pf2_misconceptions.xml

<?xml version="1.0"?>

<ruleset name="PF2 misconceptions"
    xmlns="http://pmd.sourceforge.net/ruleset/2.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://pmd.sourceforge.net/ruleset/2.0.0 https://pmd.sourceforge.io/ruleset_2_0_0.xsd">

    <description>
        Common misconceptions found in the pf2 course, or in general in novice programmers.
    </description>



    <rule name="NoFloatLiterals"
          language="java"
          message="Use float literal instead of casting a literal to float"
          class="net.sourceforge.pmd.lang.rule.XPathRule"
          >
        <description>
A float literal can be obtained by appending 'f' to the number. i.e. 1.2f
        </description>
        <priority>3</priority>
        <properties>
            <property name="xpath">
                <value>
<![CDATA[
//CastExpression[
./Type/PrimitiveType[@Image='float']
and
./PrimaryExpression/PrimaryPrefix/Literal[@FloatLiteral = false()]
]
]]>
                </value>
            </property>
            <property name="version" value="2.0"/>
        </properties>
        <example>
<![CDATA[
  public class Foo {
    public void bar() {
        float f = (float) 123;
        f = (float) 23;
        f = (float) (1.5);
    }
}
]]>
        </example>
    </rule>



    <rule name="NoLongLiterals"
          language="java"
          message="Use long literal instead of casting a literal to long"
          class="net.sourceforge.pmd.lang.rule.XPathRule">
        <description>
A long literal can be obtained by appending 'L' to the number. i.e. 10L
        </description>
        <priority>3</priority>
        <properties>
            <property name="xpath">
                <value>
<![CDATA[
//CastExpression[
./Type/PrimitiveType[@Image='long']
and
./PrimaryExpression/PrimaryPrefix/Literal[@IntLiteral = true()]
]
]]>
                </value>
            </property>
            <property name="version" value="2.0"/>
        </properties>
        <example>
<![CDATA[
  public class Foo {
    public void bar() {
        long l = (long) 123;
        l = (long) 23;
        l = (long) (12);
    }
}
]]>
        </example>
    </rule>



    <rule name="MapToBooleanWithConditionalOperator"
          language="java"
          message="The value from the ternary operator is redundant."
          class="net.sourceforge.pmd.lang.rule.XPathRule">
        <description>
The ternary operator is unnecessary use the result of a boolean expression instead use.
i.e.
boolean mybool = !a;        //instead of mybool = !a      ? true : false;
boolean mybool = b.bar();   //instead of mybool = b.bar() ? true : false;
        </description>
        <priority>3</priority>
        <properties>
            <property name="xpath">
                <value>
<![CDATA[
//ConditionalExpression[
    ./Expression/PrimaryExpression/PrimaryPrefix/Literal/BooleanLiteral
    and
    ./PrimaryExpression[2]/PrimaryPrefix/Literal/BooleanLiteral
  ]
]]>
                </value>
            </property>
        </properties>
        <example>
<![CDATA[
public class Foo {
  public void bar() {
      boolean t1 = a ? true : false;
      boolean t2 = a ? false : true;
      boolean t3 = foo() ? false : true;

      if(foo() ? false : true){
        //do something
      }
  }
}
]]>
        </example>
    </rule>



    <rule name="ComparisonWithBooleanLiteral"
          language="java"
          message="Comparisons with boolean literals are unnecessary."
          class="net.sourceforge.pmd.lang.rule.XPathRule">
        <description>
A comparison with a boolean literal such as 'bool == true' serves no
purpose (true == true is true, false == true is false), directly use the value of the variable instead.
i.e.
if(bool){
  //do something
}
        </description>
        <priority>3</priority>
        <properties>
            <property name="xpath">
                <value>
<![CDATA[
//Expression[
  ./EqualityExpression//Literal/BooleanLiteral
  or
  ./ConditionalOrExpression//Literal/BooleanLiteral
  or
  ./ConditionalAndExpression//Literal/BooleanLiteral
  or
  ./ExclusiveOrExpression//Literal/BooleanLiteral
]
]]>
                </value>
            </property>
        </properties>
        <example>
<![CDATA[
public class Foo {
  public void bar() {
    boolean bool = true;

    //unnecessary comparison with true
    if(bool == true){
      //do something
    }

    //unnecessary comparison with false
    if(bool != false){
      //do something
    }
}
]]>
        </example>
    </rule>



