InterfaceIsType

Since Checkstyle 3.1

Description

Implements Joshua Bloch, Effective Java, Item 17 - Use Interfaces only to define types.

According to Bloch, an interface should describe a type. It is therefore inappropriate to define an interface that does not contain any methods but only constants. The Standard interface javax.swing.SwingConstants is an example of an interface that would be flagged by this check.

The check can be configured to also disallow marker interfaces like java.io.Serializable, that do not contain methods or constants at all.

Properties

name description type default value since
allowMarkerInterfaces Control whether marker interfaces like Serializable are allowed. boolean true 3.1

Examples

To configure the check: 

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

Example:

class Example1 {
  // violation below, 'interfaces should describe a type and hence have methods.'
  interface Test1 {
    int a = 3;
  }

  // ok below, because marker interfaces are allowed by default.
  interface Test2 {

  }

  interface Test3 { // ok, because it has a method.
    int a = 3;
    void test();
  }
}

To configure the check to report violation so that it doesn't allow Marker Interfaces: 

<module name="Checker">
  <module name="TreeWalker">
    <module name="InterfaceIsType">
      <property name="allowMarkerInterfaces" value="false"/>
    </module>
  </module>
</module>

Example:

class Example2 {
  // violation below, 'interfaces should describe a type and hence have methods.'
  interface Test1 {
    int a = 3;
  }

  // violation below, 'interfaces should describe a type and hence have methods.'
  interface Test2 {

  }

  interface Test3 { // ok, because it has a method.
    int a = 3;
    void test();
  }
}

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