Java Can not Cast from String to Integer_Sovled
I’ve also tried Integer.parseInt(num) and Integer.decode(num) and Integer.valueof(bu)
Results as follows: 40
The problem was caused by different encoding from the txt file I’m reading from and the encoding in Elipse which is the macRoman by default.
5 Answers 5
The correct way of working is this:
This, only when you are sure that the String is representing a number the valid way.
Your exception got raised in Integer.java:458 .
Looking into the source code of Integer I see that some characters of your String «40» returned a negative value (most probably -1) for Character.digit(s.charAt(i++), radix) where i is iterating over String’s characters and radix is 10.
This should not happen normally. But it happens when the String is «4o» and not «40» like @Løkling’s wild guess in the comments.
You should debug here to see what really happens.
Of course you can’t cast from String to Integer . A String is not a Integer , and Integer is not a String .
You have to use int i = Integer.parseInt(..) . It works if the string is a properly formatted integer.
For a cast to succeed in Java, the reference being cast must point to an object that is actually an instance of the type being cast to. A reference to String can’t be cast to Integer because an object couldn’t possibly be both things.
(This is somewhat unlike a cast in C, which is basically just saying, reinterpret the data stored here according to a different datatype. It would still be incorrect in C to use casting as a method to convert a string representing a number to a numeric value.)
Integer.parseInt is what you are looking for here. What problem are you having with it?
Remember : In java, we can only cast between objects if they are EXPLICITLY of the same type ! In other languages , like python, we can do the casting you requested here, because those languages allow «duck typing».
You must use Integer.parseInt(«1234»), to create an Integer object from a String.
Alternatively you could create an Object wrapper to your data :
This would be overkill for your simple problem, however 🙂
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Explanation of ClassCastException in Java
I read some articles written on «ClassCastException», but I couldn’t get a good idea on what it means. What is a ClassCastException?
12 Answers 12
Straight from the API Specifications for the ClassCastException :
Thrown to indicate that the code has attempted to cast an object to a subclass of which it is not an instance.
So, for example, when one tries to cast an Integer to a String , String is not an subclass of Integer , so a ClassCastException will be thrown.
It’s really pretty simple: if you are trying to typecast an object of class A into an object of class B, and they aren’t compatible, you get a class cast exception.
Let’s think of a collection of classes.
- You can cast any of these things to Object, because all Java classes inherit from Object.
- You can cast either B or C to A, because they’re both «kinds of» A
- You can cast a reference to an A object to B only if the real object is a B.
- You can’t cast a B to a C even though they’re both A’s.
It is an Exception which occurs if you attempt to downcast a class, but in fact the class is not of that type.
Consider this heirarchy:
You might have a method called:
If called with this code:
It will compile just fine, but at runtime you will get a ClassCastException because o was in fact an Animal, not a Dog.
In later versions of Java you do get a compiler warning unless you do:
Consider an example,
At Another t5 = (Another) new Goat() : you will get a ClassCastException because you cannot create an instance of the Another class using Goat .
Note: The conversion is valid only in cases where a class extends a parent class and the child class is casted to its parent class.
How to deal with the ClassCastException :
- Be careful when trying to cast an object of a class into another class. Ensure that the new type belongs to one of its parent classes.
- You can prevent the ClassCastException by using Generics, because Generics provide compile time checks and can be used to develop type-safe applications.
String to Int in java — Likely bad data, need to avoid exceptions
Seeing as Java doesn’t have nullable types, nor does it have a TryParse(), how do you handle input validation without throwing an exceptions?
I could use a regex to check if it’s parseable, but that seems like a lot of overhead as well.
What’s the best practice for handling this situation?
EDIT: Rationale: There’s been a lot of talk on SO about exception handling, and the general attitude is that exceptions should be used for unexpected scenarios only. However, I think bad user input is EXPECTED, not rare. Yes, it really is an academic point.
Some of the answers demonstrate exactly what is wrong with SO. You ignore the question being asked, and answer another question that has nothing to do with it. The question isn’t asking about transition between layers. The question isn’t asking what to return if the number is un-parseable. For all you know, val = Integer.MIN_VALUE; is exactly the right option for the application that this completely context free code snippet was take from.
16 Answers 16
There is no need to write your own methods to parse numbers without throwing exceptions.
For user supplied data, Integer.parseInt is usually the wrong method because it doesn’t support internationisation. The java.text package is your (verbose) friend.
That’s pretty much it, although returning MIN_VALUE is kind of questionable, unless you’re sure it’s the right thing to use for what you’re essentially using as an error code. At the very least I’d document the error code behavior, though.
Might also be useful (depending on the application) to log the bad input so you can trace.
What’s the problem with your approach? I don’t think doing it that way will hurt your application’s performance at all. That’s the correct way to do it. Don’t optimize prematurely.
I’m sure it is bad form, but I have a set of static methods on a Utilities class that do things like Utilities.tryParseInt(String value) which returns 0 if the String is unparseable and Utilities.tryParseInt(String value, int defaultValue) which allows you to specify a value to use if parseInt() throws an exception.
