6: Use Enums instead of int constants

Author: Mark Putyato

src/main/java/by/morka/effective/java/enumsandannotations/useenumsinsteadofints/Inverse.java

@@ -0,0 +1,28 @@
+package by.morka.effective.java.enumsandannotations.useenumsinsteadofints;
+
+// Switch on an enum to simulate a missing method
+public class Inverse {
+    /**
+     * Switch is applicable when:
+     * 1. Enum is NOT UNDER OUR CONTROL
+     * 2. ENUM is UNDER OUR CONTROL but the method ISN'T USEFUL ENOUGH TO BE IN ENUM
+     */
+    public static Operation inverse(Operation op) {
+        return switch (op) {
+            case PLUS -> Operation.MINUS;
+            case MINUS -> Operation.PLUS;
+            case TIMES -> Operation.DIVIDE;
+            case DIVIDE -> Operation.TIMES;
+        };
+    }
+
+    public static void main(String[] args) {
+        double x = Double.parseDouble(args[0]);
+        double y = Double.parseDouble(args[1]);
+        for (Operation op : Operation.values()) {
+            Operation invOp = inverse(op);
+            System.out.printf("%f %s %f %s %f = %f%n",
+                    x, op, y, invOp, y, invOp.apply(op.apply(x, y), y));
+        }
+    }
+}

src/main/java/by/morka/effective/java/enumsandannotations/useenumsinsteadofints/Operation.java

@@ -0,0 +1,53 @@
+package by.morka.effective.java.enumsandannotations.useenumsinsteadofints;
+
+import java.util.Map;
+import java.util.Optional;
+import java.util.stream.Stream;
+
+import static java.util.stream.Collectors.toMap;
+
+// Enum type with constant-specific class bodies and data
+public enum Operation {
+    PLUS("+") {
+        public double apply(double x, double y) { return x + y; }
+    },
+    MINUS("-") {
+        public double apply(double x, double y) { return x - y; }
+    },
+    TIMES("*") {
+        public double apply(double x, double y) { return x * y; }
+    },
+    DIVIDE("/") {
+        public double apply(double x, double y) { return x / y; }
+    };
+
+    private final String symbol;
+
+    Operation(String symbol) {
+        // Can't access static member from constructor
+//        stringToEnum;
+        this.symbol = symbol;
+    }
+
+    @Override public String toString() { return symbol; }
+
+    public abstract double apply(double x, double y);
+
+    // Implementing a fromString method on an enum type
+    private static final Map<String, Operation> stringToEnum =
+            Stream.of(values()).collect(
+                    toMap(Object::toString, e -> e));
+
+    // Returns Operation for string, if any
+    public static Optional<Operation> fromString(String symbol) {
+        return Optional.ofNullable(stringToEnum.get(symbol));
+    }
+
+    public static void main(String[] args) {
+        double x = Double.parseDouble(args[0]);
+        double y = Double.parseDouble(args[1]);
+        for (Operation op : Operation.values())
+            System.out.printf("%f %s %f = %f%n",
+                    x, op, y, op.apply(x, y));
+    }
+}

src/main/java/by/morka/effective/java/enumsandannotations/useenumsinsteadofints/PayrollDay.java

@@ -0,0 +1,52 @@
+package by.morka.effective.java.enumsandannotations.useenumsinsteadofints;
+
+public enum PayrollDay {
+    MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY,
+    SATURDAY(PayType.WEEKEND), SUNDAY(PayType.WEEKEND);
+
+    private final PayType payType;
+
+    PayrollDay(PayType payType) {
+        this.payType = payType;
+    }
+
+    PayrollDay() {
+        this(PayType.WEEKDAY);
+    }
+
+    public int pay(int minutesWorked, int payRate) {
+        return this.payType.pay(minutesWorked, payRate);
+    }
+
+    /**
+     * Strategy enum
+     * Useful when some constants share same behaviour
+     */
+    private enum PayType {
+        WEEKDAY {
+            int overtimePay(int minsWorked, int payRate) {
+                return minsWorked <= MINS_PER_SHIFT ? 0 :
+                        (minsWorked - MINS_PER_SHIFT) * payRate / 2;
+            }
+        },
+        WEEKEND {
+            int overtimePay(int minsWorked, int payRate) {
+                return minsWorked * payRate / 2;
+            }
+        };
+
+        private static final int MINS_PER_SHIFT = 8 * 60;
+
+        abstract int overtimePay(int minutesWorked, int payRate);
+
+        public int pay(int minutesWorked, int payRate) {
+            int basePay = minutesWorked * payRate;
+            return basePay + overtimePay(minutesWorked, payRate);
+        }
+    }
+
+    public static void main(String[] args) {
+        for (PayrollDay day : values())
+            System.out.printf("%-10s%d%n", day, day.pay(8 * 60, 1));
+    }
+}

src/main/java/by/morka/effective/java/enumsandannotations/useenumsinsteadofints/Planet.java

@@ -0,0 +1,45 @@
+package by.morka.effective.java.enumsandannotations.useenumsinsteadofints;
+
+/**
+ * 1. Final class
+ * 2. Singleton
+ * 3. Exports static final instance member
+ */
+public enum Planet {
+    MERCURY(3.302e+23, 2.439e6),
+    VENUS(4.869e+24, 6.052e6),
+    EARTH(5.975e+24, 6.378e6),
+    MARS(6.419e+23, 3.393e6),
+    JUPITER(1.899e+27, 7.149e7),
+    SATURN(5.685e+26, 6.027e7),
+    URANUS(8.683e+25, 2.556e7),
+    NEPTUNE(1.024e+26, 2.477e7);
+
+    private final double mass;
+    private final double radius;
+    private final double surfaceGravity;
+
+    private static final double G = 6.67300E-11;
+
+    Planet(double mass, double radius) {
+        this.mass = mass;
+        this.radius = radius;
+        this.surfaceGravity = G * mass / (radius * radius);
+    }
+
+    public double mass() {
+        return mass;
+    }
+
+    public double radius() {
+        return radius;
+    }
+
+    public double surfaceGravity() {
+        return surfaceGravity;
+    }
+
+    public double surfaceWeight(double mass) {
+        return mass * surfaceGravity;
+    }
+}

src/test/java/by/morka/effective/java/enumsandannotations/WeightTable.java

@@ -0,0 +1,16 @@
+package by.morka.effective.java.enumsandannotations;
+
+import by.morka.effective.java.enumsandannotations.useenumsinsteadofints.Planet;
+import org.junit.jupiter.api.Test;
+
+public class WeightTable {
+
+    @Test
+    public void main() {
+        double earthWeight = Double.parseDouble("76");
+        double mass = earthWeight / Planet.EARTH.surfaceGravity();
+        for (Planet p : Planet.values())
+            System.out.printf("Weight on %s is %f%n",
+                    p, p.surfaceWeight(mass));
+    }
+}