elx/config/pmd/category/java/performance.xml

<?xml version="1.0"?>

<ruleset name="Performance"
         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>
        Rules that flag suboptimal code.
    </description>

    <rule name="AddEmptyString"
          language="java"
          since="4.0"
          message="Do not add empty strings"
          class="net.sourceforge.pmd.lang.rule.XPathRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#addemptystring">
        <description>
            The conversion of literals to strings by concatenating them with empty strings is inefficient.
            It is much better to use one of the type-specific toString() methods instead.
        </description>
        <priority>3</priority>
        <properties>
            <property name="xpath">
                <value>
                    <![CDATA[
//AdditiveExpression/PrimaryExpression/PrimaryPrefix/Literal[@Image='""']
]]>
                </value>
            </property>
        </properties>
        <example>
            <![CDATA[
String s = "" + 123;                // inefficient
String t = Integer.toString(456);   // preferred approach
]]>
        </example>
    </rule>

    <rule name="AppendCharacterWithChar"
          since="3.5"
          message="Avoid appending characters as strings in StringBuffer.append."
          class="net.sourceforge.pmd.lang.java.rule.performance.AppendCharacterWithCharRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#appendcharacterwithchar">
        <description>
            Avoid concatenating characters as strings in StringBuffer/StringBuilder.append methods.
        </description>
        <priority>3</priority>
        <example>
            <![CDATA[
StringBuffer sb = new StringBuffer();
sb.append("a");     // avoid this

StringBuffer sb = new StringBuffer();
sb.append('a');     // use this instead
]]>
        </example>
    </rule>

    <rule name="AvoidArrayLoops"
          language="java"
          since="3.5"
          message="System.arraycopy is more efficient"
          class="net.sourceforge.pmd.lang.rule.XPathRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#avoidarrayloops">
        <description>
            Instead of manually copying data between two arrays, use the efficient Arrays.copyOf or System.arraycopy method instead.
        </description>
        <priority>3</priority>
        <properties>
            <property name="xpath">
                <value>
                    <![CDATA[
//Statement[(ForStatement or WhileStatement) and
count(*//AssignmentOperator[@Image = '='])=1
and
*/Statement
[
./Block/BlockStatement/Statement/StatementExpression/PrimaryExpression
/PrimaryPrefix/Name/../../PrimarySuffix/Expression
[(PrimaryExpression or AdditiveExpression) and count
(.//PrimaryPrefix/Name)=1]//PrimaryPrefix/Name/@Image
and
./Block/BlockStatement/Statement/StatementExpression/Expression/PrimaryExpression
/PrimaryPrefix/Name/../../PrimarySuffix[count
(..//PrimarySuffix)=1]/Expression[(PrimaryExpression
or AdditiveExpression) and count(.//PrimaryPrefix/Name)=1]
//PrimaryPrefix/Name/@Image
]]
]]>
                </value>
            </property>
        </properties>
        <example>
            <![CDATA[
public class Test {
    public void bar() {
        int[] a = new int[10];
        int[] b = new int[10];
        for (int i=0;i<10;i++) {
            b[i]=a[i];
        }

        int[] c = new int[10];
        // this will trigger the rule
        for (int i=0;i<10;i++) {
            b[i]=a[c[i]];
        }
    }
}
]]>
        </example>
    </rule>

    <rule name="AvoidFileStream"
          since="6.0.0"
          message="Avoid instantiating FileInputStream, FileOutputStream, FileReader, or FileWriter"
          language="java"
          minimumLanguageVersion="1.7"
          class="net.sourceforge.pmd.lang.rule.XPathRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#avoidfilestream">
        <description>
            The FileInputStream and FileOutputStream classes contains a finalizer method which will cause garbage
            collection pauses.
            See [JDK-8080225](https://bugs.openjdk.java.net/browse/JDK-8080225) for details.

            The FileReader and FileWriter constructors instantiate FileInputStream and FileOutputStream,
            again causing garbage collection issues while finalizer methods are called.

