Unresolved compilation problem

Exception in thread "main" java.lang.Error: Unresolved compilation problem:
    Unhandled exception type Exception

A java file cannot be ran unless and until it is compilation error free. JVM compiler understands any syntax error or compile time error before it could create runable byte code (.class file). Hence JVM can run a java program only after successful compilation. Look for the compile time error if you get above error.

JVM will run the code no matter if there is any run time exception. If there is any run time exception that will handled at run time accordingly. You must ensure that atleast a code do not contains any syntax error and is compileable.

How to solve compile error?

Solving compilation error is very simple, all you need to do is follow all rules and regulation of JVM. To solve compilation error you need to check for Syntax error, Scope of a variable, Non reachable statements, Exceptions handling etc

Solving compilation error is simple because it can be viewed. A simple Google search can solve your compilation error. Also IDEs like Eclipse, NetBeans etc has advance features which can show any compile error while wring the code itself i.e. before even compilation.

How to solve run time error/exceptions?

My personal experience says solving a RUN time error could me very tough in some cases. You may even require to spend overnight to solve a simple run time exceptions because it cannot be viewed by compiler. In most cases there is a data issue. Run time exception could be simply a null pointer exception which occurs because the object is null.

Example :

Code
System.out.println(tempStr.trim());
Can throw
Exception in thread "main" java.lang.NullPointerException
because tempStr is null.

The answer to the above question is debugging i.e. try to debug your code. Put logs wherever you doubt for the values. In the above example if you print the value of tempStr you will recognize the root cause of the exception. Also try to put exception handling codes i.e. try { .. } catch (Exception ex) {..} wherever you doubt for the values.

Source File Declaration Rules in Java

C:\folder>javac test.java
test.java:9: class Test is public, should be declared in a file named Test.java
public class test
       ^
1 error

The above error appeared because the source file declaration rules in java says that If there is a public class in a file, the name of the file must match the name of the public class. For example, a class declared as public class Test { } must be in a source code file named Test.java. Also note that in Java everything is case-sensitive hence test and Test are treated as two different things.

Below are the rules associated with declaring classes, import statements, and package statements in a source file.

• There can be only one public class per source code file.
• Comments can appear at the beginning or end of any line in the source code file; they are independent of any of the positioning rules discussed here.
• If there is a public class in a file, the name of the file must match the name of the public class. For example, a class declared as public class Test { } must be in a source code file named Test.java.
• If the class is part of a package, the package statement must be the first line in the source code file, before any import statements that may be present.
• If there are import statements, they must go between the package statement (if there is one) and the class declaration. If there isn't a package statement, then the import statement(s) must be the first line(s) in the source code file. If there are no package or import statements, the class declaration must be the first line in the source code file.
• import and package statements apply to all classes within a source code file. In other words, there's no way to declare multiple classes in a file and have them in different packages, or use different imports.
• A file can have more than one nonpublic class.
• Files with no public classes can have a name that does not match any of the classes in the file

How to send mail using MAILX command in UNIX / LINUX

Simple MAILX command

Below is the syntax to send simple mail using mailx command in UNIX / Linux.
You can use it as command line or used it in your shell script.


$ mailx -s "Mail Subject" test@xyz.com
type body of the mail
...
..
EOT (Ctrl+d)
$

Send attachment using MAILX command

In some cases you may have to send attachment in the mail. Lets see the syntax of sending an attachment using mailx command.

Below is the syntax to attach a file while sending a mail using mailx command in LINUX / UNIX


( cat /root/MailBody.txt
uuencode /root/file_name.txt file_name.txt 
) | mailx -s "Mail Subject" test@xyz.com


Here file_name.txt is the attachment to be attached in the mail
AND MailBody.txt is the text body of the mail.

Simple Cryptography example in Java

The Java security APIs span a wide range of areas, including cryptography, public key infrastructure, secure communication, authentication, and access control. Java security technology provides the developer with a comprehensive security framework for writing applications, and also provides the user or administrator with a set of tools to securely manage applications.
Source : http://www.oracle.com/technetwork/java/javase/tech/index-jsp-136007.html

javax.crypto.Cipher
This class provides the functionality of a cryptographic cipher for encryption and decryption. It forms the core of the Java Cryptographic Extension (JCE) framework.
Cipher API : http://docs.oracle.com/javase/6/docs/api/javax/crypto/Cipher.html

Cipher: initialized with keys, these used for encrypting/decrypting data. There are various types of algorithms: symmetric bulk encryption (e.g. AES, DES, DESede, Blowfish, IDEA)
http://docs.oracle.com/javase/6/docs/technotes/guides/security/crypto/CryptoSpec.html

Blowfish 
Blowfish has a 64-bit block size and a variable key length from 1 bit up to 448 bits
Blowfish Wiki : http://en.wikipedia.org/wiki/Blowfish_%28cipher%29

To run this example you will need below mentioned JAR files in classpath.
jce.jar
rt.jar

If you are using JDK 6 or higher version, These JAR files are implicitly present. You can cross verify in JRE folder. Probably in "C:\Program Files\Java\jre6\lib"

Hence C:\Program Files\Java\jre6\lib\jce.jar and C:\Program Files\Java\jre6\lib\rt.jar are implicitly set in your classpath.

