Recall our discussion of module systems: a module system's fundamental purpose is to isolate pieces of a system from each other, and provide well-defined interfaces through which the isolated pieces can interact.
In Java, the module system has two levels:
...is as simple as using a package declaration at the top of a source file. When something is not labeled public, private, or protected, it has "package" visibility, and may not be accessed outside of its declared package.
// FILE 1
package whales.baleen;
public class HumpbackWhale {
public void spout() { ... }
void dive() { // Package-visible method
StealthyWhale b = new StealthyWhale();
b.encounter(this);
}
void greet() { ... } // Another package-visible method
}
class StealthyWhale { // A package-visible class
void encounter(HumpbackWhale hw) {
System.out.println("Stealthy whale encounters humpback");
hw.spout();
hw.greet();
}
}
// FILE 2
package whales;
// "import" is analogous to ML's "open ModuleName" declaration. It allows
// you to refer to names in another package, without using the fully
// qualified name. You can also import classes one at a time.
import whales.baleen.*;
public class KillerWhale {
// This method's signature is illegal, because StealthyWhale has
// package visibility to whales.baleen.
public void eat(StealthyWhale sw) { System.out.println("Drat! Foiled!"); }
// This method's signature is legal, but its implementation...
public void eat(HumpbackWhale hw) {
// ... calls the package-visibility method greet(), which is illegal.
hw.greet();
System.out.println("Drat! Foiled again!");
}
}