How to throw exceptions in java

How to throw exceptions in Java — the differences between throw and throws

You know that Java uses the throw and try-catch mechanism for handling exceptions as described in the tutorial Getting Started with Exception Handling in Java.

In this Java tutorial, I’m going to tell you how to write code that declares to throw exceptions. This involves in using the throw and throws keywords together.

1. How to throw an exception?

We want to ensure that the age of an engineer is always in a valid range, e.g. between 21 and 59. To do so, we can check the parameter age and throw an exception if it is out of range. Hence, we update the setAge() method as following:

public void setAge(int age) < if (age < 21 || age >59) < throw new IllegalArgumentException("Invalid age"); >this.age = age; >

Here, the throw keyword is used to throw a new exception object if the passed-in parameter has invalid value. When the statement started by the throw keyword gets executed, the current method stops its execution and control returned to the caller code which can have an exception handler.

The following is a test program that uses try-catch structure to handle the above exception:

public class EngineerTest < public static void main(String[] args) < int age = Integer.parseInt(args[0]); try < Engineer newEngineer = new Engineer(); newEngineer.setAge(age); >catch (IllegalArgumentException ex) < System.err.println(ex); >> >

This program tries to create a new Engineer object with age is passed from the command-line argument. Note that in the catch block, we use the System.err static class to print out the exception object. It represents the error console, which differs from the output console represented by the System.out static class.

Run the test program with the following command:

java.lang.IllegalArgumentException: Invalid age

The above examples show you how to use the throw keyword to throw an exception in your code. And which kind of exceptions can you throw?

Read on as I’m about to explain in the next section.

2. Specify exceptions to be thrown in a method

How do we know whether a method can throw exceptions or not? And how do we know which exceptions does a method throw?

The answer is, by looking for the throws clause in the method’s signature. For example:

public int read() throws IOException

I take this method from the InputStreamReader class in the java.io package. The throws clause contains one more exceptions (separated by commas) which can be thrown in the method’s body.

Here, the read() method can throw IOException while reading data from a file.

So we know that the throws keyword is used in method’s signature to specify exceptions can be thrown. Remember these rules:

• If code in the method can throw checked exceptions, the method must declare to throw those exceptions in its signature, otherwise we will get compile error. And the caller code should handle those exceptions.
• If code in the method can throw unchecked exceptions, the method is not required to declare to throw those exceptions. That’s why in the Engineer example above, the setAge() method does not declare to throw IllegalArgumentException which is an unchecked exception. And the caller code can choose to handle unchecked exceptions or not.

public int divide(int a, int b) throws Exception < if (b == 0) < throw new Exception("Cannot be divided by zero"); >return a / b; >

Here, the divide() method declares to throw an Exception if the parameter b equals to 0. Note that throwing a general exception (here Exception is the base class of all exceptions) is discouraged. In practice, we should throw more specific exceptions.

3. Some rules regarding throw and throws:

• If code in a method throws checked exceptions, the method must specify those exceptions in the throws clause.

public void deleteDir(String path) throws IOException < if (path == null) < throw new FileNotFoundException("Directory not found"); >// code to delete. >

public void encryptFile(String path) throws IOException < // no exceptions thrown here. >

Other Java Exception Handling Tutorials:

About the Author:

Nam Ha Minh is certified Java programmer (SCJP and SCWCD). He started programming with Java in the time of Java 1.4 and has been falling in love with Java since then. Make friend with him on Facebook and watch his Java videos you YouTube.

Источник

Throwing Exceptions in Java

It is important to understand how to throw exceptions in Java. This will allow you to create higher quality code where errors are checked at compile time instead of runtime, and create custom exceptions that make debugging and recovery easier.

How to throw exceptions in Java

Throwing an exception is as simple as using the «throw» statement. You then specify the Exception object you wish to throw. Every Exception includes a message which is a human-readable error description. It can often be related to problems with user input, server, backend, etc. Here is an example that shows how to throw an exception:

throw new Exception("Exception message");

It’s limiting to use a generic exception because it makes it difficult for the calling code to catch it. It’s better to throw custom exceptions, which we will come back to in a bit.

Using the Throws keyword

Throws is a keyword used to indicate that this method could throw this type of exception. The caller has to handle the exception using a try-catch block or propagate the exception. We can throw either checked or unchecked exceptions.

