I'm very new to java and the idea of try catch blocks to handle exceptions.
This roughly what I'm ending up with, and there simply has to be a better way:
try {
JSONObject jsonObject = new JSONObject(jsonString);
int aCount = jsonObject.getInt("acount");
String devTok = jsonObject.getString("dt");
String qURL = jsonObject.getString("qu");
try {
DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
Key qKey = KeyFactory.createKey("qu", qURL);
int dsACount = (Integer) datastore.get(qKey).getProperty(kLastKnownANumber);
//..etc.. more try catch blocks needed
} catch (EntityNotFoundException e) {
e.printStackTrace();
}
} catch (com.google.appengine.repackaged.org.json.JSONException e) {
e.printStackTrace();
}
There are more try catch blocks embedded in the same way, so that at the end, there is just a lump of catch blocks. How else should exceptions be handled, Eclipse keeps asking my to either surround with a try catch block or "Add throws declaration".
Sometimes I want to catch certain exceptions, for example if it can't find an entity, I want to print something like "Entity not found", and if the JSON string can't be parsed into an object, I want to print something like "Can't parse JSON".
(I'm used to objective-c where the are delegate methods for failure, or the method returns null and you have passed a pointer to [a pointer to] an NSError object, which will have been "filled", Is there somewhere to learn about try-catch?)
If all you're doing is catching them and printing the stack trace regardless of the exception type, you can just wrap the code in one large try/catch block. To save many "catches", you can catch java.lang.Throwable which is the interface that all exceptions implement. If not, you can have a catch for every type of checked exception the code you're calling throws, and handle them specifically.
Eclipse keeps asking you to do so because Java code will not compile if the checked exceptions are not caught, or declared to be thrown by the caller.
+Adding this comment to the answer (Thanks, Paul Tomblin):
In production quality apps you'd be logging the trace, adding some logic where you're handling the exception in a right way, taking an alternate flow, and/or re-wrapping it in another exception and throwing it, etc. It all depends on the particular problem you're trying to solve.
The idea of exception handling is that you can handle errors at points in your program flow where you can deal with them meaningfully. Rather than checking every function's return value like in C, where most of the time you can't do anything sensible other than passing the error further up, you install a try/catch block at sensible points in your program:
Basically, whenever there is a point where you can react meaningfully to an error, then catch that error, and pass everything else on. That way, error handling is only invoked when there is a plausible recovery from the error.
For example, worst case if any error stops your program from executing meaningfully, then you might almost not catch anything at all and just let the OS handle the situation (well, perhaps one single try/catch to produce a friendly error message).
Example (in C++, sorry, I'm can't type Java blind):
int main()
{
try {
while (masterloop()) { }
catch (...) {
LOG("Fatal program error, terminating!"); // nothing else we can do!
}
}
/* lots of program logic */
void process_image()
{
try {
Image im = load_image_from_disk();
/* ... */
}
catch (const OutOfMemoryExc & e) {
LOG("Not enough memory to process the image.");
return;
}
catch (const DataErrorExc & e) {
LOG("Could not read the image data.");
return;
}
catch (...) {
throw; // pass everything else along
}
}
In this example, we may try to process an image and fail for some anticipable reasons (out of memory, or failure to read the image). In that case we just return without doing work and let the program continue gracefully. All other errors are propagated up to a higher point. Most importantly, we do not need to litter the actual image processing function with error checks and responses all the time, it suffices for any code there to throw one of our two good exceptions and not worry any further.
Moral: If you have try/catch blocks absolutely everywhere, you're doing it wrong.
I know there's a lot of answers here, and they do a good job of covering how to structure the try/catch blocks. However, I'm thinking one of the things bothering you is the significant... indentation and code growth (... because I know it's not the indentation or amount of code, but the implied complexity by wrapping it and shifting it over and growing longer and longer between the opening try and enclosing catch, and I can't put a word to that apprehension).
