ThrowsCount

Since Checkstyle 3.2

Description

Restricts throws statements to a specified count. Methods with "Override" or "java.lang.Override" annotation are skipped from validation as current class cannot change signature of these methods.

Rationale: Exceptions form part of a method's interface. Declaring a method to throw too many differently rooted exceptions makes exception handling onerous and leads to poor programming practices such as writing code like catch(Exception ex). 4 is the empirical value which is based on reports that we had for the ThrowsCountCheck over big projects such as OpenJDK. This check also forces developers to put exceptions into a hierarchy such that in the simplest case, only one type of exception need be checked for by a caller but any subclasses can be caught specifically if necessary. For more information on rules for the exceptions and their issues, see Effective Java: Programming Language Guide Second Edition by Joshua Bloch pages 264-273.

ignorePrivateMethods - allows to skip private methods as they do not cause problems for other classes.

Properties

name description type default value since
ignorePrivateMethods Allow private methods to be ignored. boolean true 6.7
max Specify maximum allowed number of throws statements. int 4 3.2

Examples

To configure check: 

<module name="Checker">
  <module name="TreeWalker">
    <module name="ThrowsCount"/>
  </module>
</module>

Example: 

public class Example1 {
  // violation below, 'Throws count is 5 (max allowed is 4)'
  public void myFunction() throws CloneNotSupportedException,
                                ArrayIndexOutOfBoundsException,
                                StringIndexOutOfBoundsException,
                                IllegalStateException,
                                NullPointerException {
  }

  public void myFunc() throws ArithmeticException,
                                NumberFormatException,
                                NullPointerException {
  }

  private void privateFunc() throws CloneNotSupportedException,
                                ClassNotFoundException,
                                IllegalAccessException,
                                ArithmeticException,
                                ClassCastException {
  }

  private void func() throws IllegalStateException,
                                NullPointerException {
  }
}

To configure the check so that it doesn't allow more than two throws per method: 

<module name="Checker">
  <module name="TreeWalker">
    <module name="ThrowsCount">
      <property name="max" value="2"/>
    </module>
  </module>
</module>

Example: 

public class Example2 {
  // violation below, 'Throws count is 5 (max allowed is 2)'
  public void myFunction() throws CloneNotSupportedException,
                                ArrayIndexOutOfBoundsException,
                                StringIndexOutOfBoundsException,
                                IllegalStateException,
                                NullPointerException {
  }
  // violation below, 'Throws count is 3 (max allowed is 2)'
  public void myFunc() throws ArithmeticException,
                                NumberFormatException,
                                NullPointerException {
  }

  private void privateFunc() throws CloneNotSupportedException,
                                ClassNotFoundException,
                                IllegalAccessException,
                                ArithmeticException,
                                ClassCastException {
  }

  private void func() throws IllegalStateException,
                                NullPointerException {
  }
}

To configure the check so that it doesn't skip private methods: 

<module name="Checker">
  <module name="TreeWalker">
    <module name="ThrowsCount">
      <property name="ignorePrivateMethods" value="false"/>
    </module>
  </module>
</module>

Example: 

public class Example3 {
  // violation below, 'Throws count is 5 (max allowed is 4)'
  public void myFunction() throws CloneNotSupportedException,
                                ArrayIndexOutOfBoundsException,
                                StringIndexOutOfBoundsException,
                                IllegalStateException,
                                NullPointerException {
  }

  public void myFunc() throws ArithmeticException,
                                NumberFormatException,
                                NullPointerException {
  }
  // violation below, 'Throws count is 5 (max allowed is 4)'
  private void privateFunc() throws CloneNotSupportedException,
                                ClassNotFoundException,
                                IllegalAccessException,
                                ArithmeticException,
                                ClassCastException {
  }

  private void func() throws IllegalStateException,
                                NullPointerException {
  }
}

Example of Usage

Violation Messages

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.design

Parent Module

TreeWalker