            * Use `Files.newInputStream(Paths.get(fileName))` instead of `new FileInputStream(fileName)`.
            * Use `Files.newOutputStream(Paths.get(fileName))` instead of `new FileOutputStream(fileName)`.
            * Use `Files.newBufferedReader(Paths.get(fileName))` instead of `new FileReader(fileName)`.
            * Use `Files.newBufferedWriter(Paths.get(fileName))` instead of `new FileWriter(fileName)`.

            Please note, that the `java.nio` API does not throw a `FileNotFoundException` anymore, instead
            it throws a `NoSuchFileException`. If your code dealt explicitly with a `FileNotFoundException`,
            then this needs to be adjusted. Both exceptions are subclasses of `IOException`, so catching
            that one covers both.
        </description>
        <priority>1</priority>
        <properties>
            <property name="xpath">
                <value>
                    <![CDATA[
//PrimaryPrefix/AllocationExpression/ClassOrInterfaceType[
       pmd-java:typeIs('java.io.FileInputStream')
    or pmd-java:typeIs('java.io.FileOutputStream')
    or pmd-java:typeIs('java.io.FileReader')
    or pmd-java:typeIs('java.io.FileWriter')
  ]
]]>
                </value>
            </property>
        </properties>
        <example>
            <![CDATA[
    // these instantiations cause garbage collection pauses, even if properly closed

    FileInputStream fis = new FileInputStream(fileName);
    FileOutputStream fos = new FileOutputStream(fileName);
    FileReader fr = new FileReader(fileName);
    FileWriter fw = new FileWriter(fileName);

    // the following instantiations help prevent Garbage Collection pauses, no finalization

    try(InputStream is = Files.newInputStream(Paths.get(fileName))) {
    }
    try(OutputStream os = Files.newOutputStream(Paths.get(fileName))) {
    }
    try(BufferedReader br = Files.newBufferedReader(Paths.get(fileName), StandardCharsets.UTF_8)) {
    }
    try(BufferedWriter wr = Files.newBufferedWriter(Paths.get(fileName), StandardCharsets.UTF_8)) {
    }
]]>
        </example>
    </rule>

    <rule name="AvoidInstantiatingObjectsInLoops"
          since="2.2"
          message="Avoid instantiating new objects inside loops"
          class="net.sourceforge.pmd.lang.java.rule.performance.AvoidInstantiatingObjectsInLoopsRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#avoidinstantiatingobjectsinloops">
        <description>
            New objects created within loops should be checked to see if they can created outside them and reused.
        </description>
        <priority>3</priority>
        <example>
            <![CDATA[
public class Something {
    public static void main( String as[] ) {
        for (int i = 0; i < 10; i++) {
            Foo f = new Foo(); // Avoid this whenever you can it's really expensive
        }
    }
}
]]>
        </example>
    </rule>

    <rule name="AvoidUsingShortType"
          language="java"
          since="4.1"
          message="Do not use the short type"
          class="net.sourceforge.pmd.lang.rule.XPathRule"
          typeResolution="true"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#avoidusingshorttype">
        <description>
            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. Thus any storage gains found through use of the 'short' type may be offset by
            adverse impacts on performance.
        </description>
        <priority>1</priority>
        <properties>
            <property name="xpath">
                <value>
                    <![CDATA[
//FieldDeclaration/Type/PrimitiveType[@Image = 'short']
|
//ClassOrInterfaceBodyDeclaration[not(Annotation/MarkerAnnotation/Name[pmd-java:typeIs('java.lang.Override')])]
    /MethodDeclaration/ResultType/Type/PrimitiveType[@Image = 'short']
|
//ClassOrInterfaceBodyDeclaration[not(Annotation/MarkerAnnotation/Name[pmd-java:typeIs('java.lang.Override')])]
    /MethodDeclaration/MethodDeclarator/FormalParameters/FormalParameter/Type/PrimitiveType[@Image = 'short']
|
//LocalVariableDeclaration/Type/PrimitiveType[@Image = 'short']
|
//AnnotationMethodDeclaration/Type/PrimitiveType[@Image = 'short']
]]>
                </value>
            </property>
        </properties>
        <example>
            <![CDATA[
public class UsingShort {
   private short doNotUseShort = 0;

   public UsingShort() {
    short shouldNotBeUsed = 1;
    doNotUseShort += shouldNotBeUsed;
  }
}
]]>
        </example>
    </rule>

