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2) Flow Control and Exception Handling

Objective 3)

Write code that makes proper use of exceptions and exception handling clauses (try catch finally) and declares methods and overriding methods that throw exceptions.

An exception condition is a when a program gets into a state that is not quite normal. Exceptions trapping is sometimes referred to as error trapping. A typical example of an exception is when a program attempts to open a file that does not exsist or you try to refer to an element of an array that does not exist.

The try and catch statements are part of the exception handling built into Java. Neither C/C++ nor Visual Basic have direct equivalents to Javas built in exceptions. C++ does support exceptions but they are optional, and Visual Basic supports On Error/Goto error trapping, which smacks somewhat of a throwback to an earlier less flexible era of BASIC programming.

Java exceptions are a built in part of the language. For example if you are performing I/O you must put in exception handling. You can of course put in null handling that doesn't do anything. The following is a little piece of code I have used with Borland/Inprise JBuilder to temporarily halt output to the console and wait for any key to be pressed.

import java.io.*; 
public class Try{ 
    public static void main(String argv[]){
        Try t = new Try(); 
        t.go(); 
    }//End of main
public void go(){ 
     try{ 
        DataInputStream dis= new DataInputStream(System.in); 
        dis.readLine(); 
      } catch(Exception e){
     /*Not doing anything when exception occurs*/
     } //End of try
    System.out.println("Continuing"); 
   }//End of go 
}

In this case nothing is done when an error occurs, but the programmer must still acknowledge that an error might occur. If you remove the try and catch clause the code will simply not compile. The compiler knows that the I/O methods can cause exceptions and demands exception handling code.

Comparing with Visual Basic and C/C++

This is a little more rigorous than Visual Basic or C/C++ which allows you to throw together "quick and dirty" programs that pretend they are in a world where errors do not occur. Remember that the original version of DOS was called QDOS for Quick and Dirty DOS by it's creator and look how long we have lived with the legacy of that bit of hackery. By the time you have gone to the trouble of putting in a try/catch block and blank braces you may as well put in some real error tracking. It's not exactly bondage and discipline programming, it just persuasively encourages you to "do the right thing". 

The finally clause

The one oddity that you are likely to be questioned on in the exam, is under what circumstances the finally clause of a try/catch block is executed. The short answer to this is that the finally clause is almost always executed, even when you might think it would not be. Having said that, the path of execution of the try/catch/finally statements is something you really need to play with to convince yourself of what happens under what circumstances. 

The finally clause of a try catch block will always execute, even if there are any return statements in the try catch part

One of the few occasions when the finally clause will not be executed is if there is a call to 

System.exit(0);

The exam tends not to attempt to catch you out with this exception to the rule.

The exam is more likely to give examples that include return statements in order to mislead you into thinking that the code will return without running the finally statement. Do not be mislead, the finally clause will almost always run. 

The try/catch clause must trap errors in the order their natural order of hierarchy. Thus you cannot attempt to trap the generic catch all Exception before you have put in a trap for the more specific IOException.

The following code will not compile

try{
    DataInputStream dis = new DataInputStream(System.in); 
    dis.read(); 
    }catch (Exception ioe) {} 
    catch (IOException e) {//Compile time error cause} 
    finally{}

This code will give an error message at compile time that the more specific IOException will never be reached. 

Overriding methods that throw exceptions

An overriding method in a subclass may only throw exceptions declared in the parent class or children of the exceptions declared in the parent class. This is only true for overriding methods not overloading methods. Thus if a method has exactly the same name and arguments it can only throw exceptions declared in the parent class, or exceptions that are children of exceptions in the parent declaration. It can however throw fewer or no exceptions. Thus the following example will not compile

import java.io.*;
class Base{
public static void amethod()throws FileNotFoundException{}
}
public class ExcepDemo extends Base{
  //Will not compile, exception not in base version of method
  public static void amethod()throws IOException{}
}

If it were the method in the parent class that was throwing IOException and the method in the child class that was throwing FileNotFoundException this code would compile. Again, remember that this only applies to overridden methods, there are no similar rules to overloaded methods. Also an overriden method in a sub class may throw Exceptions.

