Exception Handling

In Java, exceptions represent an interruption in the normal execution of the algorithm. They are used to manage the issues that appear during the execution and to take action in order to recover and resume the program when possible. Exceptions, like everything else in Java, are objects. The base class for any type of exception is java.lang.Throwable.

From class Throwable, two other classes are derived:

• java.lang.Error - a type of exception describing a serious problem in program execution because of which the application can no longer continue; these types of exceptions usually can't be managed or recovered.
• java.lang.Exception - a class of exceptions that the application can manage and after which execution can resume correctly.

Generating (Throwing) Exceptions

An exception is generated by using keyword throw:

public void setAge(int _age) {
    if(_age < 0 || _age > 150){
        Error _e = new Error("Invalid age " + _age);
        throw _e;
    }

    age = _age;
}

An exception once thrown, the normal execution of the program is interrupted and it is resumed in a catch block associated to that type of exception (see Catching Exceptions). For example, in the case above, after the exception has been thrown, the assignment age = _age; is not executed anymore.

Handling Exceptions

In certain situations, exceptions interrupt the program for good. These exceptions are not managed (those extended from class Error). There are however, exceptions that can be managed, those extended from class Exception. There are two types of these:

• checked at compile-time (those extended directly from Exception, but NOT java.lang.RuntimeException), called checked exceptions;
• checked at runtime (those extended from java.lang.RuntimeException), called unchecked exceptions.

Rule: Checked exceptions which are not caught (managed) in the current method, must appear in the list of thrown exceptions of the method. Unchecked exceptions that are not caught (managed) in the current method don't need to appear in the list of thrown exceptions of the method. Error type exceptions are unchecked.

The list of checked exceptions generated by a method is placed after the list of arguments, but before the body of the method, using the word throws

public void setAge(int _age) throws Exception{
    if(_age < 0 || _age > 150){
        Exception _e = new Exception("Invalid age " + _age);
        throw _e;
    }

    age = _age;
}

Since class Exception is checked, it needs to appear in the list of exceptions generated by the method. However, RuntimeException can be missing:

public void setAge(int _age){
    if(_age < 0 || _age > 150){
        RuntimeException _e = new RuntimeException("Invalid age " + _age);
        throw _e;
    }

    age = _age;
}

Stack Propagation

The execution stack of a program represents the sequence of methods that called one another.

public class Person{

    private String fullName;
    private int age;
    private boolean gender;

public Person(String _fullName, int _age, boolean _gender){
    setAge(_age);
} 

public static void main(String[] _args){
    Person _p = new Person("Ghita Vasile", 20, true);
}

public void setAge(int _age){
     /* in this point, the execution stack is as follows:
      * 
      *  /---------------------------------\    ^
      *  |        void setAge(int)         |    |
      *  +---------------------------------+    |
      *  |  Person(String, int, boolean)   |    |
      *  +---------------------------------+    |
      *  |    static void main(String[])   |    |
      *  \---------------------------------/    |
      *
      *  where method main(String[]) is at the 
      *  bottom of the stack (first one called)
      *  and method setAge(int) is on the top
      *  of the stack (last one called), and it's
      *  the method that's currently in execution.
      */
     age = _age;
}

}

In the situation when the caller method does not catch the exception either, then it's considered that it throws the exception itself, in a sort of recursive manner:

public class Person{

    private String fullName;
    private int age;
    private boolean gender;

public Person(String _fullName, int _age, boolean _gender) throws Exception{
    //...
    /* setAge() throws a checked exception of type Exception
     * which isn't caught in this method, so this method will
     * throw the exception further up the stack, thus the 'throws Exception'
     * in the method declaration
     */  
    setAge(_age);
    //..
    System.out.println("If setAge() throws an exception, this line will not be executed!");
} 

public void setAge(int _age) throws Exception{
    if(_age < 0 || _age > 150){
        Exception _e = new Exception("Invalid age " + _age);
        throw _e;
    }