Below is the sample self explanatory Java program using Blowfish Cipher. Directly you can run the program and see output.

Simple Blowfish Cipher Example in java

/* SimpleCryptography.java */

import javax.crypto.Cipher;
import javax.crypto.spec.SecretKeySpec;

import sun.misc.BASE64Decoder;
import sun.misc.BASE64Encoder;

public class SimpleCryptography {
    
    private String AlgoName = "Blowfish";
    
    private String keyString = "DesireSecretKey";
    
    public String encrypt(String sValue) throws Exception {
        
        SecretKeySpec skeySpec = new SecretKeySpec(keyString.getBytes(), AlgoName);
        Cipher cipher = Cipher.getInstance(AlgoName);
        
        cipher.init(Cipher.ENCRYPT_MODE, skeySpec);
        
        byte[] encrypted = cipher.doFinal(sValue.getBytes());
        
        BASE64Encoder bASE64Encoder = new BASE64Encoder();
        String enStr = bASE64Encoder.encodeBuffer(encrypted);    
        
        return enStr;
    }
    
    public String decrypt(String sValue) throws Exception {
        
        SecretKeySpec skeySpec = new SecretKeySpec(keyString.getBytes(), AlgoName);
        Cipher cipher = Cipher.getInstance(AlgoName);
        
        BASE64Decoder bASE64Decoder = new BASE64Decoder();
        byte decrytByt[] = bASE64Decoder.decodeBuffer(sValue);
        
        cipher.init(Cipher.DECRYPT_MODE, skeySpec);
        
        byte decrypted[] = cipher.doFinal(decrytByt);
        
        String deStr = new String(decrypted).trim();
        
        return deStr;
    }
    
    public static void main(String[] args) throws Exception {
        
        SimpleCryptography obj = new SimpleCryptography();
        
        String password = "MadanChaudhary";
        
        System.out.println("password : "+password);
        
        String encrypted_password = obj.encrypt(password);
        
        System.out.println("encrypted_password : "+encrypted_password);
        
        String decrypted_password = obj.decrypt(encrypted_password);
        
        System.out.println("decrypted_password : "+decrypted_password);

    }
}

Output :

password : MadanChaudhary
encrypted_password : 0LpwIuI0V+44sbZ5w3CCZw==

decrypted_password : MadanChaudhary

Below are the known Errors and exceptions I observed while writing this program.

1.
javax.crypto.IllegalBlockSizeException: Input length must be multiple of 8 when decrypting with padded cipher
    at com.sun.crypto.provider.SunJCE_f.b(DashoA13*..)
    at com.sun.crypto.provider.SunJCE_f.b(DashoA13*..)
    at com.sun.crypto.provider.BlowfishCipher.engineDoFinal(DashoA13*..)
    at javax.crypto.Cipher.doFinal(DashoA13*..)
Exception in thread "main" javax.crypto.IllegalBlockSizeException: Input length must be multiple of 8 when decrypting with padded cipher
    at com.sun.crypto.provider.SunJCE_f.b(DashoA13*..)
    at com.sun.crypto.provider.SunJCE_f.b(DashoA13*..)
    at com.sun.crypto.provider.BlowfishCipher.engineDoFinal(DashoA13*..)
    at javax.crypto.Cipher.doFinal(DashoA13*..)

2.
java.security.InvalidKeyException: Illegal key size or default parameters
    at javax.crypto.Cipher.a(DashoA13*..)
    at javax.crypto.Cipher.a(DashoA13*..)
    at javax.crypto.Cipher.a(DashoA13*..)
    at javax.crypto.Cipher.init(DashoA13*..)
    at javax.crypto.Cipher.init(DashoA13*..)
Exception in thread "main" java.security.InvalidKeyException: Illegal key size or default parameters
    at javax.crypto.Cipher.a(DashoA13*..)
    at javax.crypto.Cipher.a(DashoA13*..)
    at javax.crypto.Cipher.a(DashoA13*..)
    at javax.crypto.Cipher.init(DashoA13*..)
    at javax.crypto.Cipher.init(DashoA13*..)

3.
java.security.NoSuchAlgorithmException: Cannot find any provider supporting MyAlgo
    at javax.crypto.Cipher.getInstance(DashoA13*..)
Exception in thread "main" java.security.NoSuchAlgorithmException: Cannot find any provider supporting MyAlgo
    at javax.crypto.Cipher.getInstance(DashoA13*..)

