Thinking in Javaでこのコードを読んで、戸惑いました。
package generics;
//: generics/Mixins.java
import java.util.*;
interface TimeStamped { long getStamp(); }
class TimeStampedImp implements TimeStamped {
private final long timeStamp;
public TimeStampedImp() {
timeStamp = new Date().getTime();
}
public long getStamp() { return timeStamp; }
}
interface SerialNumbered { long getSerialNumber(); }
class SerialNumberedImp implements SerialNumbered {
private static long counter = 1;
private final long serialNumber = counter++;
public long getSerialNumber() { return serialNumber; }
}
interface Basic {
public void set(String val);
public String get();
}
class BasicImp implements Basic {
private String value;
public void set(String val) { value = val; }
public String get() { return value; }
}
class Mixin extends BasicImp
implements TimeStamped, SerialNumbered {
private TimeStamped timeStamp = new TimeStampedImp();
private SerialNumbered serialNumber =
new SerialNumberedImp();
public long getStamp() { return timeStamp.getStamp(); }
public long getSerialNumber() {
return serialNumber.getSerialNumber();
}
}
public class Mixins {
public static void main(String[] args) {
Mixin mixin1 = new Mixin(), mixin2 = new Mixin();
mixin1.set("test string 1");
mixin2.set("test string 2");
System.out.println(mixin1.get() + " " +
mixin1.getStamp() + " " + mixin1.getSerialNumber());
System.out.println(mixin2.get() + " " +
mixin2.getStamp() + " " + mixin2.getSerialNumber());
while(true)System.out.println(new Date().getTime());
}
} /* Output: (Sample)
test string 1 1132437151359 1
test string 2 1132437151359 2
*///:~
getStamp()から返される値が同じなのはなぜですか?(1132437151359 == 1132437151359)?2つのオブジェクトが作成され、それらは異なる時間に異なるプロパティを作成します。それでは、なぜですか?