The way to get around this is to refactor into functions the distinct bits in the code. I know it's a simplistic answer, but it's a good way to isolate individual tasks and keep the error handling fairly local to the code that requires it without padding things out vertically and horizontally with nested try/catch blocks.
You can make these methods private as they are intended for internal use only, presumably.
private Integer getDatastoreACount() {
try {
DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
Key qKey = KeyFactory.createKey("qu", qURL);
return (Integer) datastore.get(qKey).getProperty(kLastKnownANumber);
//..etc.. more try catch blocks needed
} catch (EntityNotFoundException e) {
// expects an Integer return, so need to deal with this
// but for simplicity I'm just simply recycling 'e'
throw e;
}
}
public void parseJSON(String jsonString) {
try {
JSONObject jsonObject = new JSONObject(jsonString);
int aCount = jsonObject.getInt("acount");
String devTok = jsonObject.getString("dt");
String qURL = jsonObject.getString("qu");
Integer dsACount = getDatastoreACount();
//etc etc
} catch (com.google.appengine.repackaged.org.json.JSONException e) {
e.printStackTrace();
}
}
You can catch multiple exceptions in the same try e.g.
try{
xyz;
}catch(NullPointerException npx){
npx.getMessage();
}catch(ArrayOutOfBoundsException ax){
ax.getMessage();
}
Also, by declaring the Exception as throws in your method signatures you can pass the Exception up the stack.
If you're just doing something like this:
try {
do smth
try {
do smth more
...
} catch (Exception1 e1) {reaction to e1}
} catch (Exception2 e2) {reaction to e2}
You can do everything in one try-block:
try {
do smth
do smth more
...
}
catch (Exception1 e1) {reaction to e1}
catch (Exception2 e2) {reaction to e2}
You can also break this down to one catch block if you're just printing the exception:
try {
do smth
do smth more
...
}
catch (Exception e) {e.printStackTrace();}
But this doesn't if you want to do somthing more, even if e1 is thrown, like:
try {
do smth
try {
do smth more
...
} catch (Exception1 e1) {reaction to e1}
do smth even if e1 was thrown
} catch (Exception2 e2) {reaction to e2}
The last example can't be written shorter.
If you have a block of code in which more than one type of exception may be thrown, you can declare two separate catch blocks:
try {
JSONObject jsonObject = new JSONObject(jsonString);
int aCount = jsonObject.getInt("acount");
String devTok = jsonObject.getString("dt");
String qURL = jsonObject.getString("qu");
DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
Key qKey = KeyFactory.createKey("qu", qURL);
int dsACount = (Integer) datastore.get(qKey).getProperty(kLastKnownANumber);
} catch (EntityNotFoundException e) {
e.printStackTrace();
} catch (com.google.appengine.repackaged.org.json.JSONException e) {
e.printStackTrace();
}
//..etc.. as many catch blocks as needed
Alternatively, if you don't care about the exact type of the exception, you can have onyl one catch block which catches Exception (or maybe Throwable; I can't remember exactly what the superclass of exceptions is in Java).
Another point I will make now is that you might not have the most modular code. Remember, code that does one thing well makes for good, modular code. If you find that you have many nested black (whether try/catch blocks, if/else blocks, etc.) you may want to check if some of the code can be extracted into its own method. This may also make your code look better when many exceptions must be handled.
First, from a design perspective, catching and printing exceptions is not a good thing. Something went wrong, and your code just keeps going in the same way as if it went right. That is not usually correct. So: perhaps your method needs to throw these exceptions instead of catching them. Perhaps only the caller is in a position to decide what happens if something fails like this.
But otherwise, the only advice I can offer to clean up how the code looks, syntactically, is to tell you that you can write:
try {
...
} catch (...) {
...
} catch (...) {
...
}
You can also catch for a broader exception class like Exception and just write one catch block but this is bad design. In Java 7, you will be able to catch for several exception types in one block.