4.
java.security.InvalidKeyException: Invalid key length: 16 bytes
    at com.sun.crypto.provider.DESCipher.engineGetKeySize(DashoA13*..)
    at javax.crypto.Cipher.b(DashoA13*..)
    at javax.crypto.Cipher.a(DashoA13*..)
    at javax.crypto.Cipher.a(DashoA13*..)
    at javax.crypto.Cipher.a(DashoA13*..)
    at javax.crypto.Cipher.init(DashoA13*..)
    at javax.crypto.Cipher.init(DashoA13*..)
Exception in thread "main" java.security.InvalidKeyException: Invalid key length: 16 bytes
    at com.sun.crypto.provider.DESCipher.engineGetKeySize(DashoA13*..)
    at javax.crypto.Cipher.b(DashoA13*..)
    at javax.crypto.Cipher.a(DashoA13*..)
    at javax.crypto.Cipher.a(DashoA13*..)
    at javax.crypto.Cipher.a(DashoA13*..)
    at javax.crypto.Cipher.init(DashoA13*..)
    at javax.crypto.Cipher.init(DashoA13*..)

5.
java.security.NoSuchAlgorithmException: Cannot find any provider supporting DSA
    at javax.crypto.Cipher.getInstance(DashoA13*..)
Exception in thread "main" java.security.NoSuchAlgorithmException: Cannot find any provider supporting DSA
    at javax.crypto.Cipher.getInstance(DashoA13*..)

6.
javax.crypto.BadPaddingException: Given final block not properly padded
    at com.sun.crypto.provider.SunJCE_f.b(DashoA13*..)
    at com.sun.crypto.provider.SunJCE_f.b(DashoA13*..)
    at com.sun.crypto.provider.BlowfishCipher.engineDoFinal(DashoA13*..)
    at javax.crypto.Cipher.doFinal(DashoA13*..)
Exception in thread "main" javax.crypto.BadPaddingException: Given final block not properly padded
    at com.sun.crypto.provider.SunJCE_f.b(DashoA13*..)
    at com.sun.crypto.provider.SunJCE_f.b(DashoA13*..)
    at com.sun.crypto.provider.BlowfishCipher.engineDoFinal(DashoA13*..)
    at javax.crypto.Cipher.doFinal(DashoA13*..)

7.
java.lang.IllegalStateException: Cipher not initialized
    at javax.crypto.Cipher.c(DashoA13*..)
    at javax.crypto.Cipher.doFinal(DashoA13*..)
Exception in thread "main" java.lang.IllegalStateException: Cipher not initialized
    at javax.crypto.Cipher.c(DashoA13*..)
    at javax.crypto.Cipher.doFinal(DashoA13*..)

8.
Exception in thread "main" java.security.InvalidKeyException: Illegal key size or default parameters
    at javax.crypto.Cipher.a(DashoA13*..)
    at javax.crypto.Cipher.a(DashoA13*..)
    at javax.crypto.Cipher.a(DashoA13*..)
    at javax.crypto.Cipher.init(DashoA13*..)
    at javax.crypto.Cipher.init(DashoA13*..)


9. Warnings

$javac SimpleCryptography.java

SimpleCryptography.java:6: warning: sun.misc.BASE64Decoder is Sun proprietary API and may be removed in a future release
import sun.misc.BASE64Decoder;
               ^
SimpleCryptography.java:7: warning: sun.misc.BASE64Encoder is Sun proprietary API and may be removed in a future release
import sun.misc.BASE64Encoder;
               ^
SimpleCryptography.java:24: warning: sun.misc.BASE64Encoder is Sun proprietary API and may be removed in a future release
        BASE64Encoder bASE64Encoder = new BASE64Encoder();
        ^
SimpleCryptography.java:24: warning: sun.misc.BASE64Encoder is Sun proprietary API and may be removed in a future release
        BASE64Encoder bASE64Encoder = new BASE64Encoder();
                                          ^
SimpleCryptography.java:35: warning: sun.misc.BASE64Decoder is Sun proprietary API and may be removed in a future release
                BASE64Decoder bASE64Decoder = new BASE64Decoder();
                ^
SimpleCryptography.java:35: warning: sun.misc.BASE64Decoder is Sun proprietary API and may be removed in a future release
                BASE64Decoder bASE64Decoder = new BASE64Decoder();
                                                  ^
6 warnings

A pseudo attribute name is expected.

If you get below mentioned error

[Fatal Error] :1:54: A pseudo attribute name is expected.
Exception in thread "main" org.xml.sax.SAXParseException: A pseudo attribute name is expected.
    at org.apache.xerces.parsers.DOMParser.parse(Unknown Source)
    at org.apache.xerces.jaxp.DocumentBuilderImpl.parse(Unknown Source)
    at javax.xml.parsers.DocumentBuilder.parse(DocumentBuilder.java:124)

Possible reason could be the version and encoding information provided is not formatted correctly.
Please used the below syntax and try again.

?>

Note the ? at the end, I have missed that and it gave above error.