Using var As A Data Type in Programs

Overview

In this lesson, students will begin to develop an understanding of when it is appropriate to use var and when it is not appropriate.

For AP CSA teachers, the use of var is outside the scope of the AP CSA course and exam.

Learning Objectives

1.4.A.3 Use var to create variables of inferred types.

Skills

S1.C Explain the impact design has on data storage.
S2.A Write program code and implement algorithms.

Student Outcomes

Students will be able to:

determine when var can be used instead of a data type.

Duration: 1 class period

Resources

Warm-up / Motivate

The use of var for a data type works because the compiler can intuitively determine what type of data is being stored based on what is assigned to the variable. It is a short-cut that can be used. Students should have a sound understanding of data types before beginning to use var instead of explicitly defining the data type.

Learn

Either as a group or on their own, have students review the tutorial: Learn - Local Variable (var)

Practice

Have students complete practice exercises: Practice - Exploring var with the Java Playground

Apply

Incorporate var into a prior program.

Have students revisit a prior program and include var in all places that are unambiguous.
Identify places in the program where the use of var cannot be used.
Identify places in the program where the use of var can be used, but shouldn't be used.

Wrap-up / Extension

Using `var` on AP CSA Free Response Questions

How does this look on the exam? Now that we have taken a chance to understand var outside of the AP CSA exam. Let's look at an example of how students might use this on the AP Computer Science A (AP CSA) exam.

The following are examples of how students could use var on the AP CSA Free Response Exam. You can find all of the questions here

Free Response Question 1 - Bird Feeder

This question involved a simulation of a bird feeder and requires students to do the following:

generate random values
use if statement
use loops

Part A - Example Canonical Solution

public void simulateOneDay (int numBirds) {
   boolean bearPresent = false;
   double bear = Math.random() * 100;
   if (bear <= 5) {
      bearPresent = !bearPresent;
   }
   int birdConsumed = ((int) (Math.random() * 41) + 10) * numBirds;
   if (bearPresent)
   {
      currentFood = 0;
   }
   else{
      if (birdConsumed >= currentFood) {
          currentFood = 0; 
      else {
          currentFood = currentFood - birdConsumed;
   }
}

There are three variable declarations in the sample solution above:

bearPresent – is assigned to the Boolean false. The type is unambiguous with the assignment.
bear – is assigned to a double from Math.random(). You could use var, but it can be ambiguous to use with a method return.
birdConsumed – even though there is a method call, the typecast to an int makes the type clear that the value being assigned is an int.

A sample solution using var is as follows:

public void simulateOneDay (int numBirds) {
   var bearPresent = false;
   double bear = Math.random() * 100;
   if (bear <= 5) {
      bearPresent = !bearPresent;
   }
   var birdConsumed = ((int) (Math.random() * 41) + 10) * numBirds;
   if (bearPresent)
   {
      currentFood = 0;
   }
   else{
      if (birdConsumed >= currentFood) {
          currentFood = 0; 
      else {
          currentFood = currentFood - birdConsumed;
   }
}

Part B - Example Canonical Solution

public int simulateManyDays(int numBirds, int numDays) {
   int count = 1; 
   for(int days = 1; days <= numDays; days++) {
      if(currentFood == 0) {
         return days;
      }
      simulateOneDay(numBirds);
   }
   return numDays;
}

There are two variables declared in the sample solution above:

count - Since the variable is being assigned to 1, it is unambiguous and is a good place to use var.
days - The for loop counter days can be declared using var.

