extendedoperation.java

java 进阶专用。

// Subclass of extensible, serializable typesafe enum - Page 112

import java.io.*;

abstract class ExtendedOperation extends Operation {
    ExtendedOperation(String name) { super(name); }

    public static Operation LOG = new ExtendedOperation("log") {
        protected double eval(double x, double y) {
            return Math.log(y) / Math.log(x);
        }
    };
    public static Operation EXP = new ExtendedOperation("exp") {
        protected double eval(double x, double y) {
            return Math.pow(x, y);
        }
    };

    // The 4 declarations below are necessary for serialization
    private static int nextOrdinal = 0;
    private final int ordinal = nextOrdinal++;
    private static final Operation[] VALUES = { LOG, EXP };
    Object readResolve() throws ObjectStreamException {
        return VALUES[ordinal];  // Canonicalize
    }
}