Generating Random Numbers with a Specified Seed

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//Creates a Random instance with a seed of 12345.
Random random = new Random(12345L);
 
//Gets a ThreadLocalRandom instance
ThreadLocalRandom tlr = ThreadLocalRandom.current();
 
//Set the instance's seed.
tlr.setSeed(12345L);

Using the same seed to generate random numbers will return the same numbers every time, so setting a different seed for every Random instance is a good idea if you don't want to end up with duplicate numbers

A good method to get a Long that is different for every call is System.currentTimeMillis():

Random random = new Random(System.currentTimeMillis());
ThreadLocalRandom.current().setSeed(System.currentTimeMillis());

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Select random numbers without duplicates

/**
* returns a array of random numbers with no duplicates
* @param range the range of possible numbers for ex. if 100 then it can be anywhere from 1-100
* @param length the length of the array of random numbers
* @return array of random numbers with no duplicates.
*/
public static int[] getRandomNumbersWithNoDuplicates(int range, int length){
	if (length<range){
		// this is where all the random numbers
		int[] randomNumbers = new int[length];
 
		// loop through all the random numbers to set them
		for (int q = 0; q < randomNumbers.length; q++){
 
			// get the remaining possible numbers
			int remainingNumbers = range-q;
 
			// get a new random number from the remainingNumbers
			int newRandSpot = (int) (Math.random()*remainingNumbers);
 
			newRandSpot++;
 
			// loop through all the possible numbers
			for (int t = 1; t < range+1; t++){
 
				// check to see if this number has already been taken
				boolean taken = false;
				for (int number : randomNumbers){
					if (t==number){
						taken = true;
						break;
					}
				}
 
				// if it hasnt been taken then remove one from the spots
				if (!taken){
					newRandSpot--;
 
					// if we have gone though all the spots then set the value
					if (newRandSpot==0){
						randomNumbers[q] = t;
					}
				}
			}
		}
		return randomNumbers;
	} else {
		// invalid can't have a length larger then the range of possible numbers
	}
	return null;
}

The method works by looping though an array that has the size of the requested length and finds the remaining length of possible numbers. It sets a random number of those possible numbers newRandSpot and finds that number within the non taken number left. It does this by looping through the range and checking to see if that number has already been taken.

For example if the range is 5 and the length is 3 and we have already chosen the number 2. Then we have 4 remaining numbers so we get a random number between 1 and 4 and we loop through the range(5) skipping over any numbers that we have already used(2).

Now let's say the next number chosen between 1 & 4 is 3. On the first loop we get 1 which has not yet been taken so we can remove 1 from 3 making it 2. Now on the second loop we get 2 which has been taken so we do nothing. We follow this pattern until we get to 4 where once we remove 1 it becomes 0 so we set the new randomNumber to 4.

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Generating Random number using apache-common lang3

We can use org.apache.commons.lang3.RandomUtils to generate random numbers using a single line.

int x = RandomUtils.nextInt(1, 1000);

The method nextInt(int startInclusive, int endExclusive) takes a range.

Apart from int, we can generate random long, double, float and bytes using this class.

RandomUtils class contains the following methods

static byte[] nextBytes(int count) //Creates an array of random bytes.
static double nextDouble() //Returns a random double within 0 - Double.MAX_VALUE
static double nextDouble(double startInclusive, double endInclusive) //Returns a random double within the specified range.
static float nextFloat() //Returns a random float within 0 - Float.MAX_VALUE
static float nextFloat(float startInclusive, float endInclusive) //Returns a random float within the specified range.
static int nextInt() //Returns a random int within 0 - Integer.MAX_VALUE
static int nextInt(int startInclusive, int endExclusive) //Returns a random integer within the specified range.
static long nextLong() //Returns a random long within 0 - Long.MAX_VALUE
static long nextLong(long startInclusive, long endExclusive) //Returns a random long within the specified range.

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Basic Programs

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• Creating Your First Java Program
• Type Conversion
• Getters and Setters
• Reference Data Types
• Java Compiler - javac
• Documenting Java Code
• Command line Argument Processing
• Java Command - java and javaw
• Literals
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Strings in Java

• Comparing Strings
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• Finding a String Within Another String
• String pool and heap storage
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• Adding toString() method for custom objects
• Remove Whitespace from the Beginning and End of a String and Case insensitive switch
• Replacing parts of Strings and Getting the nth character in a String
• Counting occurrences of a substring or character in a string

StringBuffer and StringBuilder

• String Buffer
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Date Class

• Convert java.util.Date to java.sql.Date and Basic date
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• Comparing Date objects
• Converting String into Date and Time Zones and java.util.Date

Dates and Time (java.time.*)

• Dates and Time
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• LocalTime

BigDecimal

• Comparing and Using BigDecimals
• BigDecimal.valueOf()
• Mathematical operations with BigDecimal

BigInteger

• Initialization and Generating random BigIntegers
• BigInteger Mathematical Operations Examples
• NumberFormat

Bit Manipulation

• Checking, setting, clearing, and toggling individual bits. Using long as bit mask
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• Signed vs unsigned shift and Expressing the power of 2

Arrays

• Creating and Initializing Arrays
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Collections

• Removing items from a List within a loop
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Lists

• Sorting a generic list
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Sets

• Initialization and Basics of Set
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Maps

• Iterating Map Entries Effciently
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Object Class Methods and Constructor

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Immutable Objects

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Classes and Objects

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Alternative Collections

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super keyword

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Optional

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Calendar and its Subclasses

• Creating, Subtracting and Finding AM/PM Calendar

Using the static keyword

• Reference to non-static member from static context

Properties Class

• Reference to non-static member from static context

Lambda Expressions

• Introduction to Java lambdas
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Basic Control Structures

• Switch, do while, For Each, Continue Statement in Java
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BufferedWriter

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New File I/O

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Scanner

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Interfaces

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Regular Expressions

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Comparable and Comparator

• Sorting a List using Comparable<T> or a Comparator<T>
• compareTo, compare Methods and Natural vs explicit sorting

Java Floating Point Operations

• Comparing floating point values
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Currency and Money

• Add custom currency

Object Cloning

• Cloning performing a deep copy and copy factory
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Recursion

• Basic idea of Recursion
• StackOverflowError & recursion to loop
• Computing the Nth Fibonacci Number and power of a number

Converting to and from Strings

• Converting String to other datatypes
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Random Number Generation

• Pseudo Random Numbers in Specific Range
• Generating Random Numbers with a Specified Seed

Singletons

• Enum Singleton and Singleton without use of Enum, Thread safe Singleton
• Extending singleton (singleton inheritance)

Autoboxing

• Using int and Integer interchangeably and Auto-unboxing may lead to NullPointerException
• Different Cases When Integer and int can be used interchangeably

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