Java for Python developers

== on objects compares identity (the same reference). Two strings can look equal and still be different objects if they are built in different ways. Then == can be false. Use .equals() for value equality. Equal hashCodes are necessary but not sufficient (collisions exist).

Source: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Object.html#equals(java.lang.Object)

A record is an immutable carrier of data. Java generates the constructor and the component accessors (named like the components, for example currency()). Java also generates value-based equals/hashCode and a readable toString. It is the Java analogue of a frozen dataclass.

Source: https://docs.oracle.com/en/java/javase/21/language/records.html

Optional shows absence in the type, so the caller is pushed to handle it (orElse, map, ifPresent). Returning null invites NullPointerException. Magic sentinel values cause errors.

Source: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Optional.html

Checked exceptions extend Exception (not RuntimeException). The compiler forces you to catch or declare them. Unchecked exceptions (RuntimeException and subclasses like IllegalArgumentException) do not require a declaration. Python has no checked-exception concept.

Source: https://docs.oracle.com/javase/tutorial/essential/exceptions/catchOrDeclare.html

A lambda is a short syntax for an instance of a functional interface (an interface with exactly one abstract method), such as Function, Predicate or Comparator. It is the compiled analogue of a Python lambda.

Source: https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html

The directory structure under the source root must match the package. Package de.mastery means the file is de/mastery/Money.java. Build tools and the compiler rely on this convention.

Source: https://docs.oracle.com/javase/tutorial/java/package/managingfiles.html

Java has no truthiness. An if condition must be a boolean. Test emptiness explicitly, for example if (!list.isEmpty()). Python-style truthiness is a common bug when you port code.

Source: https://docs.oracle.com/javase/specs/jls/se21/html/jls-14.html#jls-14.9