Question 1)

What will happen when you attempt to compile and run the following code?

import java.io.*;
class Base{
public static void amethod()throws FileNotFoundException{}
}

public class ExcepDemo extends Base{
public static void main(String argv[]){
       ExcepDemo e = new ExcepDemo();
}
public static void amethod(){}
protected ExcepDemo(){
	 try{
	  DataInputStream din = new DataInputStream(System.in);
	  System.out.println("Pausing");
	  din.readChar();
	  System.out.println("Continuing");
	  this.amethod();
	  }catch(IOException ioe) {}
	}
}

1) Compile time error caused by protected constructor
2) Compile time error caused by amethod not declaring Exception
3) Runtime error caused by amethod not declaring Exception
4) Compile and run with output of "Pausing" and "Continuing" after a key is hit

Question 2)

What will happen when you attempt to compile and run the following code?

import java.io.*;
class Base{
public static void amethod()throws FileNotFoundException{}
}
public class ExcepDemo extends Base{
public static void main(String argv[]){
       ExcepDemo e = new ExcepDemo();
}

public static void amethod(int i)throws IOException{}
private ExcepDemo(){
 try{
    DataInputStream din = new DataInputStream(System.in);
    System.out.println("Pausing");
    din.readChar();
    System.out.println("Continuing");
    this.amethod();
    }catch(IOException ioe) {}
  }
}

1) Compile error caused by private constructor
2) Compile error caused by amethod declaring Exception not in base version
3) Runtime error caused by amethod declaring Exception not in base version
4) Compile and run with output of "Pausing" and "Continuing" after a key is hit

Question 3)

What will happen when you attempt to compile and run this code?

import java.io.*;
class Base{
public static void amethod()throws FileNotFoundException{}
}
public class ExcepDemo extends Base{
public static void main(String argv[]){
       ExcepDemo e = new ExcepDemo();
}
public static void amethod(int i)throws IOException{}
private boolean ExcepDemo(){
	 try{
	    DataInputStream din = new DataInputStream(System.in);
	    System.out.println("Pausing");
	    din.readChar();
	    System.out.println("Continuing");
	    this.amethod();
	    return true;
	    }catch(IOException ioe) {}
	    finally{
	    System.out.println("finally");
	    }
	   return false;
	}
}

1) Compilation and run with no output.
2) Compilation and run with output of "Pausing", "Continuing" and "finally"
3) Runtime error caused by amethod declaring Exception not in base version
4) Compile and run with output of "Pausing" and "Continuing" after a key is hit

Question 4)

What will happen when you attempt to compile and run the following code?

import java.io.*;
class Base{
public static void amethod()throws FileNotFoundException{}
}
public class ExcepDemo extends Base{
public static void main(String argv[]){
       ExcepDemo e = new ExcepDemo();
}
public boolean amethod(int i){
	 try{
	    DataInputStream din = new DataInputStream(System.in);
	    System.out.println("Pausing");
	    din.readChar();
	    System.out.println("Continuing");
	    this.amethod();
	    return true;
	    }catch(IOException ioe) {}
	    finally{
	   System.out.println("Doing finally");
	   }
	    return false;
	  }
		ExcepDemo(){
      		amethod(99);
	  }
}

1) Compile time error amethod does not throw FileNotFoundException
2) Compile, run and output of Pausing and Continuing
3) Compile, run and output of Pausing, Continuing, Doing Finally
4) Compile time error finally clause never reached

Answers

Answer to Question 1)

4) Compile and run with output of "Pausing" and "Continuing" after a key is hit

An overriden method in a sub class must not throw Exceptions not thrown in the base class. In the case of the method amethod it throws no exceptions and will thus compile without complaint. There is no reason that a constructor cannot be protected.

Answer to Question 2)

4) Compile and run with output of "Pausing" and "Continuing" after a key is hit
In this version amethod has been overloaded so there are no restrictions on what Exceptions may or may not be thrown.

Answer to Question 3)

1) Compilation and run with no output.

OK, I have wandered off topic here a little. Note that the constructor now has a return value. This turns it into an ordinary method and thus it does not get called when an instance of the class is created.

Answer to Question 4)

3) Compile, run and output of Pausing, Continuing, Doing Finally

The finally clause will always be run.

Last updated
28 Dec 1999
copyright © Marcus Green 1999
most recent version at www.jchq.net