This page contains the list of question that are generally asked in the Java General Questions. They can also be termed as questions asked on Basic Java Language.
Question 1: What is the difference between java.util.Date and java.sql.Date?
The main difference is that java.util.Date represents "date and time" stored upto a milisecond. However, java.sql.Date only stores data value which is required for the SQL DATE type. java.sql.Date has a java.sql.Time counterpart which stores only time.
Question 2: What is the difference between String, StringBuffer and String Builder?
Question 3: What are various visibility modifiers in Java and what is the difference between them?
There are four access modifiers : public, private, protected and default. There accessibility is as defined below:
public - Any class can access it directly.
protected - It is available to class, subclasses and package.
default or no-modifier: It has the package accessibility but is not available to subclasses.
private: This is only visible with the class itself.
Question 4: What is difference between .equals and "=="?
"==" checks whether the two references refer to the same instance whereas .equals check if the two different instances are equal.
Question 5: What is the relationship between .equals and hashcode()? or Why is it a good practise to either override both .equals() and hashcode() or none at all?
The relationship can be explained in simple way as, if two objects returns true for .equals() then their .hashcode() method must return the same value. However, if .hashcode() value returns same value .equals() may not return true, provided .hashcode() and .equals() uses the same fields for evaluation.
You should always override both these methods together because they should use the same fields to evaluate hashcode() and compare the two objects in .equals().
Question 6: Why is clone method "protected" in the Object class?
Default method in the Object class does not provide any implementation, so it does not make sense to make it public. Also, the object which needs to be cloned needs to implement Cloneable interface. Any object that wishes to be available for cloning can override this method and make it public.
Question 7: Is Generics a compile-time feature or runtime feature?
Generics are a compile-time feature, they help us to find some of the bugs at compile-time itself. Generics helps in ensuring the strong type checking. This information is not kept along with the compiled version of class.
Question 8: What is the use of serialVersionUId in Serializable class?
serialVersionUId is used in the serialization and de-serialization process. While serialization and de-serialization the serialVersionUId needs to be same. In case, the serialVersionUId is changed in a class the JVM will throw an "InvalidClassException". You should only change the serialVersionUId only, when your serialization class is updated by some incompatible Java types changes to a serializable class. Please refer this for further explanation on serialVersionUId.
Question 9: What is the difference between Serializable and Externalizable?
When a class implement the serializable interface, the JVM will serialize and de-serialize the class variables by itself, however, if you want to change the way serialization is done you need to implement the Externalizable interface. When you implement the Serializable interface you dont need to implement any method the JVM will use reflection to serialize your class. However, if you implement Externalizable interface, you need to implement two methods readExternal() and writeExternal(). If you have implemented Serializable you can still control the serialization by writing two methods in your class namely, readObject() and writeObject().
Question 10: How can you control which fields should be serialized?
There are three exceptions to the Serialization process. They are:
1) Static fields are not serialized.
2) transient fields are not serialized.
3) Base class variables are only serialized if the Base class itself implement Serializable.
Question 1: What is the difference between java.util.Date and java.sql.Date?
The main difference is that java.util.Date represents "date and time" stored upto a milisecond. However, java.sql.Date only stores data value which is required for the SQL DATE type. java.sql.Date has a java.sql.Time counterpart which stores only time.
Question 2: What is the difference between String, StringBuffer and String Builder?
- String is immutable whereas StringBuffer/StringBuilder are not. It means it gives faster performance while update operations.
- StringBuffer is synchronised whereas StringBuilder is not.
- Use String if you need immutability, use StringBuffer is you need mutability and thread-safety, use StringBuilder if you need mutability but not thread safety.
Question 3: What are various visibility modifiers in Java and what is the difference between them?
There are four access modifiers : public, private, protected and default. There accessibility is as defined below:
public - Any class can access it directly.
protected - It is available to class, subclasses and package.
default or no-modifier: It has the package accessibility but is not available to subclasses.
private: This is only visible with the class itself.
Question 4: What is difference between .equals and "=="?
"==" checks whether the two references refer to the same instance whereas .equals check if the two different instances are equal.
Question 5: What is the relationship between .equals and hashcode()? or Why is it a good practise to either override both .equals() and hashcode() or none at all?
The relationship can be explained in simple way as, if two objects returns true for .equals() then their .hashcode() method must return the same value. However, if .hashcode() value returns same value .equals() may not return true, provided .hashcode() and .equals() uses the same fields for evaluation.
You should always override both these methods together because they should use the same fields to evaluate hashcode() and compare the two objects in .equals().
Question 6: Why is clone method "protected" in the Object class?
Default method in the Object class does not provide any implementation, so it does not make sense to make it public. Also, the object which needs to be cloned needs to implement Cloneable interface. Any object that wishes to be available for cloning can override this method and make it public.
Question 7: Is Generics a compile-time feature or runtime feature?
Generics are a compile-time feature, they help us to find some of the bugs at compile-time itself. Generics helps in ensuring the strong type checking. This information is not kept along with the compiled version of class.
Question 8: What is the use of serialVersionUId in Serializable class?
serialVersionUId is used in the serialization and de-serialization process. While serialization and de-serialization the serialVersionUId needs to be same. In case, the serialVersionUId is changed in a class the JVM will throw an "InvalidClassException". You should only change the serialVersionUId only, when your serialization class is updated by some incompatible Java types changes to a serializable class. Please refer this for further explanation on serialVersionUId.
Question 9: What is the difference between Serializable and Externalizable?
When a class implement the serializable interface, the JVM will serialize and de-serialize the class variables by itself, however, if you want to change the way serialization is done you need to implement the Externalizable interface. When you implement the Serializable interface you dont need to implement any method the JVM will use reflection to serialize your class. However, if you implement Externalizable interface, you need to implement two methods readExternal() and writeExternal(). If you have implemented Serializable you can still control the serialization by writing two methods in your class namely, readObject() and writeObject().
Question 10: How can you control which fields should be serialized?
There are three exceptions to the Serialization process. They are:
1) Static fields are not serialized.
2) transient fields are not serialized.
3) Base class variables are only serialized if the Base class itself implement Serializable.
