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Nicolai Parlog
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[01] Demonstrate static factory methods
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‎README.md‎

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* [Amazon USA](https://amzn.to/2QI0D0S)
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* [Amazon Germany](https://amzn.to/2OvsWOu)
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* visit [courses.codefx.org](http://courses.codefx.org) for online courses on Java
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## Creating and destroying objects
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* Item 1: [Consider static factory methods instead of constructors](https://www.youtube.com/watch?v=WUROOKn2OTk) -
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[examples](src/main/java/org/codefx/demo/effective_java/_01_static_factory_methods/Main.java)

‎pom.xml‎

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<maven.compiler.target>10</maven.compiler.target>
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</properties>
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<dependencies>
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<dependency>
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<groupId>com.google.guava</groupId>
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<artifactId>guava</artifactId>
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<version>25.0-jre</version>
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</dependency>
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</dependencies>
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<build>
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<plugins>
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<plugin>

‎src/main/java/org/codefx/demo/effective_java/_01_static_factory_methods/Main.java‎

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package org.codefx.demo.effective_java._01_static_factory_methods;
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/*
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* You will find NOTEs on the following topics:
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* - basics
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* - named constructors
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* - instance-control
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* - type-control
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*/
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public class Main {
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public static void main(String[] args) {
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namedConstructors();
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instanceControl();
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typeControl();
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}
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// private static void before() {
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// // NOTE basics:
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// // * the code must read `new $CLASS` - no way to give it a nice name
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// // * the code will create a new instance - no way to, e.g., cache instances
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// // * the code will create a `Point` - no way to create a more suitable subtype
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// Point origin = new Point(0, 0);
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// Point somePoint = new Point(42, 63);
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// Rectangle rectangle = new Rectangle(origin, somePoint);
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// System.out.println(rectangle);
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// }
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private static void namedConstructors() {
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Point origin = Point.ofXY(0, 0);
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Point upperRight = Point.ofXY(42, 63);
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Rectangle someRectangle = Rectangle.fromLowerLeftToUpperRight(origin, upperRight);
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Point onXAxis = Point.ofXY(0, 63);
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Point onYAxis = Point.ofXY(42, 0);
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Rectangle otherRectangle = Rectangle.fromUpperLeftToLowerRight(onXAxis, onYAxis);
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System.out.println(someRectangle + " vs " + otherRectangle);
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}
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private static void instanceControl() {
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Point origin = Point.ofXY(0, 0);
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Point otherOrigin = Point.ofXY(0, 0);
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if (origin == otherOrigin)
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System.out.println("There can only be one origin!");
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Point somePoint = Point.ofXY(4, 2);
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Point otherPoint = Point.ofXY(4, 2);
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if (somePoint == otherPoint)
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System.out.println("There can only be one 4/2!");
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}
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private static void typeControl() {
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Point origin = Point.ofXY(0, 0);
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Point upperRight = Point.ofXY(42, 42);
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Rectangle square = Rectangle.fromLowerLeftToUpperRight(origin, upperRight);
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if (square instanceof Square)
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System.out.println(square);
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}
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}

‎src/main/java/org/codefx/demo/effective_java/_01_static_factory_methods/Point.java‎

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package org.codefx.demo.effective_java._01_static_factory_methods;
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import com.google.common.cache.Cache;
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import com.google.common.cache.CacheBuilder;
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import java.util.concurrent.ExecutionException;
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public class Point {
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// NOTE instance-control: ORIGIN is a constant
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private static final Point ORIGIN = new Point(0, 0);
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// NOTE instance-control: Points can be cached
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private static final Cache<Integer, Cache<Integer, Point>> POINTS_BY_X_Y = CacheBuilder.newBuilder().build();
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private final int x, y;
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// NOTE basics: The constructor is private to avoid accidental use; this also prevents subclassing
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private Point(int x, int y) {
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this.x = x;
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this.y = y;
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}
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// NOTE instance-control: The method's contract clarifies that there is no guarantee regarding identity
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/**
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* @return A {@code Point} instance with the specified x/y coordinates (not necessarily a new instance)
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*/
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// NOTE named constructors:
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// * `ofXY` explains very well what the parameters mean;
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// * `Rectangle`'s static factories are even more informative
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// NOTE basics: A static factory method is `static` and returns an instance of the type (Duh!)
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public static Point ofXY(int x, int y) {
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if (x == 0 && y == 0)
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return ORIGIN;
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// NOTE instance-control: Only points with specific properties (in the 20x20 box around origin) care cached
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if (Math.abs(x) <= 10 && Math.abs(y) <= 10)
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try {
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return POINTS_BY_X_Y
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.get(x, () -> CacheBuilder.newBuilder().build())
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.get(y, () -> new Point(x, y));
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} catch (ExecutionException e) {
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// do nothing, ignore the cache, and construct a new point
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}
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return new Point(x, y);
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}
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public int x() {
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return x;
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}
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public int y() {
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return y;
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}
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@Override
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public String toString() {
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return "(" + x + "/" + y + ")";
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}
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}

‎src/main/java/org/codefx/demo/effective_java/_01_static_factory_methods/Rectangle.java‎

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package org.codefx.demo.effective_java._01_static_factory_methods;
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import static org.codefx.demo.effective_java._01_static_factory_methods.Point.ofXY;
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public class Rectangle {
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protected final Point lowerLeft, upperRight;
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// NOTE basics: Constructor is protected to make accidental use less likely while allowing subclassing
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protected Rectangle(Point lowerLeft, Point upperRight) {
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this.lowerLeft = lowerLeft;
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this.upperRight = upperRight;
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}
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public static Rectangle fromLowerLeftToUpperRight(Point lowerLeft, Point upperRight) {
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// NOTE type-control: the code determines at run time what type to return
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if (upperRight.x() - lowerLeft.x() == upperRight.y() - lowerLeft.y())
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return new Square(lowerLeft, upperRight.x() - lowerLeft.x());
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return new Rectangle(lowerLeft, upperRight);
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}
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// NOTE named constructors:
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// Thanks to static factories, it is easy to distinguish the two ways to construct a `Rectangle`
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public static Rectangle fromUpperLeftToLowerRight(Point upperLeft, Point lowerRight) {
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Point lowerLeft = ofXY(upperLeft.x(), lowerRight.y());
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Point upperRight = ofXY(lowerRight.x(), upperLeft.y());
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return fromLowerLeftToUpperRight(lowerLeft, upperRight
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);
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}
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@Override
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public String toString() {
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return "[↙" + lowerLeft + " & ↗" + upperRight + "]";
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}
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}

‎src/main/java/org/codefx/demo/effective_java/_01_static_factory_methods/Square.java‎

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package org.codefx.demo.effective_java._01_static_factory_methods;
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import static org.codefx.demo.effective_java._01_static_factory_methods.Point.ofXY;
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// NOTE type-control: `Square` is package-visible, i.e. not part of the public API
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class Square extends Rectangle {
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protected final int edgeLength;
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protected Square(Point lowerLeft, int edgeLength) {
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super(lowerLeft, computeUpperRight(lowerLeft, edgeLength));
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this.edgeLength = edgeLength;
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}
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private static Point computeUpperRight(Point lowerLeft, int edgeLength) {
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return ofXY(lowerLeft.x() + edgeLength, lowerLeft.y() + edgeLength);
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}
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public static Square ofLowerLeftAndEdgeLength(Point lowerLeft, int edgeLength) {
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return new Square(lowerLeft, edgeLength);
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}
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@Override
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public String toString() {
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return "[↙" + lowerLeft + " & ⇿" + edgeLength + "]";
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}
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}

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