Chapter 5a - Random

Random Introduction

import java.util.Random;
class Main {
   public static void main(String args[]) {
       
       Random rand = new Random(); //creates object of class Random which is used to generate random number
       int randomNum = rand.nextInt(4); //generates random numbers
       System.out.println(randomNum);
   }
}

🧪 Try the code out~!

👀

Coin Simulation

import java.util.Scanner;
import java.util.Random;
class Main {
   public static void main(String args[]) {
       Scanner scan = new Scanner(System.in);
       Random rand = new Random(); //creates object of class Random which is used to generate random number
       System.out.println("This is a simulation of a coin toss.");
       System.out.println("Tossing coin now...\n");
       int randomNum = rand.nextInt(2); //generates random numbers 0 & 1
       if (randomNum == 0) {
           System.out.println("HEADS...\n");
       } else {
           System.out.println("TAILS...\n");
       }
   }
}

🧪 Try the code out~!

(i) Does the user provide any input here?

Yes

No

(ii) What are the possible outputs?

Heads or Tails

1 and 2's

Yes and No's

(iii) Which line of code requires that the random module be imported?

Scanner scan = new Scanner(System.in);

Random rand = new Random()

(iv) What are the values stored by randomNum ?

0 and 1 only

2 and 3

(v) What module is required to generate random numbers?

import java.util.Scanner

import java.util.Random

Exercise: Team Selection

tip

• Create a program to randomnly assign someone in either blue or red team

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🙋‍♀️ expected

Introduce Random Module - Formatting

import java.math.RoundingMode; 
import java.text.DecimalFormat;
class Main {
 public static void main(String[] args) {
   for(int count = 0; count< 5; count++){
     System.out.println(Math.random());
   }
  }
}

🧪 Try the code out~!

Formatting Random

import java.math.RoundingMode;
import java.text.DecimalFormat;
class Main {
public static void main(String[] args) {
    DecimalFormat df = new DecimalFormat("0.00");
    for(int count = 0; count< 5; count++){
    double d=Math.random();
    System.out.println(df.format(d)); 
  }
 }
}

🧪 Try the code out~!

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Mini quiz — Random ranges

Why we cannot autograde a random value

Random is unpredictable on purpose, so the challenges below test the decision logic around a random result, not the random number itself — that is the part you actually write.

Quiz

Random ranges

1 / 2

rand.nextInt(6) can return which set of values?

Pick an answer to see if you are right.

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Challenge: describe a die roll

You will not generate the random number here — instead write the decision logic a die-rolling program needs. Write describeRoll(int roll) where roll is already a value from 1 to 6: return "Lucky seven? No, but nice roll!" is wrong — instead return "You rolled a " + roll. Then in a real program you would call this with rand.nextInt(6) + 1.

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Challenge: format to two decimals

Read a double. Print it formatted to exactly two decimal places using DecimalFormat (the same tool the "Formatting Random" example used).

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Chapter summary

Key takeaways

1. new Random() creates a random number generator; rand.nextInt(bound) gives a value from 0 up to (not including) bound.
2. Add 1 to nextInt(n) to shift a 0..n-1 range into a 1..n range (a classic die roll).
3. Since randomness cannot be tested by comparing exact output, separate the decision logic (what to do with a roll) from the random source (how the roll was generated) — you can test the logic on its own by passing in a known value.
4. DecimalFormat("0.00") formats a double to a fixed number of decimal places, useful for cleaning up Math.random() output.

Exercise

Activity: Coin toss message

Write coinMessage(int flip) where flip is 0 or 1 (as if it came from rand.nextInt(2)). Return "HEADS" for 0, "TAILS" for 1.