Why do I get Exception must be caught or declared to be thrown when I try to compile my Java code

Question

Consider   import java awt     import javax swing    import java awt event    import javax crypto    import javax crypto spec    import java security    import java io      public class EncryptURL extends JApplet implements ActionListener        Container content      JTextField userName   new JTextField        JTextField firstName   new JTextField        JTextField lastName   new JTextField        JTextField email   new JTextField        JTextField phone   new JTextField        JTextField heartbeatID   new JTextField        JTextField regionCode   new JTextField        JTextField retRegionCode   new JTextField        JTextField encryptedTextField   new JTextField         JPanel finishPanel   new JPanel          public void init                setTitle  Book - E Project            setSize 800  600           content   getContentPane            content setBackground Color yellow           content setLayout new BoxLayout content  BoxLayout Y AXIS             JButton submit   new JButton  Submit             content add new JLabel  User Name             content add userName            content add new JLabel  First Name             content add firstName            content add new JLabel  Last Name             content add lastName            content add new JLabel  Email             content add email            content add new JLabel  Phone             content add phone            content add new JLabel  HeartBeatID             content add heartbeatID            content add new JLabel  Region Code             content add regionCode            content add new JLabel  RetRegionCode             content add retRegionCode            content add submit            submit addActionListener this               public void actionPerformed ActionEvent e             if  e getActionCommand       Submit                 String subUserName   userName getText                String subFName   firstName getText                String subLName   lastName getText                String subEmail   email getText                String subPhone   phone getText                String subHeartbeatID   heartbeatID getText                String subRegionCode   regionCode getText                String subRetRegionCode   retRegionCode getText                 String concatURL                    user     subUserName     amp f     subFName                     amp l     subLName     amp em     subEmail                     amp p     subPhone     amp h     subHeartbeatID                     amp re     subRegionCode     amp ret     subRetRegionCode               concatURL   padString concatURL       16               byte   encrypted   encrypt concatURL               String encryptedString   bytesToHex encrypted               content removeAll                content add new JLabel  Concatenated User Input -- gt     concatURL                 content add encryptedTextField               setContentPane content                        public static byte   encrypt String toEncrypt  throws Exception          try              String plaintext   toEncrypt              String key    01234567890abcde               String iv    fedcba9876543210                SecretKeySpec keyspec   new SecretKeySpec key getBytes     AES                IvParameterSpec ivspec   new IvParameterSpec iv getBytes                  Cipher cipher   Cipher getInstance  AES CBC NoPadding                cipher init Cipher ENCRYPT MODE  keyspec  ivspec               byte   encrypted   cipher doFinal toEncrypt getBytes                  return encrypted                    catch Exception e                         public static byte   decrypt byte   toDecrypt  throws Exception          String key    01234567890abcde           String iv    fedcba9876543210            SecretKeySpec keyspec   new SecretKeySpec key getBytes     AES            IvParameterSpec ivspec   new IvParameterSpec iv getBytes              Cipher cipher   Cipher getInstance  AES CBC NoPadding            cipher init Cipher DECRYPT MODE  keyspec  ivspec           byte   decrypted   cipher doFinal toDecrypt            return decrypted              public static String bytesToHex byte   data            if  data    null                        return null                    else                       int len   data length              String str                   for  int i 0  i lt len  i                                  if   data i  amp 0xFF   lt  16                      str   str    0    java lang Integer toHexString data i  amp 0xFF                   else                     str   str   java lang Integer toHexString data i  amp 0xFF                             return str                        public static String padString String source  char paddingChar  int size                int padLength   size-source length     size          for  int i   0  i  lt  padLength  i                  source    paddingChar                    return source            I m getting an unreported exception   java lang Exception  must be caught or declared to be thrown byte   encrypted   encrypt concatURL     As well as    java 109  missing return statement   How do I solve these problems

User · Answer

You ll need to decide how you d like to handle exceptions thrown by the encrypt method   Currently  encrypt is declared with throws Exception - however  in the body of the method  exceptions are caught in a try catch block  I recommend you either    remove the throws Exception clause from encrypt and handle exceptions internally  consider writing a log message at the very least   or  remove the try catch block from the body of encrypt  and surround the call to encrypt with a try catch instead  i e  in actionPerformed     Regarding the compilation error you refer to  if an exception was thrown in the try block of encrypt  nothing gets returned after the catch block finishes  You could address this by initially declaring the return value as null   public static byte   encrypt String toEncrypt  throws Exception    byte   encrypted   null    try                  encrypted             catch Exception e                    return encrypted      However  if you can correct the bigger issue  the exception-handling strategy   this problem will take care of itself - particularly if you choose the second option I ve suggested

User · Answer

In your  encrypt  method  you should either get rid of the try catch and instead add a try catch around where you call encrypt  inside  actionPerformed   or return null inside the catch within encrypt  that s the second error

