Question

Using java I want to generate some random values in one program and then use these values in some other program everytime the 2nd program is executed.

Purpose of this is to generate random values once and then hold and keep them constant for every run of the program later. Is it possible in some way? Thanks

Answer 1

When you exit a program, anything you don’t store in a file is lost.

I suspect you don’t need to worry about IO as much as you think. You should be able to read millions of values in a few milli-seconds. In fact you should be able to generate millions of random numbers in a fraction of a second.

Random random = new Random(1);
long start = System.nanoTime();
int values = 1000000;
for (int i = 0; i < values; i++)
    random.nextInt();
long time = System.nanoTime() - start;
System.out.printf("Took %.3f seconds to generate %,d values%n",
                  time / 1e9, values);

prints

Took 0.015 seconds to generate 1,000,000 values

---

Generating and writing

int values = 1000000;
ByteBuffer buffer = ByteBuffer.allocateDirect(4 * values).order(ByteOrder.nativeOrder());

Random random = new Random(1);
long start = System.nanoTime();
for (int i = 0; i < values; i++)
    buffer.putInt(random.nextInt());
buffer.flip();
FileOutputStream fos = new FileOutputStream("/tmp/random.ints");
fos.getChannel().write(buffer);
fos.close();
long time = System.nanoTime() - start;
System.out.printf("Took %.3f seconds to generate&write %,d values%n", time / 1e9, values);

prints

Took 0.021 seconds to generate&write 1,000,000 values

---

Reading the same file.

long start2 = System.nanoTime();
FileInputStream fis = new FileInputStream("/tmp/random.ints");
MappedByteBuffer buffer2 = fis.getChannel().map(FileChannel.MapMode.READ_ONLY, 0, values * 4).order(ByteOrder.nativeOrder());
for (int i = 0; i < values; i++)
    buffer2.getInt();
fis.close();
long time2 = System.nanoTime() - start2;
System.out.printf("Took %.3f seconds to read %,d values%n", time2 / 1e9, values);

prints

Took 0.011 seconds to read 1,000,000 values

---

Reading the same file repeatedly

long sum = 0;
int repeats = 1000;
for (int j = 0; j < repeats; j++) {
    buffer2.position(0);
    for (int i = 0; i < values; i++)
        sum += buffer2.getInt();
}
fis.close();
long time2 = System.nanoTime() - start2;
System.out.printf("Took %.3f seconds to read %,d values%n", time2 / 1e9, repeats * values);

prints

Took 1.833 seconds to read 1,000,000,000 values