Part 1
Interview questions on Singleton for experienced covered in this post:
What is Singleton Class? Write a program for the same?
-
Singleton means only one object can be created.
-
Following are the steps that need to take to create a Singleton
class.
1.
Create a class variable with private and static.
2.
Create a private constructor.
-
Please refer to the below example.
|
1.
Lazy Loading package simplifiedjava.crackedInterview; public class Singleton { private static Singleton INSTANCE;
private
Singleton() { }
public
static
Singleton getInstance() {
if(INSTANCE
== null) {
INSTANCE
= new
Singleton();
}
return
INSTANCE; }
public
void
m1() {
System.out.println("m1() method called.");
} } package simplifiedjava.crackedInterview; public class SingletonDemo {
public
static
void
main(String[] args) {
Singleton obj
= Singleton.getInstance();
System.out.println(obj1.hashCode());
System.out.println(obj2.hashCode());
obj1.m1();
}
}
Output:
2018699554
2018699554
m1() method called.
2. Eagar loading. package simplifiedjava.crackedInterview; public class Singleton { private static Singleton INSTANCE = new Singleton();
private
Singleton() { }
public
static
Singleton getInstance() {
if(INSTANCE
!= null) {
return
INSTANCE;
}
return
null; }
public
void
m1() {
System.out.println("m1() method called.");
} } package simplifiedjava.crackedInterview; public class SingletonDemo {
public
static
void
main(String[] args) {
Singleton obj
= Singleton.getInstance();
System.out.println(obj1.hashCode());
System.out.println(obj2.hashCode());
obj1.m1();
obj.m1();
}
}
Output:
2018699554
2018699554
m1() method called. |
 
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Can we break Singleton? How many ways we can break the singleton?
-
Yes, we can break the singleton.
-
There are three ways we can break the Singleton.
1.
Using Reflection API.
2.
Using Deserialization.
3.
Using Cloning.
WAP to break Singleton using Reflection? How to prevent Singleton to break
with Reflection?
|
package simplifiedjava.crackedInterview; public class Singleton { private static Singleton INSTANCE = new Singleton();
private
Singleton() { }
public
static
Singleton getInstance() {
if(INSTANCE
!= null) {
return
INSTANCE;
}
return
null; }
public
void
m1() {
System.out.println("m1() method called.");
} } package simplifiedjava.crackedInterview; import java.lang.reflect.Constructor; public class SingletonBreakUsingReflection { public static void main(String[] args) {
Singleton obj1
= Singleton.getInstance(); Singleton obj2 = null;
try
{
Constructor<Singleton> constructor
= Singleton.class.getDeclaredConstructor();
constructor.setAccessible(true);
obj2
= (Singleton)constructor.newInstance();
}catch(Exception e) {
e.printStackTrace(); }
System.out.println("HashCode For First Object "+ obj1.hashCode());
System.out.println("HashCode For Second Object "+ obj2.hashCode());
}
}
Output:
HashCode For First Object 2018699554
HashCode For Second Object 1311053135 |
-
We can prevent the singleton pattern to break with the
following steps.
1.
One of the best solutions is to throw any customize run-time
exception in the constructor if the instance already exists.
2.
You can refer to the code snippet for it. You have to change
the Singleton class.
|
package simplifiedjava.crackedInterview; import javax.activity.InvalidActivityException; public class Singleton { private static Singleton INSTANCE = new Singleton();
private
Singleton() {
if(INSTANCE
!= null) {
try
{
throw
new
InvalidActivityException("Can not create multiple objects.");
} catch
(InvalidActivityException e) {
e.printStackTrace();
}
} }
public
static
Singleton getInstance() {
if(INSTANCE
!= null) {
return
INSTANCE;
}
return
null; }
public
void
m1() {
System.out.println("m1() method called.");
}
}
Output:
javax.activity.InvalidActivityException: Can not create multiple objects.
at simplifiedjava.crackedInterview.Singleton.<init>(Singleton.java:12)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
HashCode For First Object 1311053135
HashCode For Second Object 118352462 |
-
Java interview questions and answers all MNC - Click here
-
Basic core java interview questions and answers for freshers
- Click here
-
Core java interview questions for 3 years experience - Click here
-
Core java interview questions and answers for 3-5 years exp
- Click here
-
Core java interview questions and Answers for 5 - 7 Years exp
- Click here
-
Basic Java Interview Questions and Answers - Click here
-
Java interview questions and answers on oops - Click here
-
Java interview questions and answers on Strings - Click here
-
Java interview questions on exception handling - Click here
-
Interview questions on multithreading in java for experienced
- Click here
-
Interview questions on serialization in java for experienced
- Click here
-
Interview questions on inner class in java for experienced
- Click here
-
Interview questions on Collections in java for experienced
- Click here
|
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