📓2.8: Wrapper Classes

Table of Contents

📖 This page is a condensed version of CSAwesome Topic 2.8

📝 Take notes in a Codespace during class, coding along with the instructor.

  1. Go to GitHub and click on your picture in the TOP RIGHT corner
  2. Select Your repositories
  3. Open CS2-Unit-2-Notes
  4. Now on your repository, click and select the Codespaces tab
  5. Click Create Codespace on main (unless you already have one listed there), wait for the environment to load, then you’re ready to code!

Wrapper Classes: Integer and Double

For every primitive type in Java, there is a built-in object type called a wrapper class. The wrapper class for int is called Integer, and for double it is called Double. Sometimes you may need to create a wrapped object for a primitive type so that you can give it to a method that is expecting an object. To wrap a value, call the constructor for the wrapper class in earlier versions of Java. In Java 9 on, this is deprecated which means it’s not the best way to do this anymore, and you should instead just set it equal to a value. The AP CSA Exam covers Java 7 which does allow using the constructor.

   // in older versions of Java (and on the AP exam)
   Integer i = new Integer(2); // create an object with 2 in it
   Double d = new Double(3.5); // create an object with 3.5 in it

   // in newer versions of Java (9+)
   Integer i = 2;
   Double d = 3.5;

These wrapper classes (defined in the java.lang included package) are also useful because they have some special values (like the minimum and maximum values for the type) and methods that you can use.

When would you ever use Integer.MIN_VALUE or Integer.MAX_VALUE? They are handy if you want to initialize a variable to the smallest possible value and then search a sequence of values for a larger value.

Autoboxing and Unboxing

Autoboxing is the automatic conversion that the Java compiler makes between primitive types and their corresponding object wrapper classes. This includes converting an int to an Integer and a double to a Double. The Java compiler applies autoboxing when a primitive value is passed as a parameter to a method that expects an object of the corresponding wrapper class or assigned to a variable of the corresponding wrapper class. Here’s an example of autoboxing.

   Integer i = 2;
   Double d = 3.5;

Unboxing is the automatic conversion that the Java compiler makes from the wrapper class to the primitive type. This includes converting an Integer to an int and a Double to a double. The Java compiler applies unboxing when a wrapper class object is passed as a parameter to a method that expects a value of the corresponding primitive type or assigned to a variable of the corresponding primitive type. Here’s an example of unboxing:

   Integer i = 2;  // autoboxing - wrap 2
   int number = i; // unboxing - back to primitive type

⭐️ Summary

  • The Integer class and Double class are wrapper classes that create objects from primitive types.

  • The following Integer methods and constructors, including what they do and when they are used, are part of the Java Quick Reference.

    • Integer(value): Constructs a new Integer object that represents the specified int value.
    • Integer.MIN_VALUE : The minimum value represented by an int or Integer.
    • Integer.MAX_VALUE : The maximum value represented by an int or Integer.
    • int intValue() : Returns the value of this Integer as an int.

  • The following Double methods and constructors, including what they do and when they are used, are part of the Java Quick Reference Guide given during the exam:

    • Double(double value) : Constructs a new Double object that represents the specified double value.
    • double doubleValue() : Returns the value of this Double as a double.

  • Autoboxing is the automatic conversion that the Java compiler makes between primitive types and their corresponding object wrapper classes. This includes converting an int to an Integer and a double to a Double.

  • The Java compiler applies autoboxing when a primitive value is:

    • Passed as a parameter to a method that expects an object of the corresponding wrapper class.
    • Assigned to a variable of the corresponding wrapper class.

  • Unboxing is the automatic conversion that the Java compiler makes from the wrapper class to the primitive type. This includes converting an Integer to an int and a Double to a double.

  • The Java compiler applies unboxing when a wrapper class object is:

    • Passed as a parameter to a method that expects a value of the corresponding primitive type.
    • Assigned to a variable of the corresponding primitive type.


🛑 When class ends, don’t forget to SAVE YOUR WORK!

  1. Navigate to the Source Control menu on the LEFT sidebar
  2. Click the button on the LEFT menu
  3. Type a brief commit message at the top of the file that opens, for example: updated Main.java
  4. Click the small ✔️ checkmark in the TOP RIGHT corner
  5. Click the button on the LEFT menu
  6. Finally you can close your Codespace!

Acknowledgement

Content on this page is adapted from Runestone Academy - Barb Ericson, Beryl Hoffman, Peter Seibel.