PMD - Controversial Rules

The Fourth installment of explanation of PMD rules covering the Controversial Rules

Controversial Rules

The Controversial Ruleset contains rules that, for whatever reason, are considered controversial. They are separated out here to allow people to include as they see fit via custom rulesets. This ruleset was initially created in response to discussions over UnnecessaryConstructorRule which Tom likes but most people really dislike :-)

UnnecessaryConstructor

This rule detects when a constructor is not necessary; i.e., when there's only one constructor, it's public, has an empty body, and takes no arguments.

Example

public class Foo {

public Foo() {}

}

NullAssignment

Assigning a "null" to a variable (outside of its declaration) is usually bad form. Some times, the assignment is an indication that the programmer doesn't completely understand what is going on in the code. NOTE: This sort of assignment may in rare cases be useful to encourage garbage collection. If that's what you're using it for, by all means, disregard this rule :-)

Example

public class Foo {

   public void bar() {

     Object x = null; // This is OK.

     x = new Object();

     // Big, complex piece of code here.

     x = null; // This is BAD.

     // Big, complex piece of code here.

   }

}

OnlyOneReturn

A method should have only one exit point, and that should be the last statement in the method.

Example

public class OneReturnOnly1 {

  public void foo(int x) {

   if (x > 0) {

    return "hey";   // oops, multiple exit points!

   }

   return "hi";

  }

}

UnusedModifier

Fields in interfaces are automatically public static final, and methods are public abstract. Classes or interfaces nested in an interface are automatically public and static (all nested interfaces are automatically static). For historical reasons, modifiers which are implied by the context are accepted by the compiler, but are superfluous.

Example

public interface Foo {

public abstract void bar(); // both abstract and public are ignored by the compiler

public static final int X = 0; // public, static, and final all ignored

public static class Bar {} // public, static ignored

public static interface Baz {} // ditto

}

public class Bar {

public static interface Baz {} // static ignored

}

AssignmentInOperand

Avoid assignments in operands; this can make code more complicated and harder to read. Always make assignment separately and check separately.

Example

public class Foo {

public void bar() {

  int x = 2;

  if ((x = getX()) == 3) {

   System.out.println("3!");

  }

}

private int getX() {

  return 3;

}

}

AtLeastOneConstructor

Each class should declare at least one constructor.

Example

public class Foo {

// no constructor!  not good!

}

DontImportSun

Avoid importing anything from the 'sun.*' packages. These packages are not portable and are likely to change.

Example

import sun.misc.foo;

public class Foo {}

SuspiciousOctalEscape

A suspicious octal escape sequence was found inside a String literal. The Java language specification (section 3.10.6) says an octal escape sequence inside a literal String shall consist of a backslash followed by: OctalDigit | OctalDigit OctalDigit | ZeroToThree OctalDigit OctalDigit Any octal escape sequence followed by non-octal digits can be confusing, e.g. "\038" is interpreted as the octal escape sequence "\03" followed by the literal character "8".

Example

public class Foo {

public void foo() {

  // interpreted as octal 12, followed by character '8'

  System.out.println("suspicious: \128");

}

}

CallSuperInConstructor

It is a good practice to call super() in a constructor. If super() is not called but another constructor (such as an overloaded constructor) is called, this rule will not report it.

Example

public class Foo extends Bar{

public Foo() {

  // call the constructor of Bar

  super();

}

public Foo(int code) {

  // do something with code

  this();

  // no problem with this

}

}

UnnecessaryParentheses

Sometimes expressions are wrapped in unnecessary parentheses, making them look like a function call.

Example

public class Foo {

      boolean bar() {

          return (true); //This could have been return true;

      }

  }

DefaultPackage

Use explicit scoping instead of the default package private level. Have all classes in some package or other.

BooleanInversion

Use bitwise inversion to invert boolean values - it's the fastest way to do this. See http://www.javaspecialists.co.za/archive/newsletter.do?issue=042&locale=en_US for specific details

Example

public class Foo {

public void main(bar) {

  boolean b = true;

  b = !b; // slow

  b ^= true; // fast

}

}

DataflowAnomalyAnalysis

The dataflow analysis tracks local definitions, undefinitions and references to variables on different paths on the data flow. From this information various problems can be found.

1.     UR - Anomaly: There is a reference to a variable that was not defined before. This is a bug and leads to an error.