I believe there are times when returning a known value on bad input is perfectly acceptable. A very contrived example: you ask the user for a date in the format YYYYMMDD and they give you bad input. It may be perfectly acceptable to do something like Utilities.tryParseInt(date, 19000101) or Utilities.tryParseInt(date, 29991231); depending on the program requirements.
I’m going to restate the point that stinkyminky was making towards the bottom of the post:
A generally well accepted approach validating user input (or input from config files, etc. ) is to use validation prior to actually processing the data. In most cases, this is a good design move, even though it can result in multiple calls to parsing algorithms.
Once you know that you have properly validated the user input, then it is safe to parse it and ignore, log or convert to RuntimeException the NumberFormatException.
Note that this approach requires you to consider your model in two pieces: the business model (Where we actually care about having values in int or float format) and the user interface model (where we really want to allow the user to put in whatever they want).
In order for the data to migrate from the user interface model to the business model, it must pass through a validation step (this can occur on a field by field basis, but most scenarios call for validation on the entire object that is being configured).
If validation fails, then the user is presented with feedback informing them of what they’ve done wrong and given a chance to fix it.
Binding libraries like JGoodies Binding and JSR 295 make this sort of thing a lot easier to implement than it might sound — and many web frameworks provide constructs that separate user input from the actual business model, only populating business objects after validation is complete.
In terms of validation of configuration files (the other use case presented in some of the comments), it’s one thing to specify a default if a particular value isn’t specified at all — but if the data is formatted wrong (someone types an ‘oh’ instead of a ‘zero’ — or they copied from MS Word and all the back-ticks got a funky unicode character), then some sort of system feedback is needed (even if it’s just failing the app by throwing a runtime exception).
Java String to Int – How to Convert a String to an Integer
String objects are represented as a string of characters.
If you have worked in Java Swing, it has components such as JTextField and JTextArea which we use to get our input from the GUI. It takes our input as a string.
If we want to make a simple calculator using Swing, we need to figure out how to convert a string to an integer. This leads us to the question – how can we convert a string to an integer?
In Java, we can use Integer.valueOf() and Integer.parseInt() to convert a string to an integer.
1. Use Integer.parseInt() to Convert a String to an Integer
This method returns the string as a primitive type int. If the string does not contain a valid integer then it will throw a NumberFormatException.
So, every time we convert a string to an int, we need to take care of this exception by placing the code inside the try-catch block.
Let’s consider an example of converting a string to an int using Integer.parseInt() :
Let’s try to break this code by inputting an invalid integer:
As you can see in the above code, we have tried to convert 25T to an integer. This is not a valid input. Therefore, it must throw a NumberFormatException.
Here’s the output of the above code:
Next, we will consider how to convert a string to an integer using the Integer.valueOf() method.
2. Use Integer.valueOf() to Convert a String to an Integer
This method returns the string as an integer object. If you look at the Java documentation, Integer.valueOf() returns an integer object which is equivalent to a new Integer(Integer.parseInt(s)) .
We will place our code inside the try-catch block when using this method. Let us consider an example using the Integer.valueOf() method:
Now, let’s try to break the above code by inputting an invalid integer number:
Similar to the previous example, the above code will throw an exception.
Here’s the output of the above code:
We can also create a method to check if the passed-in string is numeric or not before using the above mentioned methods.
I have created a simple method for checking whether the passed-in string is numeric or not.
The isNumeric() method takes a string as an argument. First it checks if it is null or not. After that we use the matches() method to check if it contains digits 0 to 9 and a period character.
This is a simple way to check numeric values. You can write or search Google for more advanced regular expressions to capture numerics depending on your use case.
It is a best practice to check if the passed-in string is numeric or not before trying to convert it to integer.
How to Handle the NumberFormat Exception in Java
Table of Contents
The NumberFormatException is an unchecked exception in Java that occurs when an attempt is made to convert a string with an incorrect format to a numeric value. Therefore, this exception is thrown when it is not possible to convert a string to a numeric type (e.g. int, float). For example, this exception occurs if a string is attempted to be parsed to an integer but the string contains a boolean value.
Since the NumberFormatException is an unchecked exception, it does not need to be declared in the throws clause of a method or constructor. It can be handled in code using a try-catch block.
What Causes NumberFormatException
There can be various cases related to improper string format for conversion to numeric values. Some of them are:
Null input string
Empty input string
Input string with leading/trailing whitespaces
Input string with inappropriate symbols
Input string with non-numeric data
Alphanumeric input string
Input string exceeding the range of the target data type
Mismatch of data type between input string and the target data type
NumberFormatException Example
Here is an example of a NumberFormatException thrown when attempting to convert an alphanumeric string to an integer:
In this example, a string containing both numbers and characters is attempted to be parsed to an integer, leading to a NumberFormatException:
Such operations should be avoided where possible by paying attention to detail and making sure strings attempted to be parsed to numeric values are appropriate and legal.
How to Handle NumberFormatException
The NumberFormatException is an exception in Java, and therefore can be handled using try-catch blocks using the following steps:
- Surround the statements that can throw an NumberFormatException in try-catch blocks
- Catch the NumberFormatException
- Depending on the requirements of the application, take necessary action. For example, log the exception with an appropriate message.
The code in the earlier example can be updated with the above steps:
Surrounding the code in try-catch blocks like the above allows the program to continue execution after the exception is encountered:
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