complete example for sect17 (packages)

src/section17/UseVessels.java

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+package section17;
+
+import section17.vessel.RectangularContainer;
+
+/**
+ * This class demonstrates the use of packages and the access right of accessing fields of classes in other package.
+ */
+class UseVessels {
+    public static void main(String[] argv) {
+        Cube testCube = new Cube(42);
+        System.out.println(testCube);
+
+        RectangularContainer testRect = testCube;   // casted to superclass
+        /**
+         * In the following line, Cube.toString() is still invoked because testRect is "actually" a Cube.
+         * The behavior is different from how *fields* are accessed (e.g. testRect.foobar).
+         */
+        System.out.println(testRect.toString());
+    }
+}
+
+// Private class based on section9.Cube
+class Cube extends RectangularContainer {
+
+    public Cube(double side) {
+        super(side, side, side);
+    }
+
+    public String toString() {
+        return String.format("Cube(side=%f)", width);   // width is accessible since it is `protected` in Rect'Container
+    }
+}

src/section17/vessel/Barrel.java

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+package section17.vessel;
+
+/**
+ * Based on section9.BarrelContainer, but with tighter member controls.
+ */
+public class Barrel extends Vessel {
+
+    private final double radius, height;        // 'private final' such that it's not directly accessible from outside
+
+    public Barrel(double radius, double height) {
+        this.radius = radius;
+        this.height = height;
+    }
+
+    public double capacity() {
+        return Math.PI * radius * radius * height;
+    }
+
+    public String toString() {
+        return String.format("section17.vessel.Barrel(radius=%f, height=%f)", radius, height);
+    }
+}

src/section17/vessel/RectangularContainer.java

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+package section17.vessel;
+
+/**
+ * Based on section9.RectangularContainer, but with different member controls.
+ */
+public class RectangularContainer extends Vessel {
+
+    protected final double length, width, height;   // Want this to be accessible in subclasses wherever they are
+
+    public RectangularContainer(double length, double width, double height) {
+        this.length = length;
+        this.width = width;
+        this.height = height;
+    }
+
+    public double capacity() {
+        return length * width * height;
+    }
+
+    public String toString() {
+        return String.format("section17.vessel.RectangularContainer(length=%f, width=%f, width=%f)",
+                length, width, height);
+    }
+
+}

src/section17/vessel/Vessel.java

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+package section17.vessel;
+
+/**
+ * Mostly the same hierarchy as section9.LiquidContainer, but with intentional package separation.
+ */
+public abstract class Vessel {
+
+    private double contents;
+    public abstract double capacity();
+
+    public final void fill(double amount) {
+        contents = Math.min(capacity(), contents + amount);
+    }
+
+    // Only defines a getter for contents
+    public final double getContents() {
+        return contents;
+    }
+
+}