To Lecture Notes

IT 372 -- May 18, 2026

Review Exercises

1. Last week we saw that to round a Kotlin Double value to two digits after the decimal point, we could use the Java String.format method:
   

   val x = 3.14159265
   val s = String.format("%.2f", x);
   

   In the Kotlin playground, test this alternative version that is similar to Python:
   

   val x = 3.14159265
   val s = "%.2f".format(x);
   

2. Unlike Java, Kotlin implements extension functions, which are a way to add instance methods to a class without using inheritance. Test this example in the Kotlin playground, which adds a dollar method to the Double class:
   

   fun Double.dollar( ) : String {
       return "$%.2f".format(this)
   }
   
   fun main( ) {
       var amount = 15.9576
       println(amount.dollar( ))
   }
   

3. Test these Kotlin List or MutableList methods: filter, map, forEach. These methods each take a lambda function parameter.
4. Check that the last parameter of the JPC Button element is named content, which accepts a lambda function. Verify that this parameter can be written in training lambda notation.
5. We know how to set up and use a JPC state variable:
   

   var stateValue = remember { mutableStateOf("abc") }
   ...
   // Usage
   val s = stateString.value  // <-- Getter
   stateString.value = "xyz"  // <-- Setter
   

   Try out these alternative formulations:
   

   // Alternative version 1:
   var stateValue by remember { mutableStateOf("abc") }
   ...
   // Usage
   val s = stateString   // <-- Getter
   stateString = "xyz"   // <-- Setter
   
   // Alternative version 2:
   val (value, setValue) = remember { mutableStateOf("abc") } 
   ...
   // Usage
   val s = value     // <-- Getter
   setValue("xyz")   // <-- Setter
   

   Alternative version 1 seems to be the most popular.

More About the Any Class

• As we know, Any is the direct or indirect base class of every Kotlin class.
• The Any Kotlin class corresponds to the Object Java class.
• The Any class defines only three instance methods:
  
  1. equals(other : Any?) : boolean   checks for equality between two any objects: the this object and the other object.
     
  2. hashCode(): Int  returns a hash code value for the object, which is used in hash-based collections like Map, MutableMap, Set, MutableSet.
  3. toString( ) : String  returns a  String representation of the object, which, if not overridden, is the Class name and hex address of the object.
• Test this Kotlin source code using the Kotlin Playground webpage:
  

  // GroceryItem Class
  class GroceryItem(var description : String, var code : Int,
      var price : Double, var onSale : Boolean) {
  
      fun salePrice(discount : Double) : Double {
          return price * (100 - discount) / 100
      }
  }
  
  fun main( ) {
      // For the GroceryItem objects, since the methods
      // equals, hashCode, and toString are not overridden,
      // the methods are called directly from the Any class.
  
      // Create four GroceryItem objects.
      var gItem1 = GroceryItem("Cabbage", 45678, 1.29, false)
      var gItem2 = GroceryItem("Green Beans", 98765, 2.45, true)
      var gItem3 = GroceryItem("Cabbage", 45678, 1.29, false)
      var gItem4 = gItem2;
      
      // Test Any equals method.
      println("${gItem1.equals(gItem2)}")
      println("${gItem1.equals(gItem3)}")
      println("${gItem2.equals(gItem4)}")
      println( )
  
      // Test Any hashCode method.
      println("${gItem1.hashCode( )}")
      println("${gItem2.hashCode( )}")
      println("${gItem3.hashCode( )}")
      println("${gItem4.hashCode( )}")
      println( )
  
      // Test Any toString method.
      println("$gItem1")
      println("$gItem2")
      println("$gItem3")
      println("$gItem4")
  }
  

• What do you observe about the behavior of these Any functions.

Data Classes

• A data class is designed to hold data.
• Usually, data classes do not define many instance methods.
• Here is the GroceryItem class defined as a data class with no defined methods:
  

  data GroceryItem(var description : String, var code : Int,
      var price : Double, var onSale : Boolean) { }
  

JetPack Compose Examples

• Look at JPCEx 19, 20, 21.
  

Practice Problems

1. Run the JPCEx ClickCounter2 Example.  Click the button several times, then rotate the emulator. Notice that the click count is reset to 0.  In a JPC app, how can we save the count in a Bundle and restore it when the app is recreated and restarted?
2. For the TheRaven Example, we used the Scanner class, which comes from Java. Test these alternative Kotlin versions for reading from a file in the raw folder:
   

   // Read the entire file into a string:
   val s = inputStream.bufferedReader().use { 
       it.readText( ) 
   }
   
   // Read the file line by line:
   var s = ""
   inputStream.bufferedReader( ).useLines { 
       lines -> lines.forEach { 
           line -> s += line + "\n"
       } 
   } 
   

Android Platform

• Some Details about the Android Platform.
• To view files on the virtual device, use the Device Explorer.