How to generate random integers by Java
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1. Java.util.Random
This Random (). NextInt (int bound) generates a random integer from 0 (inclusive) to bound (excluding).
(1) Code snippet
For getRandomNumberInRange (5,10), this produces a random integer between 5 (inclusive) and 10 (inclusive).
Private static int getRandomNumberInRange (int min, int max) {if (min > = max) {throw new IllegalArgumentException ("max must be greater than min");} Random r = new Random (); return r.nextInt ((max-min) + 1) + min;}
(2) what is (max-min) + 1) + min?
The above formula produces a random integer between min (inclusive) and max (inclusive).
/ / Random () .nextInt (int bound) = Random integer from 0 (inclusive) to bound (exclusive) / / 1.nextInt (range) = nextInt (max-min) new Random () .nextInt (5); / / [0.4] [min = 0, max = 4] new Random (). NextInt (6); / / [0.5] new Random (). NextInt (7) / / [0.6] new Random () .nextInt (8); / / [0.7] new Random () .nextInt (9); / / [0.8] new Random () .nextInt (10); / / [0.9] new Random () .nextInt (11) / / [0.10] / / 2. To include the last value (max value) = (range + 1) new Random () .nextInt (5 + 1) / / [0.5] [min = 0 Max = 5] new Random (). NextInt (6 + 1) / / [0.6] new Random (). NextInt (7 + 1) / / [0.7] new Random (). NextInt (8 + 1) / / [0.8] new Random (). NextInt (9 + 1) / / [0.9] new Random (). NextInt (10) + 1) / / [0.10] new Random () .nextInt (11 + 1) / / [0.11] / / 3. To define a start value (min value) in a range / / For example The range should start from 10 = (range + 1) + min new Random (). NextInt (5 + 1) + 10 / / [0.5] + 10 = [10.15] new Random () .nextInt (6 + 1) + 10 / / [0.6] + 10 = [10.16] new Random () .nextInt (7 + 1) + 10 / / [0.7] + 10 = [10.17] new Random (). NextInt (8 + 1) + 10 / / [0.8] + 10 = [10.18] new Random (). NextInt (9 + 1) + 10 / / [0.9] + 10 = [10.19] new Random () .nextInt (10 + 1) + 10 / / [0.10] + 10 = [10 .. 20] new Random (). NextInt (11 + 1) + 10 / / [0.11] + 10 = [10.. 21] / / Range = (max-min) / / So The final formula is ((max-min) + 1) + min / / 4. Test [10.. 30] / / min = 10, max = 30 Range = (max-min) new Random (). NextInt ((max-min) + 1) + min new Random (). NextInt ((30-10) + 1) + 10 new Random (). NextInt ((20) + 1) + 10 new Random (). NextInt (21) + 10 / / [0.20] + 10 = [10. 30] / 5. Test [15. 99] / / min = 15 Max = 99, range = (max-min) new Random (). NextInt ((max-min) + 1) + min new Random (). NextInt ((99-15) + 1) + 15 new Random (). NextInt ((84) + 1) + 15 new Random (). NextInt (85) + 15 / / [0.84] + 15 = [15.99] / / Done, understand?
(3) generate a complete example of 10 random integers ranging from 5 (inclusive) to 10 (inclusive).
Package com.mkyong.example.test;import java.util.Random;public class TestRandom {public static void main (String [] args) {for (int I = 0; I)
< 10; i++) { System.out.println(getRandomNumberInRange(5, 10)); } } private static int getRandomNumberInRange(int min, int max) { if (min >= max) {throw new IllegalArgumentException ("max must be greater than min");} Random r = new Random (); return r.nextInt ((max-min) + 1) + min;}}
Output
seven
six
ten
eight
nine
five
seven
ten
eight
five
two。 Mathematics。 Random
This Math.random () gives a random double value from 0. 0 (inclusive) to 1. 0 (excluding).
(1) Code fragment. Reference 1.2 is more or less the same formula.
(int) (Math.random () * ((max-min) + 1)) + min
(2) generate a complete example of 10 random integers in the range 16 (inclusive) to 20 (inclusive).
Package com.mkyong.example.test;public class TestRandom {public static void main (String [] args) {for (int I = 0; I)
< 10; i++) { System.out.println(getRandomNumberInRange(16, 20)); } } private static int getRandomNumberInRange(int min, int max) { if (min >= max) {throw new IllegalArgumentException ("max must be greater than min");} return (int) (Math.random () * (max-min) + 1) + min;}}
Output
seventeen
sixteen
twenty
nineteen
twenty
twenty
twenty
seventeen
twenty
sixteen
3. Java 8 Random.ints
In Java 8, a new method java.util.Random has been added
Public IntStream ints (int randomNumberOrigin, int randomNumberBound) public IntStream ints (long streamSize, int randomNumberOrigin, int randomNumberBound)
This Random.ints (int origin, int bound) or Random.ints (int min, int max) generates a random integer from the origin (inclusive) to the binding (excluding).
(1) Code fragment.
Private static int getRandomNumberInRange (int min, int max) {Random r = new Random (); return r.ints (min, (max + 1)) .findFirst () .getAsInt ();}
(2) generate a complete example of 10 random integers in the range 33 (inclusive) to 38 (inclusive).
Package com.mkyong.form.test;import java.util.Random;public class TestRandom {public static void main (String [] args) {for (int I = 0; I < 10; iTunes +) {System.out.println (getRandomNumberInRange (33,38)) }} private static int getRandomNumberInRange (int min, int max) {Random r = new Random (); return r.ints (min, (max + 1)) .limit (1) .findFirst () .getAsInt ();}}
Output
thirty-four
thirty-five
thirty-seven
thirty-three
thirty-eight
thirty-seven
thirty-four
thirty-five
thirty-six
thirty-seven
(3) extra for your own reference.
Generates a random integer in the range of 33 (inclusive) and 38 (excluding) with a stream size of 10. And print out with forEach.
/ / Java 8 only new Random () .ints (10, 33, 38) .forEach (System.out::println)
Output
thirty-four
thirty-seven
thirty-seven
thirty-four
thirty-four
thirty-five
thirty-six
thirty-three
thirty-seven
thirty-four
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