You should use try/catch blocks if you have a way to recover from the exception, for example if you want to check if a string is a valid integer, you can write a method (this is a lame method, but just to show the idea):
public boolean isInteger(String str) {
try {
new Integer(str);
}
catch(NumberFormatException e) {
return false;
}
return true;
}
If you don't have a way to recover from the exception and all you do is to print the stack trace, it is suggested to add throws declaration (as eclipse suggest) to the method, and let the caller handle the exception (or throw it to its caller).
If you want to handle some exceptions and throw some other, you can do it as well.
I like to box up the call behind a static method, just to keep it tidier. For example, here's my reduced Set Json Value call.
private static boolean setJsonValue(JSONObject j,String key,Object value)
{
try
{
if(value instanceof Integer)
{
// numbers are special. We want them unquoted.
int valueI = (Integer)value;
j.put(key,valueI);
}
else
j.put(key,value);
return true;
}
catch (JSONException e)
{
// do nothing, it turns out
return false;
}
}
...and then I ignore the return values, because I am bad.
Somewhere or other I have a similar Get method, that returns null if it fails. You get the idea.
You have two basic code-style choices here (that don't involve changing method signatures)
Method1: Put everything in the one try catch and have multiple catch blocks, like this:
try {
JSONObject jsonObject = new JSONObject(jsonString);
int aCount = jsonObject.getInt("acount");
String devTok = jsonObject.getString("dt");
String qURL = jsonObject.getString("qu");
DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
Key qKey = KeyFactory.createKey("qu", qURL);
int dsACount = (Integer) datastore.get(qKey).getProperty(kLastKnownANumber);
//..etc.. more try catch blocks needed
} catch (EntityNotFoundException e) {
e.printStackTrace();
} catch (com.google.appengine.repackaged.org.json.JSONException e) {
e.printStackTrace();
}
Method 2: Break up your code into sections that each have one catch, like this:
String qURL = null;
try {
JSONObject jsonObject = new JSONObject(jsonString);
int aCount = jsonObject.getInt("acount");
String devTok = jsonObject.getString("dt");
String qURL = jsonObject.getString("qu");
} catch (EntityNotFoundException e) {
e.printStackTrace();
}
try {
DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
Key qKey = KeyFactory.createKey("qu", qURL);
int dsACount = (Integer) datastore.get(qKey).getProperty(kLastKnownANumber);
} catch (EntityNotFoundException e) {
e.printStackTrace();
}
Method 2 is the recommended one, as it makes it obvious which lines are throwing which exceptions and generally segments the code into natural processing blocks.
Create another exception and put it below or above of the other exception. Depends on the context of your application.
Related
My function looks like:
public static double isOverturn(final String reference, final String hypothesis, FieldType fieldType) {
double overturnScore = 1.0;
if (StringUtils.isEmpty(reference) || StringUtils.isEmpty(hypothesis))
return overturnScore;
Method comparisonMethod = null;
try {
comparisonMethod = comparison(fieldType.getName());
overturnScore = (double) comparisonMethod.invoke(null, reference, hypothesis);
} catch (NoSuchMethodException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
}
return overturnScore;
}
I want to have log.info in place of e.printStackTrace. But it is giving me error. How to use log.info in static method?
Just make your log field static. This is industry standard anyway.
That is deplorable error handling. If something goes wrong with your reflective invocation, this method returns 1.0. That seems like very silly behaviour. Generally you should just throw exceptions on if there's nothing you can do to handle it properly (and 'log it and return some throwaway default value' is rarely a valid 'handling it' strategy!), because what you do results in a long chain of exceptions, or code that straight up does the wrong thing, because as they say, garbage in, garbage out, and returning 1.0 in the face of problems with definitions sure sounds like that 1.0 is just garbage, no?
You can just catch Exception if your intent is to completely ignore what it all means, shunt it off to a log, and return a default value.