    <rule name="MapToBooleanWithIf"
          language="java"
          message="Directly utilise the condition to obtain the boolean value needed."
          class="net.sourceforge.pmd.lang.rule.XPathRule">
        <description>
For a return an if statement is not necessary if we already have a
boolean expression on which we decide what to return.
i.e.

public boolean bar(boolean bool){
  return !bool || foo();
}

NOT:
public boolean bar(boolean bool){
  if(!bool || foo())
    return true;
  else
    return false;
}

there is also the case to select a value:

boolean b;
if (CONDITION) {
  b = true;
} else {
  b = false;
}

the same result could be obtained by using:
b = CONDITION;
        </description>
        <priority>4</priority>
        <properties>
            <property name="xpath">
                <value>
<![CDATA[
//IfStatement[count(./Statement) = 2][
  ./Statement[1][count(./Block/BlockStatement) = 1]/Block/BlockStatement/Statement/ReturnStatement[
    .//AllocationExpression/Arguments[(@ArgumentCount = 1) = true()]/ArgumentList/Expression//BooleanLiteral
    or
    ./Expression/PrimaryExpression/PrimaryPrefix/Literal/BooleanLiteral
  ]
  and
  ./Statement[2][count(./Block/BlockStatement) = 1]/Block/BlockStatement/Statement/ReturnStatement[
    .//AllocationExpression/Arguments[(@ArgumentCount = 1) = true()]/ArgumentList/Expression//BooleanLiteral
    or
    ./Expression/PrimaryExpression/PrimaryPrefix/Literal/BooleanLiteral
  ]
]|
//IfStatement[count(./Statement) = 2][
  ./Statement[1]/Block[count(./BlockStatement) = 1]//StatementExpression[
    ./AssignmentOperator
    and
    ./Expression/PrimaryExpression/PrimaryPrefix/Literal/BooleanLiteral
  ]
  and
  ./Statement[2]/Block[count(./BlockStatement) = 1]//StatementExpression[
    ./AssignmentOperator
    and
    ./Expression/PrimaryExpression/PrimaryPrefix/Literal/BooleanLiteral
  ]
]
]]>
                </value>
            </property>
            <property name="version" value="1.0"/>
        </properties>
        <example>
<![CDATA[
public class Foo {

  public boolean bar() {
    if(false)
      return false;
    if(false){
      return true;
    }
    //...
  }

  public Foo foo(){

    if(false)
      return new Foo(false);

    if(false){
      return new Foo(true);
    }

    //...
  }

  public Foo foo2(boolean s){

    boolean b;
    if(!s){
      b = false;
    }
    else{
      b = true;
    }

    //...
  }
}
]]>
        </example>
    </rule>



    <rule name="StringLiteralNoObject"
      language="java"
      message="Directly utilise the String literal instead of passing it as an argument to a String constructor."
      class="net.sourceforge.pmd.lang.rule.XPathRule">
    <description>
A String can be constructed directly by assigning a String literal to a String reference just as if it were a primitive type (it isn't). i.e. String s = "Hello World";
    </description>
    <priority>3</priority>
    <properties>
        <property name="xpath">
            <value>
<![CDATA[
//AllocationExpression[
  ./ClassOrInterfaceType[@Image='String']
  and
  ./Arguments/ArgumentList/Expression/PrimaryExpression/PrimaryPrefix/Literal[@StringLiteral = true()]
]
]]>
            </value>
        </property>
        <property name="version" value="2.0"/>
    </properties>
        <example>
<![CDATA[
public class Foo {
    public boolean bar() {

      String s = new String("Hello World");

      s = new String("Hello Again");
      new String("This too.");

    }
}
]]>
        </example>
    </rule>



    <rule name="CharNotNumeric"
          language="java"
          message="A char can be utilised as if it were a number."
          class="net.sourceforge.pmd.lang.rule.XPathRule">
        <description>
A char can be utilised as if it were a number, but to print a char the expression must be of type char.
i.e.
System.out.println( 'b' );               // b
System.out.println( 'a' + 1 );           // 98 (the expression type is int)
System.out.println( (char) ('a' + 1) );  // b  (the expression type is char)
System.out.println( (char) ('b' - 1) );  // a  (the expression type is char)
        </description>
        <priority>3</priority>
        <properties>
            <property name="xpath">
                <value>
<![CDATA[
//CastExpression[
	./Type/PrimitiveType[@Image='int']
	and
	./PrimaryExpression/PrimaryPrefix//Literal[@CharLiteral = true()]
]
]]>
                </value>
            </property>
            <property name="version" value="2.0"/>
        </properties>
        <example>
    <![CDATA[
public class Foo {
    public boolean bar() {

      char c = (char) (( (int)'a' ) +2);

      int i = (int) 'i';

