Make a doubleton which returns the first object in even call of
getInstance() and second instance for
odd call. See code below:
public class Doubleton
{
private Doubleton() {
}
private static int call = 0;
private static
Doubleton[] instances = { new
Doubleton(), new
Doubleton() };
public static Doubleton
getInstance() {
return instances[call++ % 2];
}
public static void main(String[] args) {
Doubleton d1 = Doubleton.getInstance();
Doubleton d2 = Doubleton.getInstance();
for (int i = 0; i < 10; i++) {
if (i % 2 == 0) {
System.out.println(d1 == Doubleton.getInstance());
} else {
System.out.println(d2 == Doubleton.getInstance());
}
}
}
}
See Thread Safe code below:
public class Doubleton
{
private static volatile Doubleton INSTANCE1;
private static volatile Doubleton INSTANCE2;
private static int call = 0;
private Doubleton() {
}
public static Doubleton
getInstance() {
if (call++ % 2 ==
0) {
if (INSTANCE1 == null) {
synchronized (Doubleton.class) {
if (INSTANCE1 == null) {
INSTANCE1 = new Doubleton();
}
}
}
return INSTANCE1;
} else {
if (INSTANCE2 == null) {
synchronized (Doubleton.class) {
if (INSTANCE2 == null) {
INSTANCE2 = new Doubleton();
}
}
}
return INSTANCE2;
}
}
}
