Java Tutorial/Development/Random

Create two random number generators with the same seed

import java.util.Random;
public class Main {
  public static void main(String[] argv) throws Exception {
    Random rand = new Random();
    long seed = rand.nextLong();
    rand = new Random(seed);
    Random rand2 = new Random(seed);
  }
}

Generate a random array of numbers

import java.util.Random;
public class Main {
  public static void main(String[] args) {
    Random r = new Random();
    // generate some random boolean values
    boolean[] booleans = new boolean[10];
    for (int i = 0; i < booleans.length; i++) {
      booleans[i] = r.nextBoolean();
    }
    for (boolean b : booleans) {
      System.out.print(b + ", ");
    }
    // generate a uniformly distributed int random numbers
    int[] integers = new int[10];
    for (int i = 0; i < integers.length; i++) {
      integers[i] = r.nextInt();
    }
    for (int i : integers) {
      System.out.print(i + ", ");
    }
    // generate a uniformly distributed float random numbers
    float[] floats = new float[10];
    for (int i = 0; i < floats.length; i++) {
      floats[i] = r.nextFloat();
    }
    for (float f : floats) {
      System.out.print(f + ", ");
    }
    // generate a Gaussian normally distributed random numbers
    double[] gaussians = new double[10];
    for (int i = 0; i < gaussians.length; i++) {
      gaussians[i] = r.nextGaussian();
    }
    for (double d : gaussians) {
      System.out.print(d + ", ");
    }
  }
}

Generating Random integer Numbers

import java.util.Random;
public class MainClass {
  public static void main(String[] args) {
    Random diceValues = new Random();
    System.out.println(diceValues.nextInt(6));
    System.out.println(diceValues.nextInt(6));
  }
}

5
3

Operations for random Strings

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 * 
 *      http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
import java.util.Random;
/**
 * Operations for random <code>String</code>s.
 * Currently <em>private high surrogate</em> characters are ignored. 
 * These are unicode characters that fall between the values 56192 (db80)
 * and 56319 (dbff) as we don"t know how to handle them. 
 * High and low surrogates are correctly dealt with - that is if a 
 * high surrogate is randomly chosen, 55296 (d800) to 56191 (db7f) 
 * then it is followed by a low surrogate. If a low surrogate is chosen, 
 * 56320 (dc00) to 57343 (dfff) then it is placed after a randomly 
 * chosen high surrogate. 
 *
 * @author 
 * @author Stephen Colebourne
 * @author Gary Gregory
 * @author Phil Steitz
 * @since 1.0
 * @version $Id: RandomStringUtils.java 471626 2006-11-06 04:02:09Z bayard $
 */
public class RandomStringUtils {
    /**
     * Random object used by random method. This has to be not local
     * to the random method so as to not return the same value in the 
     * same millisecond.
     */
    private static final Random RANDOM = new Random();
    /**
     * <code>RandomStringUtils</code> instances should NOT be constructed in
     * standard programming. Instead, the class should be used as
     * <code>RandomStringUtils.random(5);</code>.
     *
     * This constructor is public to permit tools that require a JavaBean instance
     * to operate.
     */
    public RandomStringUtils() {
      super();
    }
    // Random
    //-----------------------------------------------------------------------
    /**
     * Creates a random string whose length is the number of characters
     * specified.
     *
     * Characters will be chosen from the set of all characters.
     *
     * @param count  the length of random string to create
     * @return the random string
     */
    public static String random(int count) {
        return random(count, false, false);
    }
    /**
     * Creates a random string whose length is the number of characters
     * specified.
     *
     * Characters will be chosen from the set of characters whose
     * ASCII value is between <code>32</code> and <code>126</code> (inclusive).
     *
     * @param count  the length of random string to create
     * @return the random string
     */
    public static String randomAscii(int count) {
        return random(count, 32, 127, false, false);
    }
    
    /**
     * Creates a random string whose length is the number of characters
     * specified.
     *
     * Characters will be chosen from the set of alphabetic
     * characters.
     *
     * @param count  the length of random string to create
     * @return the random string
     */
    public static String randomAlphabetic(int count) {
        return random(count, true, false);
    }
    
