Mixed Questions
169. What are Wrapper classes in Java?
A wrapper class wraps (encloses) around a data type and gives it an object .Wrapper classes include methods to unwrap the object and give back the data type.
List of Wrapper classes:
| PRIMITIVE DATA TYPE | WRAPPER CLASS |
|---|---|
| byte | Byte |
| short | Short |
| int | Integer |
| long | Long |
| float | Float |
| double | Double |
| char | Character |
| boolean | Boolean |
Importance of Wrapper classes
There are mainly two uses with wrapper classes.
- To convert simple data types into objects, that is, to give object form to a data type; here constructors are used.
- To convert strings into data types (known as parsing operations), here methods of type parseXXX() are used.
170. What is the purpose of native method in Java?
Native methods allow you to use code from other languages such as C or C++ in your java code. You use them when java doesn't provide the functionality that you need.
For example, if I were writing a program to calculate some equation and create a line graph of it, I would use java, because it is the language I am best in. However, I am also proficient in C. Say in part of my program I need to calculate a really complex equation. I would use a native method for this, because I know some C++ and I know that C++ is much faster than java, so if I wrote my method in C++ it would be quicker. Also, say I want to interact with another program or device. This would also use a native method, because C++ has something called pointers, which would let me do that
171. What is System class?
The java.lang.System class contains several useful class fields and methods. It cannot be instantiated.Facilities provided by System − standard output. error output streams. standard input and access to externally defined properties and environment variables.
172. What is System, out and println in System.out.println method call?
System is a final class provided by java.lang package. out refers to PrintStream class and a static member of System class. println is a method of PrintStream class.
173. What is the other name of Shallow Copy in Java?
Object Cloning. A Shallow Copy just copies the values of references in a Class.
174. What is the difference between Shallow Copy and Deep Copy in Java?
A Shallow copy just copies the values of the references in the class. A Deep copy copies the values of the objects as well.
175. What is a Singleton class?
A Singleton class in Java has maximum one instance of the class present in JVM, all the time. The constructor of this class is written in such a way that it never creates more than one object of same class.
176. What is the difference between Singleton class and Static class?
A static class in Java has only static methods. It is a container of functions. It is created based on procedural programming design.
Singleton class is a pattern in Object Oriented Design. A Singleton class has only one instance of an object in JVM. This pattern is implemented in such a way that there is always only one instance of that class present in JVM.