IT 372 -- May 18, 2026 -- Notes

IT 372 -- May 18, 2026

Review Exercises

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);
// Output: 3.14
```
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( ))
}
// Output: $15.96
```

Test these Kotlin List or MutableList methods: filter, map, forEach. These methods each take a lambda function parameter. Answer:

var animals = listOf("dog", "wolf", "pig", "skunk", "cow");

// filter only keeps list items for which
// the lambda function returns true.
println(animals.filter { it.length == 3 });
// Output: [dog, pig, cow]

// map applies the lambda function to each item
// in the list, forming a new list from the results.
println(animals.map { it.uppercase( ) });
// Output: [DOG, WOLF, PIG, SKUNK, COW]

// forEach applies the lambda function to each 
// element of the list like a for loop.
animals.forEach { print("$it ") }
// Output:
dog wolf pig skunk cow

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 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( )
    // Output: false false true 

    // Test Any hashCode method.
    print("${gItem1.hashCode( )} ")
    print("${gItem2.hashCode( )} ")
    print("${gItem3.hashCode( )} ")
    println("${gItem4.hashCode( )}")
    println( )
    // Output: 122883338 500977346 20132171 500977346 

    // Test Any toString method.
    println("$gItem1")
    println("$gItem2")
    println("$gItem3")
    println("$gItem4")
}
// Output: 
GroceryItem@7530d0a
GroceryItem@1ddc4ec2 
GroceryItem@133314b 
GroceryItem@1ddc4ec2

What do you observe about the behavior of these Any functions.
Answer: they behave like the corresponding Java Object 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) { }

Test this class with the main method that we used in the previous section. Answer:

// Output
false true true

1391093775 -733823277 1391093775 -733823277

GroceryItem(description=Cabbage, code=45678, price=1.29, onSale=false)
GroceryItem(description=Green Beans, code=98765, price=2.45, onSale=true)
GroceryItem(description=Cabbage, code=45678, price=1.29, onSale=false)
GroceryItem(description=Green Beans, code=98765, price=2.45, onSale=true)

JetPack Compose Examples

Look at JPCEx 19, 20, 21, 23.