Java for Python developers

1. "Difference between == and .equals() for objects?"

== compares references. It asks: are these the same object? .equals() compares values. It asks: do these mean the same thing? For String and records, use .equals(). == is fine for primitives (int, boolean) and for identity checks like x == null.

2. "What's a record and when do you use it?"

A record is an immutable value type: record Order(String currency, double amount) {}. Java generates the constructor, the accessors, and value-based equals/hashCode/toString. Use it for data carriers (events, DTOs, keys). Use it where you would use a frozen dataclass in Python. You can add a compact constructor for validation.

3. "Checked vs unchecked exception — give an example of each and when you'd throw which."

You must catch a checked exception (IOException) or declare it with throws. Use it for recoverable I/O conditions that the caller should plan for. Unchecked exceptions (IllegalArgumentException, NullPointerException) extend RuntimeException. Throw IllegalArgumentException for a bad argument that is a programming error. If you use too many checked exceptions, APIs become noisy.

4. "How does Optional improve on returning null?"

It puts "may be absent" in the type signature. The caller must handle the empty case on purpose, with orElse, map, ifPresent, or orElseThrow. It prevents the silent NullPointerException that a bare null return invites. Don't use Optional for fields or collection elements. Prefer empty collections there.

5. "A Python dev writes if (orders) to check a non-empty list and it won't compile. Why?"

Java has no truthiness. An if needs a boolean. Write if (!orders.isEmpty()). Java also has no implicit conversion between int and bool. The Java idiom is to be explicit.

6. "Why must de/mastery/Money.java sit under that exact directory path?"

The file declares package de.mastery;. Java maps packages to directories under the source root. The compiler and every build tool rely on this. If the path does not match, the build fails to compile or to find the class.