    /**
     * Creates a random string whose length is the number of characters
     * specified.
     *
     * Characters will be chosen from the set of alpha-numeric
     * characters.
     *
     * @param count  the length of random string to create
     * @return the random string
     */
    public static String randomAlphanumeric(int count) {
        return random(count, true, true);
    }
    
    /**
     * Creates a random string whose length is the number of characters
     * specified.
     *
     * Characters will be chosen from the set of numeric
     * characters.
     *
     * @param count  the length of random string to create
     * @return the random string
     */
    public static String randomNumeric(int count) {
        return random(count, false, true);
    }
    /**
     * Creates a random string whose length is the number of characters
     * specified.
     *
     * Characters will be chosen from the set of alpha-numeric
     * characters as indicated by the arguments.
     *
     * @param count  the length of random string to create
     * @param letters  if <code>true</code>, generated string will include
     *  alphabetic characters
     * @param numbers  if <code>true</code>, generated string will include
     *  numeric characters
     * @return the random string
     */
    public static String random(int count, boolean letters, boolean numbers) {
        return random(count, 0, 0, letters, numbers);
    }
    
    /**
     * Creates a random string whose length is the number of characters
     * specified.
     *
     * Characters will be chosen from the set of alpha-numeric
     * characters as indicated by the arguments.
     *
     * @param count  the length of random string to create
     * @param start  the position in set of chars to start at
     * @param end  the position in set of chars to end before
     * @param letters  if <code>true</code>, generated string will include
     *  alphabetic characters
     * @param numbers  if <code>true</code>, generated string will include
     *  numeric characters
     * @return the random string
     */
    public static String random(int count, int start, int end, boolean letters, boolean numbers) {
        return random(count, start, end, letters, numbers, null, RANDOM);
    }
    /**
     * Creates a random string based on a variety of options, using
     * default source of randomness.
     *
     * This method has exactly the same semantics as
     * {@link #random(int,int,int,boolean,boolean,char[],Random)}, but
     * instead of using an externally supplied source of randomness, it uses
     * the internal static {@link Random} instance.
     *
     * @param count  the length of random string to create
     * @param start  the position in set of chars to start at
     * @param end  the position in set of chars to end before
     * @param letters  only allow letters?
     * @param numbers  only allow numbers?
     * @param chars  the set of chars to choose randoms from.
     *  If <code>null</code>, then it will use the set of all chars.
     * @return the random string
     * @throws ArrayIndexOutOfBoundsException if there are not
     *  <code>(end - start) + 1</code> characters in the set array.
     */
    public static String random(int count, int start, int end, boolean letters, boolean numbers, char[] chars) {
        return random(count, start, end, letters, numbers, chars, RANDOM);
    }
    /**
     * Creates a random string based on a variety of options, using
     * supplied source of randomness.
     *
     * If start and end are both <code>0</code>, start and end are set
     * to <code>" "</code> and <code>"z"</code>, the ASCII printable
     * characters, will be used, unless letters and numbers are both
     * <code>false</code>, in which case, start and end are set to
     * <code>0</code> and <code>Integer.MAX_VALUE</code>.
     *
     * If set is not <code>null</code>, characters between start and
     * end are chosen.
     *
     * This method accepts a user-supplied {@link Random}
     * instance to use as a source of randomness. By seeding a single 
     * {@link Random} instance with a fixed seed and using it for each call,
     * the same random sequence of strings can be generated repeatedly
     * and predictably.
     *
     * @param count  the length of random string to create
     * @param start  the position in set of chars to start at
     * @param end  the position in set of chars to end before
     * @param letters  only allow letters?
     * @param numbers  only allow numbers?
     * @param chars  the set of chars to choose randoms from.
     *  If <code>null</code>, then it will use the set of all chars.
     * @param random  a source of randomness.
     * @return the random string
     * @throws ArrayIndexOutOfBoundsException if there are not
     *  <code>(end - start) + 1</code> characters in the set array.
     * @throws IllegalArgumentException if <code>count</code> &lt; 0.
     * @since 2.0
     */
    public static String random(int count, int start, int end, boolean letters, boolean numbers,
                                char[] chars, Random random) {
        if (count == 0) {
            return "";
        } else if (count < 0) {
            throw new IllegalArgumentException("Requested random string length " + count + " is less than 0.");
        }
        if ((start == 0) && (end == 0)) {
            end = "z" + 1;
            start = " ";
            if (!letters && !numbers) {
                start = 0;
                end = Integer.MAX_VALUE;
            }
        }
        char[] buffer = new char[count];
        int gap = end - start;
        while (count-- != 0) {
            char ch;
            if (chars == null) {
                ch = (char) (random.nextInt(gap) + start);
            } else {
                ch = chars[random.nextInt(gap) + start];
            }
            if ((letters && Character.isLetter(ch))
                || (numbers && Character.isDigit(ch))
                || (!letters && !numbers)) 
            {
                if(ch >= 56320 && ch <= 57343) {
                    if(count == 0) {
                        count++;
                    } else {
                        // low surrogate, insert high surrogate after putting it in
                        buffer[count] = ch;
                        count--;
                        buffer[count] = (char) (55296 + random.nextInt(128));
                    }
                } else if(ch >= 55296 && ch <= 56191) {
                    if(count == 0) {
                        count++;
                    } else {
                        // high surrogate, insert low surrogate before putting it in
                        buffer[count] = (char) (56320 + random.nextInt(128));
                        count--;
                        buffer[count] = ch;
                    }
                } else if(ch >= 56192 && ch <= 56319) {
                    // private high surrogate, no effing clue, so skip it
                    count++;
                } else {
                    buffer[count] = ch;
                }
            } else {
                count++;
            }
        }
        return new String(buffer);
    }
    /**
     * Creates a random string whose length is the number of characters
     * specified.
     *
     * Characters will be chosen from the set of characters
     * specified.
     *
     * @param count  the length of random string to create
     * @param chars  the String containing the set of characters to use,
     *  may be null
     * @return the random string
     * @throws IllegalArgumentException if <code>count</code> &lt; 0.
     */
    public static String random(int count, String chars) {
        if (chars == null) {
            return random(count, 0, 0, false, false, null, RANDOM);
        }
        return random(count, chars.toCharArray());
    }
    /**
     * Creates a random string whose length is the number of characters
     * specified.
     *
     * Characters will be chosen from the set of characters specified.
     *
     * @param count  the length of random string to create
     * @param chars  the character array containing the set of characters to use,
     *  may be null
     * @return the random string
     * @throws IllegalArgumentException if <code>count</code> &lt; 0.
     */
    public static String random(int count, char[] chars) {
        if (chars == null) {
            return random(count, 0, 0, false, false, null, RANDOM);
        }
        return random(count, 0, chars.length, false, false, chars, RANDOM);
    }
    
}

Random boolean

import java.util.Random;
public class Main {
  public static void main(String[] argv) throws Exception {
    Random rand = new Random();
    boolean b = rand.nextBoolean();
    long l = rand.nextLong();
    float f = rand.nextFloat(); // 0.0 <= f < 1.0
    double d = rand.nextDouble(); // 0.0 <= d < 1.0
  }
}

Random bytes

import java.util.Random;
public class Main {
  public static void main(String[] argv) throws Exception {
    Random rand = new Random();
    byte[] bytes = new byte[5];
    rand.nextBytes(bytes);
  }
}

Random double type number

import java.util.Random;
public class Main {
  public static void main(String[] argv) throws Exception {
    Random rand = new Random();
    boolean b = rand.nextBoolean();
    long l = rand.nextLong();
    float f = rand.nextFloat(); // 0.0 <= f < 1.0
    double d = rand.nextDouble(); // 0.0 <= d < 1.0
  }
}

