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Understanding Identity vs. Equality in Java - Class Variables, Access Modifiers, and Information Hiding

In this lecture, we delve into the concepts of identity and equality in Java, highlighting the critical differences between the two. We explore class variables, also known as static variables, including how they can be shared across instances of a class. The importance of private and public accessors is discussed, emphasizing the principle of information hiding which increases module independence in large systems. We also examine best practices for comparing Strings in Java, illustrating why using `.equals()` is essential for ensuring accurate equality checks.

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Understanding Identity vs. Equality in Java - Class Variables, Access Modifiers, and Information Hiding

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  1. CS1054: Lecture 11 More Sophisticated Behavior (contd..)

  2. Today’s topics • Identity vs. equality • Class variables or static variables • Private vs. public accessors

  3. Side note: String equality String input = “bye”; if(input == "bye") { tests identity ... } if(input.equals("bye")) { tests equality ... } • Strings should (almost) always be compared with.equals

  4. Identity vs equality 1 Other (non-String) objects: :Book :Book “Lord of the Rings” “Harry Porter” book1 book2 book1 == book2 ?

  5. Identity vs equality 2 Other (non-String) objects: :Book :Book “Harry Porter” “Harry Porter” book1 book2 book1 == book2 ?

  6. Identity vs equality 3 Other (non-String) objects: :Book :Book “Harry Porter” “Harry Porter” book1 book2 book1 == book2 ?

  7. Identity vs equality (Strings) String input = reader.getInput(); if(input == "bye") { ... } == tests identity :String :String == ? "bye" "bye" input  (may be) false!

  8. Identity vs equality (Strings) equals tests equality String input = reader.getInput(); if(input.equals("bye")) { ... } :String :String ? equals "bye" "bye" input  true!

  9. Class variables.. aka.. static variables • ‘static’ keyword is used to define class variables • class variables are common to the entire class • All the objects share this variable • A static variable can be accessed from any of the class instances • Besides static variables … there are static methods (later in the semester).

  10. Constants private final int SIZE = 10; • private: access modifier, as usual • static: class variable • final: constant

  11. Public vs private • Public attributes (fields, constructors, methods) are accessible to other classes. • Fields should not be public. • Private attributes are accessible only within the same class. • Only methods that are intended for other classes should be public.

  12. Information hiding • Data belonging to one object is hidden from other objects. • Know what an object can do, not how it does it. • Information hiding increases the level of independence. • Independence of modules is important for large systems and maintenance.

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