    }
}
    ]]>
        </example>
    </rule>

<!-- only with pmd:check, compilation fails otherwise-->
    <rule name="LargeIntegerLong"
          language="java"
          message="A large integer literal bigger than 32bit is not a Long"
          class="net.sourceforge.pmd.lang.rule.XPathRule">
        <description>
A value bigger than the maximal value of int is not represented as long but overflows.
Use a literal of type long instead
i.e.
long l = 1234567876543245678L;
        </description>
        <priority>1</priority>
        <properties>
            <property name="xpath">
                <value>
<![CDATA[
//PrimaryPrefix/Literal[@IntLiteral = true()][
	@ValueAsDouble > 2147483647 or
	@ValueAsDouble < -2147483648
]
]]>
                </value>
            </property>
            <property name="version" value="2.0"/>
        </properties>
        <example>
<![CDATA[
public class Foo {
    public boolean bar() {
        long i = 1234567876543245678;
    }
}
]]>
        </example>
    </rule>



    <rule name="ReturnCall"
          language="java"
          message="The return statement does not need parenthesis as it is NOT a method."
          class="net.sourceforge.pmd.lang.rule.XPathRule">
        <description>
The return statement only requires an expression, no parenthesis are needed.
i.e.
return 3;
        </description>
        <priority>4</priority>
        <properties>
            <property name="xpath">
                <value>
<![CDATA[
//ReturnStatement/Expression/PrimaryExpression/PrimaryPrefix/Expression
]]>
                </value>
            </property>
            <property name="version" value="2.0"/>
        </properties>
        <example>
<![CDATA[
public class Foo {
    public boolean bar() {
        return (true);
    }
}
]]>
        </example>
    </rule>



    <rule name="ThisExistsInStaticMethod"
          language="java"
          message=" this only exists in instance methods."
          class="net.sourceforge.pmd.lang.rule.XPathRule">
        <description>
"this" only exists in instance methods as it the refers to the object on which the
method is called.

        </description>
        <priority>2</priority>
        <properties>
            <property name="xpath">
                <value>
<![CDATA[
//MethodDeclaration[@Static = true()]//PrimaryPrefix[@ThisModifier = true()]
]]>
                </value>
            </property>
            <property name="version" value="2.0"/>
        </properties>
        <example>
<![CDATA[
public class Foo {
    int a;
    public Foo void bar() {
        this.a = 12;
        return this;
    }
}
]]>
        </example>
    </rule>



    <rule name="DeferredReturn"
          language="java"
          message="A return statement immediately returns."
          class="net.sourceforge.pmd.lang.rule.XPathRule">
        <description>
When a return statement is reached it immediately return unless inside a try in a
try{}catch(){}finally{}.

        </description>
        <priority>2</priority>
        <properties>
            <property name="xpath">
                <value>
<![CDATA[
//Block/BlockStatement[position() != last()]/Statement/ReturnStatement
]]>
                </value>
            </property>
            <property name="version" value="2.0"/>
        </properties>
        <example>
<![CDATA[
public class Foo {
    int i = 0;
    public int bar() {
        return i;
        i++;
    }
}
]]>
        </example>
    </rule>



    <rule name="ForEachVariableIsElement"
          language="java"
          message="The variable of a for-each statement is not an alias for the element, therefore a for-each cannot be used to initialise the elements of an array."
          class="net.sourceforge.pmd.lang.rule.XPathRule">
        <description>
The variable of a for-each statement can be seen as a new variable to which it's
assigned the value currently in the collection, it's therefore a distinct variable
not an alias for the variable holding the element.

        </description>
        <priority>2</priority>
        <properties>
            <property name="xpath">
                <value>
<![CDATA[
//ForStatement[
	  .//LocalVariableDeclaration//VariableDeclaratorId/@Image =
    .//StatementExpression[ ./AssignmentOperator ]/PrimaryExpression[not(./PrimarySuffix)]//Name/@Image
  and
    not( .//LocalVariableDeclaration//VariableInitializer )
]
]]>
                </value>
            </property>
            <property name="version" value="2.0"/>
        </properties>
        <example>
<![CDATA[
public class Foo {
    int i = 0;
    public int bar() {
      Foo[] foos = new Foo[10];
      for (Foo foo : foos) {
        foo = new Foo();        //violation
      }

      int[] ages = new int[10];
      for (int age : ages) {
        age = 19;               //violation
      }
    }
}
]]>
        </example>
    </rule>


</ruleset>