Android tricks: the ideal way to write try/catch clauses

Exceptions and exception handling are an important part in most applications. And modern programming languages support exceptions very well. Most of them provide try/catch/finally clauses for developers to handle exceptions. Here we talk about how to write try/catch/finally clauses in a more elegant way.

Here, we take I/O operations as our illustrated example, because I/O conerns three important steps when using: initialization, exception handing and cleanup.

Generally, some programmer might write like this:

// Declaration of objects
InputStream input = null;
OutputStream output = null;
try {
     // Initialization input and output
     input = new FileInputStream("readme.txt");
     output = new FileOutputStream("ic_card.txt");
     
     // Operations
     int seg = 0;
     byte[] buf = new byte[4096];
     while ((seg = input.read(buf)) != -1) {
          output.write(buf, 0, seg);
     }
} catch (FileNotFoundException e) {
     // handling exception
} catch (IOException e) {
     // handling exception
} finally {
     // cleanup
     try {
          if (input != null) {
             input.close();
          }
          if (output != null) {
             output.close();
          }
      } catch (IOException e) { 
          // handling exception
      }
}

Such style has the following problems:

1. There are extra try/catch inside outer try/catch to catch the same exception, which is might be redundant
2. The variable input/output are declared beyond where they are needed, which have unnecessary life circle
3. Two try/catch and try/catch in finally, which might be not elegant enough

The suggested way is like this:

try {
     // Declarations
     InputStream input = null;
     OutputStream output = null;
     try {
           // Initialization input and output
           input = new FileInputStream("readme.txt");
           output = new FileOutputStream("ic_card.txt");
           // Operations
           int seg = 0;
           byte[] buf = new byte[4096];
           while ((seg = input.read(buf)) != -1) {
               output.write(buf, 0, seg);
           }
      } finally {
           // cleanup codes
           if (input != null) {
               input.close();
           }
           if (output != null) {
               output.close();
           }
      }
} catch (FileNotFoundException e) {
     // handling exception
} catch (IOException e) {
     // handling exception
}

This try/finally try/catch style is superior to previous version is because, it finishes all the tasks: declaration, initialization, operations and cleanup. All blocks are inside big try/catch so that all exceptions are get caught; Cleanup is inside try/finally so that we ensure to release taken resources. It also looks elegant.

Such try/finally try/catch clauses, however, have a fatal defect that exceptions could be ignored and lost. In try/finally, finally block gets executed even if when exceptions are thrown in try block. If finally block also throws exceptions, ones thrown in try blocks are ignored and lost. Outer catch clauses will never catch them. In all, this style might not catch all exceptions thrown in try blocks.

You can apply this try/finally try/catch style when:

1. cleanup codes are not much and quite simple, like closing IO streams, shutdown connection.etc.
2. cleanup codes are not likely to throw exceptions, or much less than initializations and operations.
3. you do not care what exceptions are, but whether there are exceptions thrown

Besides these, you should use try/catch/finally, when specific exceptions are important in particular.

// Delcarations
try {
     // Initializations
     
     // Operations
} catch {
     // handling exceptions
} finally {
     // cleanup
}