    <rule name="BigIntegerInstantiation"
          since="3.9"
          message="Don't create instances of already existing BigInteger and BigDecimal (ZERO, ONE, TEN)"
          class="net.sourceforge.pmd.lang.java.rule.performance.BigIntegerInstantiationRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#bigintegerinstantiation">
        <description>
            Don't create instances of already existing BigInteger (BigInteger.ZERO, BigInteger.ONE) and
            for Java 1.5 onwards, BigInteger.TEN and BigDecimal (BigDecimal.ZERO, BigDecimal.ONE, BigDecimal.TEN)
        </description>
        <priority>3</priority>
        <example>
            <![CDATA[
BigInteger bi = new BigInteger(1);       // reference BigInteger.ONE instead
BigInteger bi2 = new BigInteger("0");    // reference BigInteger.ZERO instead
BigInteger bi3 = new BigInteger(0.0);    // reference BigInteger.ZERO instead
BigInteger bi4;
bi4 = new BigInteger(0);                 // reference BigInteger.ZERO instead
]]>
        </example>
    </rule>

    <rule name="BooleanInstantiation"
          since="1.2"
          message="Avoid instantiating Boolean objects; reference Boolean.TRUE or Boolean.FALSE or call Boolean.valueOf() instead."
          class="net.sourceforge.pmd.lang.java.rule.performance.BooleanInstantiationRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#booleaninstantiation">
        <description>
            Avoid instantiating Boolean objects; you can reference Boolean.TRUE, Boolean.FALSE, or call Boolean.valueOf() instead.
            Note that new Boolean() is deprecated since JDK 9 for that reason.
        </description>
        <priority>2</priority>
        <example>
            <![CDATA[
Boolean bar = new Boolean("true");        // unnecessary creation, just reference Boolean.TRUE;
Boolean buz = Boolean.valueOf(false);    // ...., just reference Boolean.FALSE;
]]>
        </example>
    </rule>

    <rule name="ByteInstantiation"
          language="java"
          since="4.0"
          message="Avoid instantiating Byte objects. Call Byte.valueOf() instead"
          class="net.sourceforge.pmd.lang.rule.XPathRule"
          typeResolution="true"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#byteinstantiation">
        <description>
            Calling new Byte() causes memory allocation that can be avoided by the static Byte.valueOf().
            It makes use of an internal cache that recycles earlier instances making it more memory efficient.
            Note that new Byte() is deprecated since JDK 9 for that reason.
        </description>
        <priority>2</priority>
        <properties>
            <property name="xpath">
                <value>
                    <![CDATA[
//AllocationExpression
[not (ArrayDimsAndInits)
and ClassOrInterfaceType[pmd-java:typeIs('java.lang.Byte')]]
]]>
                </value>
            </property>
        </properties>
        <example>
            <![CDATA[
public class Foo {
    private Byte i = new Byte(0); // change to Byte i = Byte.valueOf(0);
}
]]>
        </example>
    </rule>

    <rule name="ConsecutiveAppendsShouldReuse"
          language="java"
          since="5.1"
          message="StringBuffer (or StringBuilder).append is called consecutively without reusing the target variable."
          class="net.sourceforge.pmd.lang.java.rule.performance.ConsecutiveAppendsShouldReuseRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#consecutiveappendsshouldreuse">
        <description>
            Consecutive calls to StringBuffer/StringBuilder .append should be chained, reusing the target object. This can improve the performance
            by producing a smaller bytecode, reducing overhead and improving inlining. A complete analysis can be found [here](https://github.com/pmd/pmd/issues/202#issuecomment-274349067)
        </description>
        <priority>3</priority>
        <example>
            <![CDATA[
String foo = " ";

StringBuffer buf = new StringBuffer();
buf.append("Hello"); // poor
buf.append(foo);
buf.append("World");

StringBuffer buf = new StringBuffer();
buf.append("Hello").append(foo).append("World"); // good
]]>
        </example>
    </rule>

