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 + "&f=" + subFName +                 "&l=" + subLName + "&em=" + subEmail +                 "&p=" + subPhone + "&h=" + subHeartbeatID +                 "&re=" + subRegionCode + "&ret=" + subRetRegionCode;              concatURL = padString(concatURL, ' ', 16);             byte[] encrypted = encrypt(concatURL);             String encryptedString = bytesToHex(encrypted);             content.removeAll();             content.add(new JLabel("Concatenated User Input -->" + 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<len; i++)             {                 if ((data[i]&0xFF) < 16)                     str = str + "0" + java.lang.Integer.toHexString(data[i]&0xFF);                 else                     str = str + java.lang.Integer.toHexString(data[i]&0xFF);             }             return str;         }     }       public static String padString(String source, char paddingChar, int size)     {         int padLength = size-source.length() % size;         for (int i = 0; i < 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?

Answer 1

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 -->" + 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