This sample solution using var is as follows:

public int simulateManyDays(int numBirds, int numDays) {
   var count = 1; 
   for(var days = 1; days <= numDays; days++) {
      if(currentFood == 0) {
         return days;
      }
      simulateOneDay(numBirds);
   }
   return numDays;
}

Free Response Question 2 - Scoreboard

In this free response question, students are asked to write a class for a scoreboard with two teams. Students will need to:

declare instance variables
write constructors
write methods
use if statements

The following is a sample solution:

public class Scoreboard {
   private String team1; 
   private String team2; 
   private String activeTeam; 
   private int team1Score;
   private int team2Score;

   public Scoreboard(String t1, String t2) {
      team1 = t1; 
      team2 = t2; 
      activeTeam = team1; 
      team1Score = 0;
      team2Score = 0;  
   }

   public void recordPlay(int points) {
      if (points == 0) {
         activeTeam = (team1.equals(activeTeam)) ? team2 : team1;
      } else {
         if (team1.equals(activeTeam)) {
            team1Score += points; 
         } else {
            team2Score += points;
         }
      }  
   }
 
   public String getScore() {
      String retVal = team1Score + " – " + team2Score + " – " ;
      retVal += (team1.equals(activeTeam)) ? team1 : team2;  
      return retVal;
   }
}

In this solution, we have many data types where students might be tempted to use var. However, you cannot use var for the following:

instance variables
constructor or method parameters
method return types

If a student were to use var in these places, they will likely lose points.

There is only one place in the solution above where a student could use var if they wanted to do so. In the getScore method the String variable, retVal, is declared and assigned the concatenation of team scores and a string hyphen. Since the concatenation of strings with int values creates a String, it is unambiguous what type we are using and var can be used appropriately.

A sample solution using var is as follows:

public class Scoreboard {
   private String team1; 
   private String team2; 
   private String activeTeam; 
   private int team1Score;
   private int team2Score;

   public Scoreboard(String t1, String t2) {
      team1 = t1; 
      team2 = t2; 
      activeTeam = team1; 
      team1Score = 0;
      team2Score = 0;  
   }

   public void recordPlay(int points) {
      if (points == 0) {
         activeTeam = (team1.equals(activeTeam)) ? team2 : team1;
      } else {
         if (team1.equals(activeTeam)) {
            team1Score += points; 
         } else {
            team2Score += points;
         }
      }  
   }
 
   public String getScore() {
      var retVal = team1Score + " – " + team2Score + " – " ;
      retVal += (team1.equals(activeTeam)) ? team1 : team2;  
      return retVal;
   }
}

NOTE: In this solution we have used the ternary conditional operator. This operator is not a part of the AP Java Subset. You do not need to teach this operator.

Learn more on the Ternary Conditional Operator.

Free Response Question 3 - WordList

In this free response students will need to:

loop over a list
access adjacent elements of a list
use string methods
add values to a list
create a new list

Part A - Sample Canonical Solution

In part A of this free response question, students are asked to determine if the previous word is a substring of the next word.

public boolean isWordChain() {
   for (int index = 0; index < wordlist.size() – 1; index++) {
      String nextWord = wordlist.get(index + 1); 
      String currentWord = wordlist.get(index);
      if(nextWord.indexOf(previous) == -1) {
         return false;
      }
   }
   return true;
}

In this solution, there are three variables being declared:

index - the for loop counter index can be declared with var
nextWord and currentWord - students could use var to declare these variables, but this would be considered less readable, since the value these are being assigned comes from a method call. In this case, we are using common method calls and will likely know that wordlist stores String values, but it is still considered less than desirable to use it in these two cases.

A sample solution using var is as follows:

public boolean isWordChain() {
   for (var index = 0; index < wordlist.size() – 1; index++) {
      String nextWord = wordlist.get(index + 1); 
      String currentWord = wordlist.get(index);
      if(nextWord.indexOf(previous) == -1) {
         return false;
      }
   }
   return true;
}

Part B - Sample Canonical Solution

In part B, students need to make a list of all word that have the target word at the beginning, but with the beginning of the word is removed.

public ArrayList<String> createList(String target) {
   ArrayList<String> retList = new ArrayList<String> ();
   for (String word : wordList) {
      if (word.indexOf(target) == 0) {
         retList.add(word.substring(target.length()));
      }   
   }
   return retList;
}

In this solution, var could be used in two places:

retList - a new ArrayList that is able to store string values
word - used in the enhanced-for loop to store the values in wordList, which are strings

