2.7 Comparing Objects

== compares references, not content

For object variables, the == operator checks whether two references point to the same object in memory, not whether their data is equal. Two different `String` objects with identical characters can be `==` false, even though their contents look the same.

equals() compares values

The .equals() method is used to compare the actual content of objects, like two `String` objects for identical characters. `String` overrides `.equals()` for this purpose. Always use `.equals()` when checking if two objects have the same value.

compareTo() for ordering

The .compareTo() method compares objects for relative order, returning a negative number, zero, or a positive number. For `String`, it uses lexicographic (dictionary-like) order based on Unicode values. `a.compareTo(b)` returns negative if `a` comes before `b`, zero if equal, positive if `a` comes after `b`.

Key terms

== (for objects)

Checks whether two references point to the same object in memory (reference equality).

.equals()

Method that compares the actual content/value of two objects; String overrides it for character comparison.

.compareTo()

Method that returns a negative, zero, or positive number indicating relative order; used for Strings in lexicographic order.

Lexicographic order

Dictionary-like ordering of Strings based on the Unicode values of their characters.

new String("cat") == new String("cat")

false — they are two distinct objects in memory, even though their content is identical.

Try an AP-style question

String a = new String("hi");
String b = new String("hi");
System.out.println(a == b);

What is printed?

1. hi
2. An error
3. false
4. true