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If/Else

Table of contents

  1. If (if/else) Control Flow Statement
  2. If Example
  3. If/else Example
  4. If/else-if/else Example
  5. Java Ternary Operator
  6. Can we throw an exception from a ternary operator?

If (if/else) Control Flow Statement

The following program simulates a player playing a dice game. Two dice are rolled and their sum, referred to as the score, is printed.

package demo;

import java.util.Random;

public class App {

  public static void main( final String[] args ) {
    final int a = rollDice();
    final int b = rollDice();
    final int score = a + b;

    System.out.printf( "You scored %d%n", score );
  }

  private static int rollDice() {
    return random.nextInt( 6 ) + 1;
  }

  private static final Random random = new Random();
}

If Example

The player will win the game if the player rolls 10 or higher. Change the program, such that, it only prints the following message when the player wins.

You won!! You scored 10

The if statement can be used

if ( score >= 10 ) {
  System.out.printf( "You won!! You scored %d%n", score );
}

Complete solution

package demo;

import java.util.Random;

public class App {

  public static void main( final String[] args ) {
    final int a = rollDice();
    final int b = rollDice();
    final int score = a + b;

    if ( score >= 10 ) {
      System.out.printf( "You won!! You scored %d%n", score );
    }
  }

  private static int rollDice() {
    return random.nextInt( 6 ) + 1;
  }

  private static final Random random = new Random();
}

Refactoring

  1. Create a function that returns true if the score is high enough, false otherwise

       private static boolean hasWon( final int score ) {
     return score >= 10;
   }

  2. Use the hasWon() function instead

         if ( hasWon( score ) ) {

    Complete example

     package demo;

 import java.util.Random;

 public class App {

   public static void main( final String[] args ) {
     final int a = rollDice();
     final int b = rollDice();
     final int score = a + b;

     if ( hasWon( score ) ) {
       System.out.printf( "You won!! You scored %d%n", score );
     }
   }

   public static boolean hasWon( final int score ) {
     return score >= 10;
   }

   public static int rollDice() {
     return random.nextInt( 6 ) + 1;
   }

   public static final Random random = new Random();
 }

If/else Example

After trying the program, it was notices that nothing is shown on the screen when the player loses.

Display the follow message whenever the player scores lower than 10.

Better luck next time!! You scored too low 9

The else statement can be added to the existing if statement

if ( hasWon( score ) ) {
  System.out.printf( "You won!! You scored %d%n", score );
} else {
  System.out.printf( "Better luck next time!! You scored too low (%d)%n", score );
}

Note that an else cannot exists without an if as is always the last block.

Complete example

package demo;

import java.util.Random;

public class App {

  public static void main( final String[] args ) {
    final int a = rollDice();
    final int b = rollDice();
    final int score = a + b;

    if ( hasWon( score ) ) {
      System.out.printf( "You won!! You scored %d%n", score );
    } else {
      System.out.printf( "Better luck next time!! You scored too low (%d)%n", score );
    }
  }

  public static boolean hasWon( final int score ) {
    return score >= 10;
  }

  private static int rollDice() {
    return random.nextInt( 6 ) + 1;
  }

  private static final Random random = new Random();
}

If/else-if/else Example

Improve the program such that if a player scores 12, then the program displays a message

Max score. You won a bonus!!  Well done.

If a player scores between 10 and 11 (both inclusive), the program shows the original message.

You won!! You scored 11

If the player scores below 10, the program shows the original message.

Better luck next time!! You scored too low (9)

The if/else-if/else statement can be used in this case

if ( hasWonWithBonus( score ) ) {
  System.out.println( "Max score. You won a bonus!!  Well done." );
} else if ( hasWon( score ) ) {
  System.out.printf( "You won!! You scored %d%n", score );
} else {
  System.out.printf( "Better luck next time!! You scored too low (%d)%n", score );
}

Complete solution

package demo;

import java.util.Random;

public class App {

  public static void main( final String[] args ) {
    final int a = rollDice();
    final int b = rollDice();
    final int score = a + b;

    if ( hasWonWithBonus( score ) ) {
      System.out.println( "Max score. You won a bonus!!  Well done." );
    } else if ( hasWon( score ) ) {
      System.out.printf( "You won!! You scored %d%n", score );
    } else {
      System.out.printf( "Better luck next time!! You scored too low (%d)%n", score );
    }
  }

  public static boolean hasWonWithBonus( final int score ) {
    return score == 12;
  }

  public static boolean hasWon( final int score ) {
    return score >= 10;
  }

  private static int rollDice() {
    return random.nextInt( 6 ) + 1;
  }

  private static final Random random = new Random();
}

Tip

The Random produces a sequence based on its seed. For example, if we seed Random with the seed 63, then the player will score 12 and wins with bonus.

private static final Random random = new Random( 63 );

The following table list some seeds that can be used to produce a deterministic result.

seedabscore
17112
36224
3336
33448
85510
345611
56511
636612

Random number generation is a hard problem and in some cases hardware random number generators are required for security or legislative purposes.

Java Ternary Operator

Java has one ternary operator.

package demo;

import java.util.Random;

public class App {

  public static void main( String[] args ) {
    final Random r = new Random();
    final int a = r.nextInt( 10 );
    final String s = a % 2 == 0 ? "even" : "odd";
    System.out.printf( "The number %d is an %s number%n", a, s );
  }
}

The above program will print.

The number 8 is an even number

The ternary operator has one catch that takes many by surprise. Consider the following example, where the second and third statements are of different types.

package demo;

public class App {

  public static void main( String[] args ) {
    char x = 'X';
    int i = 0;
    System.out.print( true ? x : 0 );
    System.out.print( false ? i : x );
  }
}

Will print.

X88

This example was taken from PUZZLE 8: DOS EQUIS in Java™ Puzzlers: Traps, Pitfalls, and Corner Cases.

  1. “The answer lies in a dark corner of the specification for the conditional operator JLS 15.25. Note that the types of the second and third operands are different from each other in both of the conditional expressions: x is of type char, whereas 0 and i are both of type int. Mixed-type computation can be confusing. Nowhere is this more apparent than in conditional expressions. You might think that the result types of the two conditional expressions in this program would be identical, as their operand types are identical, though reversed, but it isn’t so.”

  2. “Putting the final modifier on the declaration for i would turn i into a constant expression, causing the program to print XX, but it would still be confusing. To eliminate the confusion, it is best to change the type of i from int to char, avoiding the mixed-type computation.”

Can we throw an exception from a ternary operator?

🚧 Pending 🚧