Building Java Programs

Lab: if/else

Except where otherwise noted, the contents of this document are Copyright 2019 Stuart Reges and Marty Stepp.

Lab goals

Goals for this problem set:

• use if/else statements to select between multiple code actions
• factor out common redundant code between if/else statements

if/else Statements

An if/else statement lets your program choose between 2 or more options.

if (test) {
    statement(s);
} else {
    statement(s);
}

Example:

if (gpa >= 2.0) {
    System.out.println("Welcome to Mars University!");
} else {
    System.out.println("Please apply again soon.");
}

Exercise : if/else mystery

Consider the following Java code. Fill in the boxes with the output produced by each of the method calls.

public static void mystery(int n) {
    System.out.print(n + " ");
    if (n > 10) {
        n = n / 2;
    } else {
        n = n + 7;
    }
    if (n * 2 < 25) {
        n = n + 10;
    }
    System.out.println(n);
}

40 20

8 15

0 17

12 16

20 20

Exercise : numUnique

Write a method named numUnique that accepts three integers as parameters and that returns the number of unique integers among the three. For example, the call numUnique(18, 3, 4) should return 3 because the parameters have 3 different values. By contrast, the call numUnique(6, 7, 6) would return 2 because there are only 2 unique numbers among the three parameters: 6 and 7.

Compare your solution to your neighbors'. Did you all solve it the same way?

Exercise : quadrant

Write a method quadrant that accepts a pair of real numbers x and y and returns the quadrant for that point:

For example, quadrant(-2.3, 14.2) returns 2. If the point falls directly on either axis, return 0.

if/else factoring

• It's easy to introduce redundancy in the branches of an if/else. For example:

  if (x < 30) {
      a = 2;
      x++;
      System.out.println("CS is awesome! " + x);
  } else {
      a = 2;
      System.out.println("CS is awesome! " + x);
  }
  

• The code can be factored out to become: (notice that the else went away!)

  a = 2;
  if (x < 30) {
      x++;
  }
  System.out.println("CS is awesome! " + x);
  

Exercise : if/else Factoring

• Download the following program FactorExample.java to your machine and open it with your editor.
• The program's method is redundant. Factor the method, restructuring the code to eliminate unnecessary statements while retaining the same behavior.
• Insert some test calls of the method in main and run it to make sure it works properly.

Exercise : Syntax errors

• The following Java program has 7 errors. Can you find all of them?

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

  public class IfOops { public static void main(String[] args) { int a = 7, b = 42; minimum(a, b); if {smaller = a} { System.out.println("a is the smallest!"); } } public static void minimum(int a, int b) { // returns which int is smaller if (a < b) { int smaller = a; } else (a => b) { int smaller = b; } return int smaller; } }

• Copy and paste the code into your editor and see if you can fix the errors.

Exercise - answer

1. line 5: if statement should use () parentheses, not {} brackets
2. line 5: = should be ==
3. line 5: smaller is out of scope here
4. line 10: void should be int
5. line 13: => should be >= (or better yet, no if test is needed)
6. line 16: should not write variable's type of int when returning it
7. line 16: int smaller is out of scope here (declare outside if or return directly)

Exercise - Corrected version

• public class IfOops {
      public static void main(String[] args) {
          int a = 7, b = 42;
          int smaller = minimum(a, b);
          if (smaller == a) {
              System.out.println("a is the smallest!");
          }
      }
  
      public static void int minimum(int a, int b) {  // returns which int is smaller
          int smaller;
          if (a < b) {
              int smaller = a;
          } else if (a >= b) {
              int smaller = b;
          }
          return int smaller;
      }
  }
  

Exercise : AgeCheck

• Copy/paste and save the following program in your editor, then see the instructions on the next slide.

  public class AgeCheck {
      public static void main(String[] args) {
          int myAge = 19;   // I am 19; let me see if I can drive
          message(myAge);
      }
  
      // Displays message about driving to user based on given age
      public static void message(int age) {
          if (myAge >= 16) {
              System.out.println("I'm old enough to drive!");
          }
          if (myAge <= 16) {
              System.out.println("Not old enough yet... :*(");
          }
      }
  }
  

Exercise - things to fix

• The program has a few syntax errors. Fix them until it compiles.
• The code has a logic problem. (For some value(s), it prints the wrong answer.) Find any such problems and fix them. (You may need to run the program a few times and try different values to see which ones fail.)
• The program uses if and else in a clumsy way. Improve the style of the code.

Exercise - answer

• The following is a corrected version of the program:

  public class AgeCheck {
      public static void main(String[] args) {
          int myAge = 19;   // I am 19; let me see if I can drive
          message(myAge);
      }
  
      // Displays a message about driving to user based on given age
      public static void message(int age) {
          if (age >= 16) {
              System.out.println("I'm old enough to drive!");
          } else {
              System.out.println("Not old enough yet... :*(");
          }
      }
  }
  

Exercise : AgeCheck2

• Now replace your previous AgeCheck program's message method with:

      // Possibly prints some message(s) to the user based on the given age
      public static void message(int age) {
          if (age >= 21) {
              System.out.println("I can legally purchase alcohol!");
          } else if (age >= 17) {
              System.out.println("I can purchase a ticket to an R-rated movie.");
          } else if (age >= 16) {
              System.out.println("I can get my driver's license!");
          }
      }
  

• For some age(s), the message(s) printed are not accurate. (In some cases, it should print no messages; in some cases, one message; and in others, many messages.) Improve the usage of ifs and elses in this method to behave properly.

Exercise : seeMovie

You're thinking about going with your friends to a movie. Write a Java method seeMovie that accepts two parameters: the cost of a ticket in dollars, and the rating number of stars the movie received out of 5. The method should print how interested you are (very, sort-of, or not). Use the following criteria:

• You like bargains. Any movie that costs less than $5.00 is one that you want to see very much.
• You dislike expensive movies. You are not interested in seeing any movie that costs $12.00 or more, unless it got 5 stars (and even then, you are only sort-of interested).
• You like quality. You are very interested in seeing 5-star movies that cost under $12.00.
• You are sort-of interested in seeing movies costing between $5.00 - $11.99 that also got between 2-4 stars inclusive.
• You are not interested in seeing any other movies not described previously.