    <rule name="ConsecutiveLiteralAppends"
          since="3.5"
          message="StringBuffer (or StringBuilder).append is called {0} consecutive times with literals. Use a single append with a single combined String."
          class="net.sourceforge.pmd.lang.java.rule.performance.ConsecutiveLiteralAppendsRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#consecutiveliteralappends">
        <description>
            Consecutively calling StringBuffer/StringBuilder.append(...) with literals should be avoided.
            Since the literals are constants, they can already be combined into a single String literal and this String
            can be appended in a single method call.
        </description>
        <priority>3</priority>
        <example>
            <![CDATA[
StringBuilder buf = new StringBuilder();
buf.append("Hello").append(" ").append("World");    // poor
buf.append("Hello World");                          // good

buf.append('h').append('e').append('l').append('l').append('o'); // poor
buf.append("hello");                                             // good

buf.append(1).append('m');  // poor
buf.append("1m");           // good
]]>
        </example>
    </rule>

    <rule name="InefficientEmptyStringCheck"
          since="3.6"
          message="String.trim().length() == 0 / String.trim().isEmpty() is an inefficient way to validate a blank String."
          class="net.sourceforge.pmd.lang.java.rule.performance.InefficientEmptyStringCheckRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#inefficientemptystringcheck">
        <description>
            <![CDATA[
String.trim().length() == 0 (or String.trim().isEmpty() for the same reason) is an inefficient
way to check if a String is really blank, as it creates a new String object just to check its size.
Consider creating a static function that loops through a string, checking Character.isWhitespace()
on each character and returning false if a non-whitespace character is found. A Smarter code to
check for an empty string would be:

```java
private boolean checkTrimEmpty(String str) {
    for(int i = 0; i < str.length(); i++) {
        if(!Character.isWhitespace(str.charAt(i))) {
            return false;
        }
    }
    return true;
}
```

You can refer to Apache's StringUtils#isBlank (in commons-lang),
Spring's StringUtils#hasText (in the Spring framework) or Google's
CharMatcher#whitespace (in Guava) for existing implementations (some might
include the check for != null).
]]>
        </description>
        <priority>3</priority>
        <example>
            <![CDATA[
public void bar(String string) {
    if (string != null && string.trim().length() > 0) {
        doSomething();
    }
}
]]>
        </example>
    </rule>

    <rule name="InefficientStringBuffering"
          since="3.4"
          message="Avoid concatenating nonliterals in a StringBuffer/StringBuilder constructor or append()."
          class="net.sourceforge.pmd.lang.java.rule.performance.InefficientStringBufferingRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#inefficientstringbuffering">
        <description>
            Avoid concatenating non-literals in a StringBuffer constructor or append() since intermediate buffers will
            need to be be created and destroyed by the JVM.
        </description>
        <priority>3</priority>
        <example>
            <![CDATA[
// Avoid this, two buffers are actually being created here
StringBuffer sb = new StringBuffer("tmp = "+System.getProperty("java.io.tmpdir"));

// do this instead
StringBuffer sb = new StringBuffer("tmp = ");
sb.append(System.getProperty("java.io.tmpdir"));
]]>
        </example>
    </rule>

    <rule name="InsufficientStringBufferDeclaration"
          since="3.6"
          message="StringBuffer constructor is initialized with size {0}, but has at least {1} characters appended."
          class="net.sourceforge.pmd.lang.java.rule.performance.InsufficientStringBufferDeclarationRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#insufficientstringbufferdeclaration">
        <description>
            Failing to pre-size a StringBuffer or StringBuilder properly could cause it to re-size many times
            during runtime. This rule attempts to determine the total number the characters that are actually
            passed into StringBuffer.append(), but represents a best guess "worst case" scenario. An empty
            StringBuffer/StringBuilder constructor initializes the object to 16 characters. This default
            is assumed if the length of the constructor can not be determined.
        </description>
        <priority>3</priority>
        <example>
            <![CDATA[
StringBuffer bad = new StringBuffer();
bad.append("This is a long string that will exceed the default 16 characters");

StringBuffer good = new StringBuffer(41);
good.append("This is a long string, which is pre-sized");
]]>
        </example>
    </rule>