So I have the following bit of code:
public static Image getImage(String filepath, Class cl) {
try {
return ImageIO.read(cl.getResource(filepath));
}
catch (IOException e) {
e.printStackTrace();
}
return null; // Will never execute
}
It's a basic try-catch block. If I am unable to read the image and return it, I immediately go into my catch block. However, because my return is within the scope of the try block and not the entire function, my compiler issues an error when I try to compile and run because it sees that it's possible that I never hit a return statement. Therefore, I've added the return null; line to suppress this warning, but I'd rather have a neater way of doing this without putting code that will never run. I've tried adding
#SuppressWarnings("all")
To my code, but it still gives me an error. Any ideas? I feel like there should be a way to tell the compiler to ignore errors like this.
Also, if it is of any use, I am using IntelliJ as my IDE.
I would suggest what #LuCio eagerly in the comments tried to say. Just don't catch the Exception and pass it upwards:
public static Image getImage(String filePath, Class<?> clazz) throws IOException {
return ImageIO.read(clazz.getResource(filePath));
}
That way you have created an easy helper method. If you would return null, you'd have to document that in JavaDoc and every caller will have to use a not-null assertion logic to then throw an error if it is null.
A try catch block does the same. So instead of passing null upwards you just propagate the exception upwards. You somewhere said that you want to assign the Image to a static field, so you can do that easily like this:
static {
try {
MY_IMAGE = getImage("somepath", MyClass.class);
} catch(IOException e){
throw new IOError(e); // will kill the Vm with an error
}
}
But maybe somewhere you have another action. Than to just kill the VM. Maybe use a default image:
final Image image;
try {
image = getImage("somepath", MyClass.class);
} catch(IOException e){
e.printStacktrace();
image = new SomeDefaultImage();
}
// do something with image
Which all in all is the way to go. You can't have a helper method to decide what to do when it fails. That should always be done by the calling code.
Ok so, I believe I was confusing the purpose of the catch block. Thank you to #Ben and #Zephyr and everybody else for your help. I will be amending my code to:
public static Image getImage(String filepath, Class cl) {
try {
return ImageIO.read(cl.getResource("hello"));
} catch (IOException e) {
System.out.println(e.getMessage());
e.printStackTrace();
throw new IOError(e);
}
}
Edit: After some more discussions, and looking through other options other people have posted, I have updated my code above, which satisfies the compiler. Note that replacing the line
throw new IOError(e)
with
System.exit(0);
does not fix the error because, as far as I know, the compiler cannot tell at compile time whether the program would end. It would've been helpful to have a way of suppressing the warning, since we know that at runtime the program will always (or practically always) end, but alas #SuppressWarnings is of no use.
I have several functions where I am casting an object to an explicit type.
The problem is, when this object is created, the type may not match the type I am casting to. Somewhere along the line there are some conversions that take place. However, it is possible the code never reaches these conversions. The fact that it doesn't is not an issue.
So when the code reaches the point where the casting will occur, all I need to do is put the part into a try catch block. In fact I don't even need to handle the exception in any special way.
But I have a lot of these functions. I would like to know if there is some way to wrap lines of code with a try catch block without actually writing out the try catch block. If I can call some function some how that will do it automatically.
try
{
// cast something
}
catch( ClassCastException e )
{
}
Instead I want to call a method that will put the try catch block around // cast something. Is there any way to do something like this?
Note: I don't do anything when I catch the ClassCastException. It is okay if I can't cast the object correctly, but I need to catch the exception so the code execution won't jump to some other place.
You can write a method like this to do the casting for you while ignoring any ClassCastException.
public static <I, O> O cast(I input, Class<O> outClass) {
try {
return outClass.cast(input);
} catch (ClassCastException e) {
return null;
}
}
You can use it like this:
Number n = new Integer(1);
Integer i = cast(n, Integer.class);
But you can also improve the code to avoid exceptions:
public static <I, O> O cast(I input, Class<O> outClass) {
if(outClass.isAssignableFrom(input.getClass())) {
return outClass.cast(input);
} else {
return null;
}
}
Not really, because this doesn't make any sense. If there's nothing you need to do when you catch the exception, then don't call the operation at all and delete all the code after. If it doesn't matter whether the operation succeeds or fails then don't call it in the first place.