The throws keyword allows the compiler to help you write code that handles this type of error, but it does not prevent the abnormal termination of the program. With the help of the throws keyword, we can provide information to the caller of the method about the types of exceptions the method might throw.

type method_name(parameters) throws exception_list

In the above syntax, exception_list is a comma-separated list of all the exceptions a method might throw. For example:

void testMethod() throws ArithmeticException, ArrayIndexOutOfBoundsException < // rest of code >

In the example below, we have created a test method to demonstrate throwing an exception. The toString() method returns a textual representation of an object, but in this case the variable is null. Calling a method on a null reference or trying to access a field of a null reference will trigger a NullPointerException .

static void testMethod() throws Exception

This must be handled with a try/catch block:

public class Example < public static void main(String[] arg) < try < testMethod(); >catch (Exception e) < e.printStackTrace(); >> >

The Exception class

To use exceptions within your application more effectively, it is important to understand how to create and throw your own. But before we get into throwing exceptions, let’s first take under the hood: We’ll describe what an exception is and how to define your own, starting with the global exception class that all Java exceptions stem from:

package java.lang; public class Exception extends Throwable < static final long serialVersionUID = -3387516993124229948L; public Exception() < super(); >public Exception(String message) < super(message); >public Exception(String message, Throwable cause) < super(message, cause); >public Exception(Throwable cause) < super(cause); >protected Exception(String message, Throwable cause, boolean enableSuppression, boolean writableStackTrace) < super(message, cause, enableSuppression, writableStackTrace); >>

The Exception class is the superclass of all classes that represent recoverable exceptions. When exceptions are thrown, they may be caught by the application code. The exception class extends Throwable . The constructor contains two parameters: message and cause. The detailMessage parameter gives the details of the message for this exception, and the throwable parameter gives the cause of this exception.

Types of exceptions

There are two types of exceptions in Java: checked (compile time) exceptions and unchecked (runtime) exceptions. For clarity, we’ll also discuss how errors are different than exceptions in Java.

Checked exception (compile time exception)

Checked exceptions must be caught and handled during compile time. If the compiler does not see a try or catch block or throws keyword to handle a checked exception, it throws a compilation error. Checked exceptions are generally caused by faults outside code like missing files, invalid class names, and networking errors.

FileInputStream fis = null; try < fis = new FileInputStream("B:/myfile.txt"); >catch (FileNotFoundException e)

Unchecked exception (runtime exception)

Unchecked exceptions do not need to be explicitly handled; they occur at the time of execution, also known as run time. These exceptions can usually be avoided by good coding practices. They are typically caused by programming bugs, such as logic errors or improper use of APIs. These exceptions are ignored at the time of compilation. For example:

The example above will cause an ArithmeticException at the time of program, since a number can’t be divided by 0. It would throw an unhandled exception and the program would end.

Errors

People often refer to «errors» and “exceptions” as the same thing colloquially. However, in Java these are separate concepts. Errors are thrown by the Java Virtual Machine and cannot be caught or handled. They derive from java.lang.Error and they occur because of some fault in the environment in which the application is running. For example, stack overflows and out of memory exceptions are environment errors that result in the application exiting.

Custom exceptions

Java’s built-in exceptions don’t always provide the information we need. So, we sometimes need to supplement these exceptions with our own. During some specific operation, if an exception occurs in your application, you need to recover and make the user know about it. A custom exception gives you more control to provide extra data about the problem and to handle the exception in your code.

The best practice is to extend the java.lang.Exception class with a new class, following the general naming convention as provided by the JDK (Java Development Kit). The new class requires a constructor that will take a string as the error message—it is called the parent class constructor.

public class HandledException extends Exception < private String code; public HandledException(String code, String message) < super(message); this.setCode(code); >public HandledException(String code, String message, Throwable cause) < super(message, cause); this.setCode(code); >public String getCode() < return code; >public void setCode(String code) < this.code = code; >>

For example, let’s say a program fails to connect to a database. You could use a custom exception to collect information like the database URL, username, password, etc. In the catch block, you could write this information to the log and display a basic message to the user like «failed to connect to database.»

public class MainClass < private static String DATABASE_EXCEPTION = "DATABASE_EXCEPTION"; private static final Logger logger = Logger.getLogger(MainClass.class); public static void main(String[] args) < try < makeDatabaseConnection(); >catch (HandledException e) < rollbar.error(e, "Hello, Rollbar"); // Display custom message to the user System.out.println("Code: "+e.getCode()+” Exception Message : ”+e.getMessage()); // Log the exception detail logger.error("Exception: ", e); >> static void makeDatabaseConnection() throws HandledException < String dbURL = "jdbc:sqlserver://localhost\sqlexpress"; String userName = "sa"; String password = "secret"; Connection conn = null; try < conn = DriverManager.getConnection(dbURL, userName, password); >catch (SQLException e) < rollbar.error(e, "Hello, Rollbar"); throw new HandledException(DATABASE_EXCEPTION,"Failed to connect to database", e); >> >

Managing errors and exceptions in your code is challenging. It can make deploying production code an unnerving experience. Rollbar can help throw Java exceptions as well as track, analyze, and manage errors in real-time to help you to proceed with more confidence. Try it today!

Источник