java

http://www.25hoursaday.com/CsharpVsJava.html

NOTE: Although goto and const are Java language keywords they are unused in the Java language.

NOTE: The [NonSerialized] attribute in C# is equivalent to the transient keyword in Java.

Arrays Can Be Jagged

In languages like C and C++, each subarray of a multidimensional array must have the same dimensions. In Java and C# arrays do not have to be uniform because jagged arrays can be created as one-dimensional arrays of arrays. In a jagged array the contents of the array are arrays which may hold instances of a type or references to other arrays. For this reason the rows and columns in a jagged array need not have uniform length as can be seen from the following code snippet:

int [][]myArray = new int[2][];

myArray[0] = new int[3];

myArray[1] = new int[9];

The above code snippet is valid for both C# and Java.

Strings Are Immutable

C# has a System.String class which is analogous to the java.lang.String class. Both classes are immutable meaning that the values of the strings cannot be changed once the strings have been created. In both instances methods that appear to modify the actual content of a string actually create a new string to return, leaving the original string unchanged. Thus the following C# and Java code does not modify the string in either case

C# Code

String csString = "Apple Jack";

csString.ToLower(); /* Does not modify string, instead returns lower case copy of string */

Java Code

String jString = "Grapes";

jString.toLowerCase(); /* Does not modify string, instead returns lower case copy of string */

To create a string-like object that allows modification in C# it is advisable to use the System.Text.StringBuilder class whereas in Java one would use the java.lang.StringBuffer class.

Inheritance Syntax

C# uses C++ syntax for inheritance, both for class inheritance and interface implementation as opposed to the extends and implements keywords.

C# Code

using System;

class B:A, IComparable{

int CompareTo(){}

public static void Main(String[] args){

Console.WriteLine("Hello World");

}

}

Java Code

class B extends A implements Comparable{

int compareTo(){}

public static void main(String[] args){

System.out.println("Hello World");

}

}

Virtual Methods (and final ones too)

Polimorphism enables one to interact with members of a type hierarchy as generic types instead of dealing with specific types. The means of implementing polimorphism typically involves having methods in a base class that may be overidden by derived classes. These methods can be invoked even though the client has a reference to a base class type which points to an object of the derived class. Such methods are bound at runtime instead of being bound during compilation and are typically called virtual methods.

In Java all methods are virtual methods while in C#, as in C++, one must explicitly state which methods one wants to be virtual since by default they are not. To mark a method as virtual in C#, one uses the virtual keyword. Also, implementers of a child class can decide to either explicitly override the virtual method by using the override keyword or explicitly choose not to by using the new keyword instead. By default, in C#, the behavior of methods in a derived class that have the same signature as those in the base class is as if they were declared with the new keyword.

It is possible to mark methods as final in Java which means that the method cannot be overridden by derived classes. In C# this can be done by not marking the method as virtual. The major difference is that in C#, the class can still define the method but the base class version is the one that will be called if the object is used via a base class reference. Java disallows the derived class from containing a method that has the same signature as the final base class method.

Synchronizing Methods and Code Blocks

In Java it is possible to specify synchronized blocks of code that ensure that only one thread can access a particular object at a time and then create a critical section of code. C# provides the lock statement which is semantically identical to the synchronized statement in Java.

C# Code

public void WithdrawAmount(int num){

lock(this){

if(num < this.amount) this.amount -= num;

}

}

Java Code

public void withdrawAmount(int num){

synchronized(this){

if(num < this.amount) this.amount -= num;

}

}

Both C# and Java support the concept of synchronized methods. Whenever a synchronized method is called, the thread that called the method locks the object that contains the method. Thus other threads cannot call a synchronized method on the same object until the object is unlocked by the first thread when it finishes executing the synchronized method. Synchronized methods are marked in Java by using the synchronized keyword while in C# it is done by annotating the method with the [MethodImpl(MethodImplOptions.Synchronized)] attribute. Examples of synchronized methods are shown below

