Java Reference
In-Depth Information
Random random= new Random(long);
where long is any long integer, or
Random random= new Random();
In the first case, the seed used to start the sequence of random numbers is based
on argument long . In the second case, the seed is the time in milliseconds at
which the new-expression was evaluated.
Use the first case when testing a program because you may need to repeat
an execution in order to help find errors. Use the second case when running so
that you always use a new seed and thus get a new sequence of random numbers.
Now, whenever a new random number is generated, use one of the follow-
ing function calls. There are several possibilities because one might want to gen-
erate sequences of random values in different types and ranges:
random.nextBoolean() = a boolean value
random.nextDouble() = a double d satisfying 0≤d<1
random.nextFloat() = a float f satisfying 0≤f<1
random.nextInt() = an int
random.nextInt(n) = an int i satisfying 0≤i<n
random.nextLong() = a long
Class Random has other instance methods, but the ones discussed above will
be used most frequently.
Exercises with random numbers
E1. Write a function oneOrTwo that uses function Math.random to return a ran-
dom integer in the range 1..2 . Test it.
E2. Function oneOrTwo produces either 1 or 2 , randomly. One can think of 1 as
“heads” and 2 as “tails”, so we can think of a call of function oneOrTwo as sim-
ulating a flip of a coin. If we flip a coin 100 times, or 1,000 times, we would
assume that half the tosses are “heads” and half are “tails”. Write a program to
test whether oneOrTwo is really fair, in this sense. The program will call
oneOrTwo a certain number of times and report back how many of the tosses
were “heads” and how many were “tails”. Experiment with this program.
E3. Write a function to “throw a die (meaning one of a pair of dice)” —it should
produce an integer in the range 1..6 .
E4. Write a program that throws a die n times (for some given n ) and counts how
many times one roll is followed by exactly the same roll. E.g. the answer for the
7 -roll sequence 3, 2, 2, 4, 4, 4, 3 is 3 , since a 2 is followed by 2 , a 4 is fol-
lowed by 4 , and that 4 is followed by 4 . After you test it, experiment with it.
E5. Write a program that rolls a die until a 6 is rolled. Print out how many rolls
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