Packages, builds, and tests

Compilation and execution are different boundaries

A source file declaring a public class has that class's filename. A package contributes to its fully qualified name; directory layout conventionally mirrors the package. javac -d out writes class files under package directories, and java -cp out package.Main loads that named class. Classpath roots are directories/jars, not package directories. Java 9+ modules add explicit dependency/export boundaries; ordinary classpath code remains useful for small exercises.

package sample;
public class Tool {
    public static String name() { return "tool"; }
}

Compile that source into an output root and import sample.Tool from consumers. Imports resolve names; they do not install dependencies. Maven/Gradle coordinate dependency resolution, reproducible builds, test execution, and packaging. Pin versions and compiler release, and distinguish the JDK used to run a build from the bytecode/API level it targets. This course uses the standard library and Java 17, with default-package Exercise/Main so each download compiles independently.

Test the public contract

A compiler error is a source/API failure, not an algorithm test failure. A process that runs zero tests must not report success. Java assert statements are disabled unless enabled with -ea; this course's harness explicitly throws AssertionError and exits nonzero. Named checks should isolate behavior, compare values, and exercise invalid input without corrupting subsequent state.

Tests should make failure precedence visible. If a parser sees an unknown flag after a valid option, it must reject the request rather than publish partial configuration. Table-driven tests cover boundary values; a handful of clear state transitions often explains more than a large collection of implementation-mirroring checks. Parsing is separated from side effects here, so argument errors cannot accidentally start a server or mutate global settings. A pure options value also makes repeated calls independent.