InterfaceMemberImpliedModifier

Since Checkstyle 8.12

Description

Checks for implicit modifiers on interface members and nested types.

This check is effectively the opposite of RedundantModifier. It checks the modifiers on interface members, ensuring that certain modifiers are explicitly specified even though they are actually redundant.

Methods in interfaces are public by default, however from Java 9 they can also be private. This check provides the ability to enforce that public is explicitly coded and not implicitly added by the compiler.

From Java 8, there are three types of methods in interfaces - static methods marked with static, default methods marked with default and abstract methods which do not have to be marked with anything. From Java 9, there are also private methods marked with private. This check provides the ability to enforce that abstract is explicitly coded and not implicitly added by the compiler.

Fields in interfaces are always public static final and as such the compiler does not require these modifiers. This check provides the ability to enforce that these modifiers are explicitly coded and not implicitly added by the compiler.

Nested types within an interface are always public static and as such the compiler does not require the public static modifiers. This check provides the ability to enforce that the public and static modifiers are explicitly coded and not implicitly added by the compiler.

public interface AddressFactory {
  // check enforces code contains "public static final"
  public static final String UNKNOWN = "Unknown";

  String OTHER = "Other";  // violation

  // check enforces code contains "public" or "private"
  public static AddressFactory instance();

  // check enforces code contains "public abstract"
  public abstract Address createAddress(String addressLine, String city);

  List<Address> findAddresses(String city);  // violation

  // check enforces default methods are explicitly declared "public"
  public default Address createAddress(String city) {
    return createAddress(UNKNOWN, city);
  }

  default Address createOtherAddress() {  // violation
    return createAddress(OTHER, OTHER);
  }
}
        

Rationale for this check: Methods, fields and nested types are treated differently depending on whether they are part of an interface or part of a class. For example, by default methods are package-scoped on classes, but public in interfaces. However, from Java 8 onwards, interfaces have changed to be much more like abstract classes. Interfaces now have static and instance methods with code. Developers should not have to remember which modifiers are required and which are implied. This check allows the simpler alternative approach to be adopted where the implied modifiers must always be coded explicitly.

Properties

name description type default value since
violateImpliedPublicField Control whether to enforce that public is explicitly coded on interface fields. Boolean true 8.12
violateImpliedStaticField Control whether to enforce that static is explicitly coded on interface fields. Boolean true 8.12
violateImpliedFinalField Control whether to enforce that final is explicitly coded on interface fields. Boolean true 8.12
violateImpliedPublicMethod Control whether to enforce that public is explicitly coded on interface methods. Boolean true 8.12
violateImpliedAbstractMethod Control whether to enforce that abstract is explicitly coded on interface methods. Boolean true 8.12
violateImpliedPublicNested Control whether to enforce that public is explicitly coded on interface nested types. Boolean true 8.12
violateImpliedStaticNested Control whether to enforce that static is explicitly coded on interface nested types. Boolean true 8.12

Examples

This example checks that all implicit modifiers on methods, fields and nested types are explicitly specified in interfaces.

Configuration:

<module name="InterfaceMemberImpliedModifier"/>
        

Code:

public interface AddressFactory {

  public static final String UNKNOWN = "Unknown";  // valid

  String OTHER = "Other";  // violation

  public static AddressFactory instance();  // valid

  public abstract Address createAddress(String addressLine, String city);  // valid

  List<Address> findAddresses(String city);  // violation

  interface Address {  // violation

    String getCity();  // violation
  }
}
        

This example checks that all implicit modifiers on methods and fields are explicitly specified, but nested types do not need to be.

Configuration:

<module name="InterfaceMemberImpliedModifier">
  <property name="violateImpliedPublicNested" value="false"/>
  <property name="violateImpliedStaticNested" value="false"/>
</module>
        

Code:

public interface RoadFeature {

  String STOP = "Stop";  // violation

  enum Lights {  // valid because of configured properties

    RED, YELLOW, GREEN;
  }
}
        

Example of Usage

Error 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.modifier

Parent Module

TreeWalker

ModifierOrder

Description

Since Checkstyle 3.0

Checks that the order of modifiers conforms to the suggestions in the Java Language specification, sections 8.1.1, 8.3.1, 8.4.3 and 9.4. The correct order is:

  1. public
  2. protected
  3. private
  4. abstract
  5. default
  6. static
  7. final
  8. transient
  9. volatile
  10. synchronized
  11. native
  12. strictfp

ATTENTION: We skip type annotations from validation.

Examples

To configure the check:

<module name="ModifierOrder"/>
        

Error 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.modifier

Parent Module

TreeWalker

RedundantModifier

Description

Since Checkstyle 3.0

Checks for redundant modifiers in:

  1. Interface and annotation definitions.
  2. Final modifier on methods of final and anonymous classes.
  3. Inner interface declarations that are declared as static.
  4. Class constructors.
  5. Nested enum definitions that are declared as static.

Rationale: The Java Language Specification strongly discourages the usage of public and abstract for method declarations in interface definitions as a matter of style.

Interfaces by definition are abstract so the abstract modifier on the interface is redundant.

Classes inside of interfaces by definition are public and static, so the public and static modifiers on the inner classes are redundant. On the other hand, classes inside of interfaces can be abstract or non abstract. So, abstract modifier is allowed.

Fields in interfaces and annotations are automatically public, static and final, so these modifiers are redundant as well.

As annotations are a form of interface, their fields are also automatically public, static and final just as their annotation fields are automatically public and abstract.

Enums by definition are static implicit subclasses of java.lang.Enum<E>. So, the static modifier on the enums is redundant. In addition, if enum is inside of interface, public modifier is also redundant.

Enums can also contain abstract methods and methods which can be overridden by the declared enumeration fields. See the following example:

public enum EnumClass {
  FIELD_1,
  FIELD_2 {
    @Override
    public final void method1() {} // violation expected
  };

  public void method1() {}
  public final void method2() {} // no violation expected
}
        

Since these methods can be overridden in these situations, the final methods are not marked as redundant even though they can't be extended by other classes/enums.

Nested enum types are always static by default.

Final classes by definition cannot be extended so the final modifier on the method of a final class is redundant.

Public modifier for constructors in non-public non-protected classes is always obsolete:

public class PublicClass {
  public PublicClass() {} // OK
}

class PackagePrivateClass {
  public PackagePrivateClass() {} // violation expected
}
        

There is no violation in the following example, because removing public modifier from ProtectedInnerClass constructor will make this code not compiling:

package a;
public class ClassExample {
  protected class ProtectedInnerClass {
    public ProtectedInnerClass () {}
  }
}

package b;
import a.ClassExample;
public class ClassExtending extends ClassExample {
  ProtectedInnerClass pc = new ProtectedInnerClass();
}
        

Examples

To configure the check:

<module name="RedundantModifier"/>
        

To configure the check to check only methods and not variables:

<module name="RedundantModifier">
  <property name="tokens" value="METHOD_DEF"/>
</module>
        

Example of Usage

Error 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.modifier

Parent Module

TreeWalker