📓3.7: Comparing Objects

Table of Contents

📖 This page is a condensed version of CSAwesome Topic 3.7

📝 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-3-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!

Comparing Objects

Comparing objects is a little different than comparing primitive typed values like numbers. Objects can be very complex and have many attribute values or instance variables inside them. For example, the Turtle objects have many instance variables like name, width, height, xPos, yPos, etc. When comparing two Turtle objects, we need a specially written equals method to compare all of these values. In this lesson, we will take a look at String objects and the difference between comparing them with == vs. the equals method.

String Equality

The equals method for Strings compares two strings letter by letter. s1.equals(s2) is true if s1 and s2 have all the same characters in the same order. With Strings and other objects, you almost always use equals instead of == to check their equality.

When the operator == is used to compare object variables, it returns true when the two variables refer to the same object. These variables are called object references and aliases for the same object. With strings this happens when one string variable is set to another.

image

Equality with New Strings

If you use the new keyword to create a string, it will always create a new string object. So, even if we create two string objects with new that contain all the same characters in the same order, they will not refer to the same object.

Watch this video to see how this code works in memory.

Since we used the new keyword, two different String objects will be created that each have the characters Hello in them. So s1 == s2 will be false since they don’t refer to the same object, but s1.equals(s2) is true since the two different objects contain the same characters in the same order.

Here is the representation of these String objects in memory:

image

Only use == with primitive types like int or to test if two strings (or objects) refer to the same object. Use equals, not ==, with strings to test if they are equal letter by letter.

Comparing with null

One common place to use == or != with objects is to compare them to null to see if they really exist. Sometimes short-circuit evaluation is used to avoid an error if the object doesn’t exist. Remember that short-circuit evaluation is used with && in Java meaning that if the first part of the if condition is false, it doesn’t even have to check the second condition and it knows the whole && test is false.

⭐️ Summary

  • Often classes have their own equals method, which can be used to determine whether two objects of the class are equivalent.

  • Two object references are considered aliases when they both reference the same object.

  • Object reference values can be compared, using == and !=, to identify aliases.

  • A reference value can be compared with null, using == or !=, to determine if the reference actually references an object.

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

  1. Navigate to the Source Control menu on the LEFT sidebar
  2. Type a brief commit message in the box, for example: updated Main.java
  3. Click the button on the LEFT menu
  4. Click the button on the LEFT menu
  5. Finally you can close your Codespace!

Acknowledgement

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