timeMillisEnd = System.currentTimeMillis();
        
        long time = timeMillisEnd - timeMillisStart;
        System.out.println(time + "ms. " + time/1000 + "s" );
    }
}