Running Java in Zerethon IDE

• Run (Ctrl/⌘+Enter) compiles every .java file with javac (Java 25), then runs the entry file's class — Main.java → Main, app/Main.java → app.Main. Make another file the entry with the ▶ button next to it in the explorer.
• The entry Name.java must declare public class Name with public static void main(String[] args).
• A compile error stops the run with “Compilation failed · nothing was run” and javac's messages (file and line).
• A run is stopped after 15 seconds (compiling included). Memory is limited, there is no network, and project files are read-only (write to /tmp).
• Run tests (Ctrl/⌘+Shift+Enter) compiles once, then runs the main of every class in a file named …Test.java, each in its own JVM. A test passes when main finishes without an exception.

import shapes.Geometry;

public class GeometryTest {
    public static void main(String[] args) {
        check(Geometry.area(2, 3) == 6, "2 x 3");
        System.out.println("geometry ok");
    }

    static void check(boolean ok, String what) {
        if (!ok) throw new AssertionError("failed: " + what);
    }
}

Watch out: Java's assert statement is switched off unless the JVM is started with -ea, and tests here run without it — an assert never fails. Throw an exception instead, as check does above.