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L2.3 — The Java Library recaps encapsulation and constructors, then covers the Java Library (importing classes, packages, implementation vs interface), String comparison pitfalls (identity vs equality with four scenarios), and introduces HashMaps as a generic key-value collection with lookup, autoboxing, and practical usage.

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📋 At a Glance

• [ ] Recall what protected means (class + sub-classes only)
• [ ] Know the five rules for constructors
• [ ] Understand that the Java Library is a collection of pre-built classes included with every JVM
• [ ] Import classes individually or as whole packages
• [ ] Distinguish implementation (internal details) from interface (public-facing methods)
• [ ] Know that Strings are objects (from java.lang, auto-imported)
• [ ] Always use .equals() to compare string values, not ==
• [ ] Explain all four identity vs equality scenarios
• [ ] Create and use a HashMap with generics
• [ ] Perform lookup with get() and insertion with put()
• [ ] Recognise autoboxing in HashMap usage

1. Encapsulation Recap

The protected Keyword

The protected keyword tells Java that only methods in the class and its sub-classes can access the variable. The opposite is public, which allows all other methods to access it.

Why Encapsulation Matters

Suppose we originally store age as an int: