Error message unreported exception java io IOException must be caught or declared to be thrown

Question

Error      filecontent java 15  unreported exception java io IOException  must be   caught or        declared to be thrown      showfile                                    filecontent java 78  unreported exception java io IOException  must be caught or  declared to be   thrown      showfile                                      I have already thrown java io IOException  but still it shows these errors   My code   import java awt    import java awt event    import java io     class filecontent extends Frame implements ActionListener       TextField t     new TextField 4       TextArea ta     new TextArea 4       Button submit      Panel p1      filecontent                 setGUI            setRegister            showfile            setTitle  FileData            setVisible true           setSize 300  300               addWindowListener new WindowAdapter                                  public void windowClosing WindowEvent we                                         System exit 0                                                              void setGUI                 p1   new Panel            p1 setLayout new GridLayout 5  4  10  10            for int i 0  i lt 4  i                          t i    new TextField 10               ta i    new TextArea                  p1 add t i                p1 add ta i                      submit   new Button  Submit            p1 add submit              void setRegister                 submit addActionListener this              void showfile   throws java io IOException               FileReader fin     new FileReader 4           FileReader fn               br   new BufferedReader 4            for int i 0 i lt 4 i                          fin i  new FileReader t i  getText                       int cnt   1          String s          fn   fin 0           BufferedReader br   new BufferedReader fn           while cnt  lt   4                        if  s br readLine       null                                ta cnt-1  append s                                else                               cnt                    fn   fin cnt-1                   ta cnt-1  setText                                         public void actionPerformed ActionEvent ae                if ae getSource    submit                        showfile                         public static void main String ar                  new filecontent

User · Answer

When the callee throws an exception i e  void showfile   throws java io IOException the caller should handle it or throw it again   And also learn naming conventions  A class name should start with a capital letter

User · Answer

The error message means that any method that calls showfile   must either declare that it  in turn  throws IOException  or the call must be inside a try block that catches IOException  When you call showfile    you do neither of these  for example  your filecontent constructor neither declares IOException nor contains a try block   The intent is that some method  somewhere  should contain a try block  and catch and handle this exception  The compiler is trying to force you to handle the exception somewhere   By the way  this code is  sorry to be so blunt  horrible  You don t close any of the files you open  the BufferedReader always points to the first file  even though you seem to be trying to make it point to another  the loops contain off-by-one errors that will cause various exceptions  etc  When you do get this to compile  it will not work as you expect  I think you need to slow down a little

User · Answer

Exceptions bubble up the stack  If a caller calls a method that throws a checked exception  like IOException  it must also either catch the exception  or itself throw it   In the case of the first block   filecontent         setGUI        setRegister        showfile        setTitle  FileData        setVisible true       setSize 300  300                   addWindowListener new WindowAdapter                         public void windowClosing WindowEvent we                                System exit 0                                        You would have to include a try catch block   filecontent         setGUI        setRegister        try           showfile              catch  IOException e               Do something here           setTitle  FileData        setVisible true       setSize 300  300                   addWindowListener new WindowAdapter                         public void windowClosing WindowEvent we                                System exit 0                                        In the case of the second   public void actionPerformed ActionEvent ae        if  ae getSource      submit                showfile              You cannot throw IOException from this method as its signature is determined by the interface  so you must catch the exception within   public void actionPerformed ActionEvent ae        if ae getSource    submit                try               showfile                      catch  IOException e                   Do something here                     Remember  the showFile   method is throwing the exception  that s what the  throws  keyword indicates that the method may throw that exception  If the showFile   method is throwing  then whatever code calls that method must catch  or themselves throw the exception explicitly by including the same throws IOException addition to the method signature  if it s permitted   If the method is overriding a method signature defined in an interface or superclass that does not also declare that the method may throw that exception  you cannot declare it to throw an exception

User · Answer

Your method showFile   declares that it can throw an IOException  Since this is a checked exception  any call to showFile   method must handle the exception somehow  One option is to wrap the call to showFile   in a try-catch block    try        showFile        catch IOException e           Code to handle an IOException here

User · Answer

void showfile   throws java io IOException   lt -----   Your showfile   method throws IOException  so whenever you use it you have to either catch that exception or again thorw it  Something like   try     showfile      catch IOException e      e printStackTrace          You should learn about exceptions in Java

[java] Error message "unreported exception java.io.IOException; must be caught or declared to be thrown"

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