    age = _age;
}

}

Catching Exceptions

Catching an exception is done in a try-catch block. Instructions that can throw exceptions (calls to methods that throw exceptions or throw instructions) are placed in a try block, after which one or more catch blocks are placed, where the exception is effectively managed:

public class Person{

    private String fullName;
    private int age;
    private boolean gender;

public Person(String _fullName, int _age, boolean _gender) throws Exception{
    //...
    /* setAge() throws a checked exception of type Exception
     * which isn't caught in this method, so this method will
     * throw the exception further up the stack, thus the 'throws Exception'
     * in the method declaration
     */  
    setAge(_age);
    //..
    System.out.println("If setAge() throws an exception, this line will not be executed!");
} 

public static void main(String[] _args){
    try{
        Person _p = new Person("Ghita Vasile", -30, true);
    }catch(Exception _e){
        /* if an exception is thrown by the constructor, the execution
         * is continued with this catch block. The reference _e is to 
         * the object that has been thrown. In this particular case, 
         * the instantiation of the object is not complete, therefor
         * _p is still null.
         */
         System.out.println("Can't create Person object: " + _e.getMessage());
    }

    /* after the catch block completes, or in case no exception is thrown,
     * the execution continues from here.
     */
    System.out.println("Program done!");
}

public void setAge(int _age) throws Exception{
    if(_age < 0 || _age > 150){
        Exception _e = new Exception("Invalid age " + _age);
        throw _e;
    }

    age = _age;
}

}

A method can throw multiple exceptions. These can be, together or separately, caught or throws further up the stack. Therefore, the principle of polymorphism applies here as well:

import java.io.*;

public class Person{

public static void main(String[] _args){
    try{
        Person _p = new Person();
        int _age = _p.readAgeFromFile("ageFile.bin");
    }catch(IOException _e){
         System.out.println("Unable to open file for reading: " + _e.getMessage());
    }catch(Exception _e){
         System.out.println("The age is invalid: " + _e.getMessage());
    }

    System.out.println("Program done!");
}

public int readAgeFromFile(String _fileName) throws IOException, Exception{
    FileInputStream _fileInputStream = new FileInputStream(_fileName);
    int _age = _fileInputStream.read();
    _fileInputStream.close();

    if(_age < 0 || _age > 150){
        throw new Exception("Invalid age read from file: " + _age);
    }

    return _age;
}

}

Since java.io.IOException is extended from java.lang.Exception, then it would have been sufficient for the method readAgeFromFile() to only declare java.lang.Exception as thrown. However, if the catch block that handle the exception of type java.lang.Exception would have been the first after the try block, then this catch block would also catch exceptions of type java.io IOException, so the order of the catch blocks if often very important.

The finally Block and try-with-resources

There are situations where a certain part of the program needs to be executed even if an exception has been thrown or not. In this case, a finally block is used, after the last catch block. Usually, the finally block is used to free any allocated resources:

public void writeAgeToFile(int _age, String _filename) throws IOException{
    FileOutputStream _file = null;

    try{
        _file = new FileOutputStream(_filename);
        _file.write(100 / _age);        
    }catch(ArithmeticException e){
        System.err.println("Caught ArithmeticException: " +  e.getMessage());
    }catch(IOException e){
        System.err.println("Caught IOException: " +  e.getMessage());
    }finally{
        if(_file != null){
             _file.close();
        }
    }
}

Starting with Java 7, a new type of block has been introduced: try-with-resources. This takes as arguments a series of expressions used to initialize objects. The classes of these objects must implement the interface java.lang.AutoClosable. The advantage is that at the end of the try-with-resources block, these resources are freed automatically:

static String readFirstLineFromFile(String _path) throws IOException{
    try(BufferedReader _reader = new BufferedReader(new FileReader(_path))){
        return _reader.readLine();
    }
}

is the same as:

static String readFirstLineFromFileWithFinallyBlock(String _path) throws IOException{
    BufferedReader _reader = new BufferedReader(new FileReader(_path));
    try{
        return _reader.readLine();
    }finally{
        if (_reader != null){
            _reader.close();
        }
    }
}

More about exceptions on the official Oracle website.