C# Code

using System;

using System.Runtime.CompilerServices;

public class BankAccount{

[MethodImpl(MethodImplOptions.Synchronized)]

public void WithdrawAmount(int num){

if(num < this.amount) this.amount - num;

}

}//BankAccount

Java Code

public class BankAccount{

public synchronized void withdrawAmount(int num){

if(num < this.amount) this.amount - num;

}

}//BankAccount

Access Modifiers

Below is a table mapping C# access modifiers to Java's. This means that a protected member can only be accessed by member methods in that class or member methods in derived classes but is inaccessible to any other classes. The internal modifier means that the member can be accessed from other classes in the same assembly as the class. The internal protected modifier means that a member can be accessed from classes that are in the same assembly or from derived classes.

C# access modifier Java access modifier

private private

public public

internal protected

protected N/A

internal protected N/A

NOTE: The default accessibility of a C# field or method when no access modifier is specified is private while in Java it is protected (except that derived classes from outside the package cannot inherit the field).

Reflection

The ability to discover the methods and fields in a class as well as invoke methods in a class at runtime, typically called reflection, is a feature of both Java and C#. The primary difference between reflection in Java versus reflection in C# is that reflection in C# is done at the assembly level while reflection in Java is done at the class level. Since assemblies are typically stored in DLLs, one needs the DLL containing the targeted class to be available in C# while in Java one needs to be able to load the class file for the targeted class. The examples below which enumerate the methods in a specified class should show the difference between reflection in C# and Java.

Exceptions

Checked exceptions are exceptions that the calling method must handle either by catching the exception or declaring that the exception should be handled by its calling method via the throws clause. On the other hand, unchecked exceptions (also called runtime exceptions) do not have to be caught or declared in the throws clause. In a sense unchecked exceptions are similar to return codes in that they can be ignored without warnings or errors being issued by the compiler. Although if an exception is ignored at runtime, your program will terminate.

Checked exceptions are typically used to indicate to a calling method that the callee failed in its task as well as pass information as to how and why it failed. One must either catch a checked exception or declare it in the throws clause of the method or the Java compiler will issue an error. The reasoning behind the fact that the exception must be declared in the throws clause is that handling whatever errors that can occur when the method is used is just as important as knowing what parameters it accepts and the kind of type it returns.

Unchecked exceptions are typically exceptions that can occur in most parts of the program and thus the overhead of explicitly checking for them outweighs their usefulness due to the massive code bloat that would ensue. Examples of situations that throw unchecked exceptions are accessing a null object reference, trying to access an out of bounds index of an array or a division by zero. In all of the aforementioned cases it would be cumbersome to put try...catch blocks around every the code (for instance a try...catch block around every object access or every array access) and they are better off as unchecked exceptions.

Q&A

* Q1. How could Java classes direct program messages to the system console, but error messages, say to a file?

A. The class System has a variable out that represents the standard output, and the variable err that represents the standard error device. By default, they both point at the system console. This how the standard output could be re-directed:

Stream st = new Stream(new FileOutputStream("output.txt")); System.setErr(st); System.setOut(st);

* Q2. What's the difference between an interface and an abstract class?

A. An abstract class may contain code in method bodies, which is not allowed in an interface. With abstract classes, you have to inherit your class from it and Java does not allow multiple inheritance. On the other hand, you can implement multiple interfaces in your class.

* Q3. Why would you use a synchronized block vs. synchronized method?

A. Synchronized blocks place locks for shorter periods than synchronized methods.

* Q4. Explain the usage of the keyword transient?

A. This keyword indicates that the value of this member variable does not have to be serialized with the object. When the class will be de-serialized, this variable will be initialized with a default value of its data type (i.e. zero for integers).

* Q5. How can you force garbage collection?

A. You can't force GC, but could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately.

* Q6. How do you know if an explicit object casting is needed?

