Understanding Equality

In Chapter 2, you learned how to use == to compare numbers and that object references refer to the same object.

StringBuilder one = new StringBuilder();
StringBuilder two = new StringBuilder();
StringBuilder three = one.append("a");
System.out.println(one == two); // false
System.out.println(one == three); // true

Since this example isn't dealing with primitives, we know to look for whether the references are referring to the same object. one and two are both completely separate StringBuilders, giving us two objects. Therefore, the first print statement gives us false. three is more interesting. Remember how StringBuilder methods like to return the current reference for chaining? This means one and three both point to the same object and the second print statement gives us true.

Let's now visit the more complex and confusing scenario, String equality, made so in part because of the way the JVM reuses String literals:

String x = "Hello World...