Unit 2 Lesson RACHIT • 14 min read

2.8 Wrapper Classes: Integer and Double

You probably know what an integer is (if you don’t see me after class immediatley!), and you probably know what a double is (again, if you don’t see me after class immediatley!).

But did you know...

Integers and Doubles are classes called wrapper classes!

Why would we want to do this? Answer the question in this cell.

Answer

These classes convert primitive data types (int/double) to a reference data type (object). We convert these because different data structures in java require different types of variables, some of which are only objects. By parsing the values of the primitive data to an object, we can send values to methods and structures that only take object values, like ArrayList, for example.

The integer Wrapper Class

The Integer class in Java is a wrapper class for the primitive data type int. It provides several methods that can be used to perform operations on int values. The constructor looks like this:

Integer(int value)

|   Integer(int value)

'.class' expected

An example of this running is:

Integer hungerRating = new Integer(5); // Send to an Object
int hungerRatingNum = hungerRating.intValue(); // Turn back into a primitive

An example of it being utilized is:

ArrayList<int> list = new ArrayList<int>();  // This will cause a compile-time error

|   ArrayList<int> list = new ArrayList<int>();  // This will cause a compile-time error

unexpected type

  required: reference

  found:    int



|   ArrayList<int> list = new ArrayList<int>();  // This will cause a compile-time error

unexpected type

  required: reference

  found:    int

The above code will not work because ArrayList requires on object type, while we are passing a primitive type.

ArrayList<Integer> list = new ArrayList<Integer>();
list.add(5);  // Autoboxing will automatically convert the int 5 to an Integer object
System.out.println(list)

[5]

This code works because the 5 is being converted by the list.add, since the list uses integer objects.

Challenge: In the cell below, demonstrate a function failing with an int and a function succeding with an integer. This cannot be an ArrayList. You are NOT allowed to use ChatGPT, Google AI, Llama 2, Bing AI, etc, however, you can Google for answers.

Moreover, we can also get other data from the integer class, which is related to how Java functions.

System.out.println("Integer Min Value: " + Integer.MIN_VALUE);
System.out.println("Integer Max Value: " + Integer.MAX_VALUE);

Integer Min Value: -2147483648
Integer Max Value: 2147483647

These numbers are the bounds of integers in java. For example, we can try going outside the range to see what happens.

Integer pleaseBreak = new Integer(2147483648)

|   Integer pleaseBreak = new Integer(2147483648);

integer number too large

Challenge: Can we have a primitive int that goes past that bound? Try and find out in the cell below.

Double Wrapper Classes

Double wrapper classes are basically the same thing as integer classes, however, they deal with double varaiables instead of integer variables. You write it like this:

Double(Double Value)

Here is an example of using Double object:

// My actual height in real life (in feet)
Double height = new Double(6.6); // Send to an Object
double primitiveHeight = height.doubleValue(); // Turn back into a primitive

And here is an example of when it works and when it does not.

ArrayList<double> list = new ArrayList<double>();  // This will cause a compile-time error

|   ArrayList<double> list = new ArrayList<double>();  // This will cause a compile-time error

unexpected type

  required: reference

  found:    double



|   ArrayList<double> list = new ArrayList<double>();  // This will cause a compile-time error

unexpected type

  required: reference

  found:    double

Again, the ArrayList needs an object type, but we are passing a primitive type.

ArrayList<Double> list = new ArrayList<Double>();
list.add(3.14);  // Autoboxing will automatically convert the double 3.14 to a Double object

true

This code works because the 3.14 is being converted by the list.add, since the list uses double objects.

Autoboxing

Autoboxing is when Java automatically changes a basic data type into its object form. The Java compiler does this for us. Think of it as putting the data into a box. Java can also unbox an object, which is the exact opposite of autoboxing. When an Integer object is assigned to a primitive int type, Java will automatically use the primitive int version of the number and assign it to the int variable.

// Boxing
int a = 5;
Integer b = a;

// Unboxing
Integer x = new Integer(10);
int y = x;

2.9 Using the Math Class

Java has a built in math class called ___? It is a part of the java.lang class.

System.out.println(Math.abs(-5)) // Absolute Value

5

System.out.println(Math.pow(5, 2)) // Power: base, exponent

25.0

// Two ways to calculate square roots
System.out.println(Math.pow(25, 0.5)); // Allows for different roots
System.out.println(Math.sqrt(25)) // Square Root

5.0


5.0

Finally, arguably the most important class in math: The random class! Many purposes, especially in game development. What is one purpose?

System.out.println((int)Math.random()) // Always returns zero because the range of math.random is [0, 1)

0

In the cell below, experiment and try to figure out a way to call Math.random and impliment bounds, with your bounds being integer bounds of a and b.