A. If you assign a superclass object to a variable of a subclass's data type, you need to do explicit casting. For example:

Object a; Customer b; b = (Customer) a;

When you assign a subclass to a variable having a supeclass type, the casting is performed automatically.

* Q7. What's the difference between the methods sleep() and wait()

A. The code sleep(1000); puts thread aside for exactly one second. The code wait(1000), causes a wait of up to one second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.

* Q8. Can you write a Java class that could be used both as an applet as well as an application?

A. Yes. Add a main() method to the applet.

* Q9. What's the difference between constructors and other methods?

A. Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times.

* Q10. Can you call one constructor from another if a class has multiple constructors

A. Yes. Use this() syntax.

* Q11. Explain the usage of Java packages.

A. This is a way to organize files when a project consists of multiple modules. It also helps resolve naming conflicts when different packages have classes with the same names. Packages access level also allows you to protect data from being used by the non-authorized classes.

* Q12. If a class is located in a package, what do you need to change in the OS environment to be able to use it?

A. You need to add a directory or a jar file that contains the package directories to the CLASSPATH environment variable. Let's say a class Employee belongs to a package com.xyz.hr; and is located in the file c:\dev\com\xyz\hr\Employee.java. In this case, you'd need to add c:\dev to the variable CLASSPATH. If this class contains the method main(), you could test it from a command prompt window as follows:

c:\>java com.xyz.hr.Employee

* Q13. What's the difference between J2SDK 1.5 and J2SDK 5.0?

A.There's no difference, Sun Microsystems just re-branded this version.

* Q14. What would you use to compare two String variables - the operator == or the method equals()?

A. I'd use the method equals() to compare the values of the Strings and the == to check if two variables point at the same instance of a String object.

* Q15. Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?

A. Yes, it does. The FileNoFoundException is inherited from the IOException. Exception's subclasses have to be caught first.

* Q16. Can an inner class declared inside of a method access local variables of this method?

A. It's possible if these variables are final.

* Q17. What can go wrong if you replace && with & in the following code:

String a=null; if (a!=null && a.length()>10) {...}

A. A single ampersand here would lead to a NullPointerException.

* Q18. What's the main difference between a Vector and an ArrayList

A. Java Vector class is internally synchronized and ArrayList is not.

* Q19. When should the method invokeLater()be used?

A. This method is used to ensure that Swing components are updated through the event-dispatching thread.

* Q20. How can a subclass call a method or a constructor defined in a superclass?

A. Use the following syntax: super.myMethod(); To call a constructor of the superclass, just write super(); in the first line of the subclass's constructor.

** Q21. What's the difference between a queue and a stack?

A. Stacks works by last-in-first-out rule (LIFO), while queues use the FIFO rule

** Q22. You can create an abstract class that contains only abstract methods. On the other hand, you can create an interface that declares the same methods. So can you use abstract classes instead of interfaces?

A. Sometimes. But your class may be a descendent of another class and in this case the interface is your only option.

** Q23. What comes to mind when you hear about a young generation in Java?

A. Garbage collection.

** Q24. What comes to mind when someone mentions a shallow copy in Java?

A. Object cloning.

** Q25. If you're overriding the method equals() of an object, which other method you might also consider?

A. hashCode()

** Q26. You are planning to do an indexed search in a list of objects. Which of the two Java collections should you use:

ArrayList or LinkedList?

A. ArrayList

** Q27. How would you make a copy of an entire Java object with its state?

A. Have this class implement Cloneable interface and call its method clone().

** Q28. How can you minimize the need of garbage collection and make the memory use more effective?

A. Use object pooling and weak object references.

** Q29. There are two classes: A and B. The class B need to inform a class A when some important event has happened. What Java technique would you use to implement it?

A. If these classes are threads I'd consider notify() or notifyAll(). For regular classes you can use the Observer interface.

** Q30. What access level do you need to specify in the class declaration to ensure that only classes from the same directory can access it?

A. You do not need to specify any access level, and Java will use a default package access level.