User · Answer

The first error     java lang Exception  must be caught or declared to be thrown byte   encrypted   encrypt concatURL     means that your encrypt method throws an exception that is not being handled or declared by the actionPerformed method where you are calling it   Read all about it at the Java Exceptions Tutorial   You have a couple of choices that you can pick from to get the code to compile    You can remove throws Exception from your encrypt method and actually handle the exception inside encrypt  You can remove the try catch block from encrypt and add throws Exception and the exception handling block to your actionPerformed method    It s generally better to handle an exception at the lowest level that you can  instead of passing it up to a higher level   The second error just means that you need to add a return statement to whichever method contains line 109  also encrypt  in this case    There is a return statement in the method  but if an exception is thrown it might not be reached  so you either need to return in the catch block  or remove the try catch from encrypt  as I mentioned before

User · Answer

The problem is in this method     public static byte   encrypt String toEncrypt  throws Exception    This is the method signature which pretty much says    what the method name is  encrypt what parameter it receives  a String named toEncrypt its access modifier  public static and if it may or not throw an exception when invoked    In this case the method signature says that when invoked this method  could  potentially throw an exception of type  Exception                 concatURL   padString concatURL       16       byte   encrypted   encrypt concatURL    lt -- HERE          String encryptedString   bytesToHex encrypted       content removeAll                 So the compilers is saying  Either you surround that with a try catch construct or you declare the method   where is being used   to throw  Exception  it self   The real problem is the  encrypt  method definition  No method should ever return  Exception   because it is too generic and may hide some other kinds of exception better is to have an specific exception   Try this   public static byte   encrypt String toEncrypt        try        String plaintext   toEncrypt        String key    01234567890abcde         String iv    fedcba9876543210          SecretKeySpec keyspec   new SecretKeySpec key getBytes     AES          IvParameterSpec ivspec   new IvParameterSpec iv getBytes            Cipher cipher   Cipher getInstance  AES CBC NoPadding          cipher init Cipher ENCRYPT MODE keyspec ivspec         byte   encrypted   cipher doFinal toEncrypt getBytes            return encrypted        catch   NoSuchAlgorithmException nsae                 What can you do if the algorithm doesn t exists              this usually won t happen because you would test             your code before shipping              So in this case is ok to transform to another kind          throw new IllegalStateException  nsae          catch   NoSuchPaddingException nspe                What can you do when there is no such padding   whatever that means                I guess not much  in either case you won t be able to encrypt the given string         throw new IllegalStateException  nsae                 line 109 won t say it needs a return anymore        Basically in this particular case you should make sure the cryptography package is available in the system    Java needs an extension for the cryptography package  so  the exceptions are declared as  checked  exceptions  For you to handle when they are not present    In this small program you cannot do anything if the cryptography package is not available  so you check that at  development  time  If those exceptions are thrown when your program is running is because you did something wrong in  development  thus a RuntimeException subclass is more appropriate    The last line don t need a return statement anymore  in the first version you were catching the exception and doing nothing with it  that s wrong   try           risky code        catch  Exception e             a bomb has just exploited        you should NOT ignore it         The code continues here  but what should it do      If the code is to fail  it is better to Fail fast  Here are some related answers    Catching Exceptions in Java When to choose checked and unchecked exceptions  Why don   t you have to explicitly declare that you might throw some built in exceptions in Java  Exception other than RuntimeException

User · Answer

In actionPerformed ActionEvent e  you call encrypt    which is declared to throw Exception   However  actionPerformed neither catches this Exception  with try catch around the call to encrypt    nor declares that it throws Exception itself   Your encrypt method  however  does not truly throw Exception   It swallows all Exceptions without even as much as logging a complaint    Bad practice and bad style    Also  your encrypt method does the following   public static byte   encrypt String toEncrypt  throws Exception     try               return encrypted     HERE YOU CORRECTLY RETURN A VALUE     catch Exception e             YOU DO NOT RETURN ANYTHING HERE     That is  if you do catch any Exception  you discard it silently and then fall off the bottom of your encrypt method without actually returning anything   This won t compile  as you see   because a method that is declared to return a value must either return a value or throw an Exception for every single possible code path

User · Answer

All your problems derive from this  byte   encrypted   cipher doFinal toEncrypt getBytes     return encrypted    Which are enclosed in a try  catch block  the problem is that in case the program found an exception you are not returning anything  Put it like this  modify it as your program logic stands    public static byte   encrypt String toEncrypt  throws Exception      try          String plaintext   toEncrypt          String key    01234567890abcde           String iv    fedcba9876543210            SecretKeySpec keyspec   new SecretKeySpec key getBytes     AES            IvParameterSpec ivspec   new IvParameterSpec iv getBytes              Cipher cipher   Cipher getInstance  AES CBC NoPadding            cipher init Cipher ENCRYPT MODE keyspec ivspec           byte   encrypted   cipher doFinal toEncrypt getBytes              return encrypted        catch Exception e           return null                Always must return something           For the second one you must catch the Exception from the encrypt method call  like this  also modify it as your program logic stands    public void actionPerformed ActionEvent e                  try           byte   encrypted   encrypt concatURL           String encryptedString   bytesToHex encrypted           content removeAll            content add new JLabel  Concatenated User Input -- gt     concatURL             content add encryptedTextField       setContentPane content         catch  Exception exc               TODO  handle exception           The lessons you must learn from this    A method with a return-type must always return an object of that type  I mean in all possible scenarios All checked exceptions must always be handled

[java] Why do I get "Exception; must be caught or declared to be thrown" when I try to compile my Java code?

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