A sample solution using var is as follows:

public ArrayList<String> createList(String target) {
  var retList = new ArrayList<String> ();
   for (var word : wordList) {
      if (word.indexOf(target) == 0) {
         retList.add(word.substring(target.length()));
      }   
   }
   return retList;
}

Free Response Question 4 - GridPath

In this free response students will need to:

access elements in a 2D array
use a provided class
use conditionals and loops

Part A - Sample Canonical Solution

In this part, students need to return the smaller of two values, either below or to the right if they exist.

public Location getNextLoc(int row, int col) {
   int numRows = grid.length;
   int numCols = grid[0].length;

   if (row + 1 == numRows) 
      return new Location(row, col + 1); 
   if (col + 1 == numCols) 
      return new Location(row + 1, col);

   int below = grid[row + 1][col];
   int right = grid[row][col + 1];
   return (below < right) ? new Location(row + 1, col) : new Location(row, col + 1);
}

In this solution, we have declared four variables:

numRows and numCols - The value assigned to numRows and numCols is from the constant length. Since this is really common, it is most likely readable to use var in these cases. However, if there is any doubt about the readability, you will want to use the type and be clear.
below and right - The values assigned to below and right are what is being stored in grid and is not ambiguous.

A sample solution with var is as follows:

public Location getNextLoc(int row, int col) {
   var numRows = grid.length;
   var numCols = grid[0].length;

   if (row + 1 == numRows) 
      return new Location(row, col + 1); 
   if (col + 1 == numCols) 
      return new Location(row + 1, col);

   var below = grid[row + 1][col];
   var right = grid[row][col + 1];
   return (below < right) ? new Location(row + 1, col) : new Location(row, col + 1);
}

NOTE: In this solution we have used the ternary conditional operator. This operator is not a part of the AP Java Subset. You do not need to teach this operator.

Learn more on the Ternary Conditional Operator

Part B - Sample Canonical Solution

In this solution, students are summing the values along a path using the return value from getNextLocation.

public int sumPath(int row, int col) {
   int total = 0;

   while (row < grid.length – 1 || col < grid[0].length – 1) {
      total += grid[row][col];
      Location next = getNextLocation(row, col); 
      row = next.getRow();
      col = next.getCol();
   }
   return total;
}

In this solution, we have two variables that are being declared and assigned values:

total - which is unambiguous being assigned to 0.
next - assigned a returned type from a method call. This method is a local method and would be easy to find, however, it is still considered less readable to use var in this case.

A sample solution with var is as follows:

public int sumPath(int row, int col) {
   var total = 0;

   while (row < grid.length – 1 || col < grid[0].length – 1) {
      total += grid[row][col];
      Location next = getNextLocation(row, col); 
      row = next.getRow();
      col = next.getCol();
   }
   return total;
}

Final Thoughts on Using var with AP CSA

Use of var and the AP CSA Free Response Questions

Using var is OUTSIDE the AP CSA Java Subset
Students can write correct solutions using var
Using var can be used in any FRQ. Students need to be careful using it with FRQ 2, since they cannot use it with instance variables, parameters, and return types.

Scoring and Teaching

Student solutions that use correct Java syntax can earn full credit for their solution
Questions are written with an in AP Java subset solution in mind
Students should be encouraged to write solutions within the AP Java Subset
Students could lose points if they use var and do not assign the variable to a value that is not unambiguous.

Using var As A Data Type in Programs

Overview

Learning Objectives

Skills

Student Outcomes

Duration: 1 class period

Resources

Warm-up / Motivate

Learn

Practice

Apply

Wrap-up / Extension

Using var on AP CSA Free Response Questions

Free Response Question 1 - Bird Feeder

Part A - Example Canonical Solution

Part B - Example Canonical Solution

Free Response Question 2 - Scoreboard

Free Response Question 3 - WordList

Part A - Sample Canonical Solution

Part B - Sample Canonical Solution

Free Response Question 4 - GridPath

Part A - Sample Canonical Solution

Part B - Sample Canonical Solution

Final Thoughts on Using var with AP CSA

Use of var and the AP CSA Free Response Questions

Scoring and Teaching

Using `var` on AP CSA Free Response Questions