How to Generate Random Numbers in Java between Range - Example Tutorial

In software development and programming world we often needs to generate
random numbers, some time random integers in a range e.g. 1 to 100 etc.
Thankfully, Random number generation in Java is easy as Java API provides good
support for random numbers via java.util.Random class, Math.random() utility
method and recently ThreadLocalRandom class in Java 7, along with more
popular features like String in Switch and ARM blocks. While random()
method seems most convenient way of generating
randoms in Java it only return random doubles, on the other hand by using Random,  you can generate pseudo-random integer,
floating point numbers e.g. double and even random boolean values. In this
article Java tutorial, we will see how to generate random numbers in Java, examples
to generating random integers and real numbers, and random numbers within a
range e.g. between 1 to 6. we will also explore difference between Math.random() and java.util.Random class in
Java.

Random Numbers in Java with Example

As I said earlier Random class in Java is used to create
random numbers. you can create instance of java.util.Random class by
default seed or you can  provide your own
seed by calling special constructor of this class, Random(long
seed). This class provides methods to return random integers, doubles, float
and  boolean values. here is an simple
example of generating random numbers in Java using java.util.Random class:

Random random = new Random();

      

 for(int i =0; i<5; i++){

      int randomInteger = random.nextInt();

      System.out.println("Random
Integer in Java: " + randomInteger);

 }

Output:

Random Integer in Java: -1194538938

Random Integer in Java: -973476571

Random Integer in Java: -1283186469

Random Integer in Java: 1176677327

Random Integer in Java: 265551019

You can see java.util.Random  by default generates random numbers in with range of integers in Java
which is -2^31 to 2^31-1, which consists both positive and
negative  integers. By the way, you can
also use Math.random() to create random numbers, here is
an example of generating random numbers using Math.random() in Java:  

for(int i =0; i<3; i++){

     double randomDouble = Math.random();

     System.out.println("Random
Number in Java: " + randomDouble);

}

Output:

Random Number in Java: 0.767752638941705

Random Number in Java: 0.482517390182052

Random Number in Java: 0.28243911879792283

As you can see Math.random() code will return random numbers
which are greater than or equal to 0.0 and less than 1.0

How to
generate random Integers in Java

If you want to create random numbers in the range of integers in Java
than best is to use random.nextInt() method it will return all
integers with equal probability. You can also use Math.random() method to
first create random number as double and than scale that number into int
later. So you can create random integers in two step process.

Generating
random Double in Java

Similar to random integers in Java, java.util.Random class
provides method nextDouble() which can return uniformly
distributed pseudo random double values between 0.0 and 1.0.

Generating
random Boolean values in Java

Use java.util.Random method nextBoolean() to
generate random boolean values in Java. here is an example of generating
boolean values randomly in Java:

Generating random number in a range in Java
– between two numbers

You can use both Math.random() and java.util.Random to
generate random numbers between a range. Most of the time we need Integer values
and since Math.random() return a floating point number,
precisely a double value,  we need to change that into an integer by
casting it. Here is a Java code example of using both Random class and random() method for generating random number in range: 

for(int i =0; i<3; i++){

    int randomInteger = random.nextInt(10);

    System.out.println("pseudo
random int in a range : " + randomInteger);

}

Output:

pseudo random int in a range : 7

pseudo random int in a range : 1

pseudo random int in a range : 3

Random.nextInt(10) method returns random
numbers between 0 (included) and 10 (excluded)

Random
number between 0 and 10 – Java Example

Here is a code snippet, which can be used to generate random numbers in a
range between 0  to 10, where 0 is
inclusive and 10 is exclusive. This code uses Math.random() method,
which returns pseudo-random number in a range 0.0 to 1.0,
where later is exclusive, by multiplying output with and then type casting into int, we can
generate random integers in any range. If you need pseudo random number between
1 to 100, you can simply multiply output of random() method
with 100 and then cast it into int for integer result. 

for(int i =0; i<3; i++){

      int randomInt = (int)(10.0 * Math.random());

      System.out.println("pseudo
random int between 1 and 10 : " + randomInt );

}

Output:

pseudo random int between 1 and 10 : 4

pseudo random int between 1 and 10 : 0

pseudo random int between 1 and 10 : 2

Difference between Math.random() and java.util.Random
class in Java

Though you can generate random numbers by using either ways in Java ,
there is slight difference in terms of usage and behavior between Math.random() method and
java.util.Random class:

1) In order to generate random numbers, these are actually pseudo random
numbers, by using java.util.Random class you need to create an
instance of this class, while in case of Math.random() method, instance of Random number
generator is created, when you first call it. This pseudo random number
generator is equivalent to new Random(), and only used exclusively here.

2) java.util.Random class has support for generating
random integers, longs, float, double and boolean while Math.random() only
returns random double values greater than or equal to 0.0 and less than 1.0.

3) You can not change seed for generating random numbers in case of Math.random(), which is created
at first time any one call this method. This method is also synchronized to allow proper
use in multithreaded environment, but that can lead to contention, when number
of thread grows. By the way, you can also use ThreadLocalRandom from JDK
1.7 for better performance, it you need to create random numbers simultaneously
in multiple threads.

4) Math.random() is more of utility method while java.util.Random is actual
random number generator class, which provides range of method to generate
numbers in different data types.

Summary

1. Random numbers in Java can be generated using either java.util.Random , ThreadLocalRandom class or
by using Math.random() method. ThreadLocalRandom is only
available from Java 7.

2. Random class can generate random integer, double, float and booleans.

3. Random numbers generated are pseudo random,  created with equal probabilities and in attempt
of uniformly distribution. So Random.nextInteger() can return
any random integer number between 2^32 values
with equal probability.

4. Math.random() only generates double value
greater than or equal to 0.0 and less than 1.0.

5. Math.random() is a thread-safe method and can be
called form multiple threads but its good idea to have separate random
number  generators for separate thread to
reduce contention. ThreadLocalRandom from Java 1.7 could be another
choice if you are sharing your Random number generator among multiple threads.

6. Though Random number can accept a long seed(64 bits) it only uses 48
bits for generating random numbers.

7. Random is not a final class and you can
extends it to implement your own algorithm or to use all 64 bits of seed.

8. Easy and convenient way to create random numbers in java is Math.random() method.

That’s all on How to generate random numbers in Java. We have seen
examples of generating random integers in a range say 1 to 10, which is quite
common and very useful as well. You can even use ThreadLocalRandom from Java
1.7, which is a Random number generator isolated to a particular thread, which
reduces contention, if used in multi-threaded environment.  You can even employ the method here to
generate random number in a range or without repetition by adding those extra
logic on top of this.

Source:http://javarevisited.blogspot.com/2013/05/how-to-generate-random-numbers-in-java-between-range.html