More seriously - ahem, that was serious - you can not catch the exception and let the caller deal with it.
Try to create a common method that does this casting for you
private MyTypeOfObject cast (Object obj) {
try {
// your casting code
return newObj;
} catch (ClassCastException ee) {
// log and ignore
return null; // ????
}
}
There's nothing that I know of to do this (other than what #Scary_Wombat said) but if I were in your shoes I would just write a simple code generator to handle these repetitive/boilerplate cases using some templating engine like Freemarker or something more advanced like Antlr. There are tutorials about the place on each.
End of try block, or after all blocks?
I read a lot of answers on stack overflow about what will happen in each case. However, I could not find out which one is the commonly accepted "corrected" place to return.
Object o = null;
try {
Connection c = getConnection();
o = c.getThing();
return o;
} catch (Exception e) {
//handle exception
return null;
} finally {
c.close();
}
vs
Object o = null;
try {
Connection c = getConnection();
o = c.getThing();
} catch (Exception e) {
//handle exception
return null;
} finally {
c.close();
}
return o;
It completely depends on what you are trying to do. See the following code for some examples of the possibilities (not all of which should be combined in any single piece of code!).
try {
// do something that may fail
return 0; // return a normal value
} catch(SomeException e) {
// maybe log an error
return -1 // maybe return a default or error value
} finally {
// maybe clean up resources
// finally will be executed even if you return in try or catch
// a return here will trump a return in try/catch. This is generally regarded as a bad idea
// see https://stackoverflow.com/questions/65035/does-finally-always-execute-in-java
}
return 1 // return a normal value here instead of in the try/catch.
// May be clearer than multiple return statements
// Also useful if return value does not depend on the try/catch outcome
Update for your updated question. It's partly a matter of preference, though there are significant reasons for each choice. Many people prefer a single point of return, so would do this:
Object o = null;
try {
Connection c = getConnection();
o = c.getThing();
} catch (Exception e) {
//handle exception; leave o as null
} finally {
c.close();
}
return o;
Although this sounds like it should be clearer, of course one still has to examine the code to determine what value(s) o may end up with, so the advantage isn't great.
However, the other options have the advantage that the returning of null is very explicit. This is good if you are one of the many programmers who consider returning null values to be generally a bad idea (see Tony Hoare's "billion dollar mistake").
Instead of returning null one can use "Optional", "Option" or "Maybe" types (available in e.g. Java 8, Scala, Haskell and the Functional Java library), or use the Null Object pattern.
I try to remove all PMD warnings in my project and have some difficulties. I have a method that returns a Container object. This was my original method (simplified):
try {
return setResult();
} catch (Exception ex) {
}
return null;
PMD warned that there are 2 return statements. So I tried this:
Container result = null;
try {
result = setResult();
} catch (Exception ex) {
}
return result;
=> PMD warned about initializing with null. So I tried this:
Container result;
try {
result = setResult();
} catch (Exception ex) {
}
return result;
=> Eclipse did not compile this, suggesting to use the "= null" variation.
What am I missing here? How do I have to write this method without PMD warnings?
I'd use the second approach you show:
Container result = null;
try {
result = setResult();
} catch (Exception ex) {
}
return result;
Because, as PMD points out, it is important to avoid unnecessary return statements for the purpose of reducing complexity.
The PMD documentation classifies the NullAssignment rule as "controversial". It's usefulness very much depends on the context. Here is the sample code they provide:
public void bar() {
Object x = null; // this is OK
x = new Object();
// big, complex piece of code here
x = null; // this is not required
// big, complex piece of code here
}
So, unless you can assign the Container variable with a meaningful value beforehand - you should probably ignore this rule for this code fragment.
If you want to completely avoid the null assignment, you could use Guava's Optional. In this case it has the following advantages:
It forces you to think about the absence of a value
It makes the code more understandable
It makes PMD happy