Random float type number

import java.util.Random;
public class Main {
  public static void main(String[] argv) throws Exception {
    Random rand = new Random();
    boolean b = rand.nextBoolean();
    long l = rand.nextLong();
    float f = rand.nextFloat(); // 0.0 <= f < 1.0
    double d = rand.nextDouble(); // 0.0 <= d < 1.0
  }
}

Random Gaussian values.

import java.util.Random;
class RandDemo {
  public static void main(String args[]) {
    Random r = new Random();
    double val;
    double sum = 0;
    int bell[] = new int[10];
    for (int i = 0; i < 100; i++) {
      val = r.nextGaussian();
      sum += val;
      double t = -2;
      for (int x = 0; x < 10; x++, t += 0.5)
        if (val < t) {
          bell[x]++;
          break;
        }
    }
    System.out.println("Average of values: " + (sum / 100));
    for (int i = 0; i < 10; i++) {
      for (int x = bell[i]; x > 0; x--)
        System.out.print("*");
      System.out.println();
    }
  }
}

Random integers that range from from 0 to n

import java.util.Random;
public class Main {
  public static void main(String[] argv) throws Exception {
    Random rand = new Random();
    int n = 10;
    int i = rand.nextInt(n + 1);
    System.out.println(i);
  }
}

Random long type number

import java.util.Random;
public class Main {
  public static void main(String[] argv) throws Exception {
    Random rand = new Random();
    boolean b = rand.nextBoolean();
    long l = rand.nextLong();
    float f = rand.nextFloat(); // 0.0 <= f < 1.0
    double d = rand.nextDouble(); // 0.0 <= d < 1.0
  }
}

Random.nextInt(n) returns a distributed int value between 0 (inclusive) and n (exclusive).

// random number between 0 AND 10
import java.util.Random;
public class Main {
  public static void main(String[] argv) {
    Random r = new Random();
    int randint = r.nextInt(10);
    System.out.println(randint);
  }
}

Random number between 0 AND 10

import java.util.Random;
public class Main {
  public static void main(String[] argv) {
    Random r = new Random();
    int randint = Math.abs(r.nextInt()) % 11;
    System.out.println(randint);
  }
}

Random numbers between 0.0 and 1.0

public class Main {
  public static void main(String[] args) {
    
    for (int i = 0; i < 5; i++){
      System.out.println("Random Number [" + (i + 1) + "] : " + Math.random());
    }
  }
}

Roll a six-sided die imes

import java.util.Random;
public class MainClass
{
   public static void main( String args[] )
   {
      Random randomNumbers = new Random(); // random number generator
      int frequency1 = 0; // count of 1s rolled
      int frequency2 = 0; // count of 2s rolled
      int frequency3 = 0; // count of 3s rolled
      int frequency4 = 0; // count of 4s rolled
      int frequency5 = 0; // count of 5s rolled
      int frequency6 = 0; // count of 6s rolled
      int face; // stores most recently rolled value
   
      // summarize results of 6000 rolls of a die
      for ( int roll = 1; roll <= 6000; roll++ ) 
      {
         face = 1 + randomNumbers.nextInt( 6 ); // number from 1 to 6
   
         // determine roll value 1-6 and increment appropriate counter
         switch ( face ) 
         {   
            case 1:
               ++frequency1; // increment the 1s counter
               break; 
            case 2:
               ++frequency2; // increment the 2s counter
               break;
            case 3:
               ++frequency3; // increment the 3s counter
               break;
            case 4:
               ++frequency4; // increment the 4s counter
               break;
            case 5:
               ++frequency5; // increment the 5s counter
               break;
            case 6:
               ++frequency6; // increment the 6s counter
               break; // optional at end of switch
         }
      }
      System.out.println( "Face\tFrequency" ); // output headers
      System.out.printf( "1\t%d\n2\t%d\n3\t%d\n4\t%d\n5\t%d\n6\t%d\n",
         frequency1, frequency2, frequency3, frequency4,
         frequency5, frequency6 );
   }
}

Face  Frequency
1 1005
2 1028
3 1027
4 989
5 994
6 957