Explain why using the '==' operator for object comparison often leads to incorrect results or undesirable behavior.

In Java, the == operator when comparing objects checks for reference identity, meaning whether the variable points to the same object in memory, not the contents of the objects. Therefore, if two different objects have the same data, == will return false.

To compare the contents of objects, you need to override the equals() method and use it. For example:

String a = new String("test");
String b = new String("test");

System.out.println(a == b);       // false, different objects
System.out.println(a.equals(b));  // true, same content

If you do not override equals(), comparing user-defined class objects with == will almost always be incorrect, as it compares references, not the data inside the objects.