2.     DU - Anomaly: A recently defined variable is undefined. These anomalies may appear in normal source text.

3.     DD - Anomaly: A recently defined variable is redefined. This is ominous but don't have to be a bug.

Example

public class Foo {

    public void foo() {

         int buz = 5;

         buz = 6; // redefinition of buz -> dd-anomaly

         foo(buz);

         buz = 2;

    } // buz is undefined when leaving scope -> du-anomaly

}

AvoidFinalLocalVariable

Avoid using final local variables, turn them into fields.

Example

public class MyClass {

    public void foo() {

        final String finalLocalVariable;

    }

}

AvoidUsingShortType

Java uses the 'short' type to reduce memory usage, not to optimize calculation. In fact, the jvm does not have any arithmetic capabilities for the short type: the jvm must convert the short into an int, do the proper calculation and convert the int back to a short. So, the use of the 'short' type may have a greater impact than memory usage.

Example

public class UsingShort

    {

        private short doNotUseShort = 0;

               public UsingShort() {

                       short shouldNotBeUsed = 1;

                       doNotUseShort += shouldNotBeUsed;

               }

       }

AvoidUsingVolatile

Use of the keyword 'volatile' is general used to fine tune a Java application, and therefore, requires a good expertise of the Java Memory Model. Moreover, its range of action is somewhat misknown. Therefore, the volatile keyword should not be used for maintenance purpose and portability. Not everybody understands the concept of volatile and the JVM implementation of JVM also changed from one version to another. It is better to avoid usage of volatile rather than try to understand it.

Example

public class ThrDeux {

        private volatile String var;

}

AvoidUsingNativeCode

As JVM and Java language offer already many help in creating application, it should be very rare to have to rely on non-java code. Even though, it is rare to actually have to use Java Native Interface (JNI). As the use of JNI make application less portable, and harder to maintain, it is not recommended.

Example

public class SomeJNIClass {

        public SomeJNIClass() {

               System.loadLibrary("nativelib");

        }

        static {

               System.loadLibrary("nativelib");

        }

        public void invalidCallsInMethod() throws SecurityException, NoSuchMethodException {

               System.loadLibrary("nativelib");

        }

}

AvoidAccessibilityAlteration

Methods such as getDeclaredConstructors(), getDeclaredConstructor(Class[]) and setAccessible(), as the interface PrivilegedAction, allow to alter, at runtime, the visibility of variable, classes, or methods, even if they are private. Obviously, no one should do so as such behavior is against everything encapsulation principal stands for.

Example

import java.lang.reflect.AccessibleObject;

import java.lang.reflect.Method;

import java.security.PrivilegedAction;

public class Violation {

        public void invalidCallsInMethod() throws SecurityException, NoSuchMethodException {

               // Possible call to forbidden getDeclaredConstructors

               Class[] arrayOfClass = new Class[1];

               this.getClass().getDeclaredConstructors();

               this.getClass().getDeclaredConstructor(arrayOfClass);

               Class clazz = this.getClass();

               clazz.getDeclaredConstructor(arrayOfClass);

               clazz.getDeclaredConstructors();

               // Possible call to forbidden setAccessible

               clazz.getMethod("", arrayOfClass).setAccessible(false);

               AccessibleObject.setAccessible(null, false);

               Method.setAccessible(null, false);

               Method[] methodsArray = clazz.getMethods();

               int nbMethod;

               for ( nbMethod = 0; nbMethod < methodsArray.length; nbMethod++ ) {

                       methodsArray[nbMethod].setAccessible(false);

                }

               // Possible call to forbidden PrivilegedAction

               PrivilegedAction priv = (PrivilegedAction) new Object();

               priv.run();

        }

}

DoNotCallGarbageCollectionExplicitly

Calls to System.gc(), Runtime.getRuntime().gc(), and System.runFinalization() are not advised. Code should have the same behavior whether the garbage collection is disabled using the option -Xdisableexplicitgc or not. Moreover, "modern" jvms do a very good job handling garbage collections. If memory usage issues unrelated to memory leaks develop within an application, it should be dealt with JVM options rather than within the code itself.

Example

public class GCCall {

        public GCCall() {

               // Explicit gc call !

               System.gc();

        }

        public void doSomething() {

               // Explicit gc call !

               Runtime.getRuntime().gc();

        }

        public explicitGCcall() { // Explicit gc call ! 

               System.gc();

        }

        public void doSomething() {// Explicit gc call !

               Runtime.getRuntime().gc();

        }

}