    <rule name="IntegerInstantiation"
          language="java"
          since="3.5"
          message="Avoid instantiating Integer objects. Call Integer.valueOf() instead."
          class="net.sourceforge.pmd.lang.rule.XPathRule"
          typeResolution="true"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#integerinstantiation">
        <description>
            Calling new Integer() causes memory allocation that can be avoided by the static Integer.valueOf().
            It makes use of an internal cache that recycles earlier instances making it more memory efficient.
            Note that new Integer() is deprecated since JDK 9 for that reason.
        </description>
        <priority>2</priority>
        <properties>
            <property name="xpath">
                <value>
                    <![CDATA[
//AllocationExpression
  [not (ArrayDimsAndInits)
   and ClassOrInterfaceType[pmd-java:typeIs('java.lang.Integer')]]
]]>
                </value>
            </property>
        </properties>
        <example>
            <![CDATA[
public class Foo {
    private Integer i = new Integer(0); // change to Integer i = Integer.valueOf(0);
}
]]>
        </example>
    </rule>

    <rule name="LongInstantiation"
          language="java"
          since="4.0"
          message="Avoid instantiating Long objects.Call Long.valueOf() instead"
          class="net.sourceforge.pmd.lang.rule.XPathRule"
          typeResolution="true"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#longinstantiation">
        <description>
            Calling new Long() causes memory allocation that can be avoided by the static Long.valueOf().
            It makes use of an internal cache that recycles earlier instances making it more memory efficient.
            Note that new Long() is deprecated since JDK 9 for that reason.
        </description>
        <priority>2</priority>
        <properties>
            <property name="xpath">
                <value>
                    <![CDATA[
//AllocationExpression
[not (ArrayDimsAndInits)
and ClassOrInterfaceType[pmd-java:typeIs('java.lang.Long')]]
]]>
                </value>
            </property>
        </properties>
        <example>
            <![CDATA[
public class Foo {
    private Long i = new Long(0); // change to Long i = Long.valueOf(0);
}
]]>
        </example>
    </rule>

    <rule name="OptimizableToArrayCall"
          language="java"
          since="1.8"
          minimumLanguageVersion="1.6"
          message="This call to Collection.toArray() may be optimizable"
          class="net.sourceforge.pmd.lang.rule.XPathRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#optimizabletoarraycall">
        <description>
            Calls to a collection's `toArray(E[])` method should specify a target array of zero size. This allows the JVM
            to optimize the memory allocation and copying as much as possible.

            Previous versions of this rule (pre PMD 6.0.0) suggested the opposite, but current JVM implementations
            perform always better, when they have full control over the target array. And allocation an array via
            reflection is nowadays as fast as the direct allocation.

            See also [Arrays of Wisdom of the Ancients](https://shipilev.net/blog/2016/arrays-wisdom-ancients/)

            Note: If you don't need an array of the correct type, then the simple `toArray()` method without an array
            is faster, but returns only an array of type `Object[]`.
        </description>
        <priority>3</priority>
        <properties>
            <property name="xpath">
                <value>
                    <![CDATA[
//PrimaryExpression
[PrimaryPrefix/Name[ends-with(@Image, 'toArray')]]
[
PrimarySuffix/Arguments/ArgumentList/Expression
 /PrimaryExpression/PrimaryPrefix/AllocationExpression
 /ArrayDimsAndInits/Expression/PrimaryExpression/PrimaryPrefix[not(Literal[@Image='0'])]
]
]]>
                </value>
            </property>
        </properties>
        <example>
            <![CDATA[
List<Foo> foos = getFoos();

// much better; this one allows the jvm to allocate an array of the correct size and effectively skip
// the zeroing, since each array element will be overridden anyways
Foo[] fooArray = foos.toArray(new Foo[0]);

// inefficient, the array needs to be zeroed out by the jvm before it is handed over to the toArray method
Foo[] fooArray = foos.toArray(new Foo[foos.size()]);
]]>
        </example>
    </rule>

    <rule name="RedundantFieldInitializer"
          language="java"
          since="5.0"
          message="Avoid using redundant field initializer for ''${variableName}''"
          class="net.sourceforge.pmd.lang.java.rule.performance.RedundantFieldInitializerRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#redundantfieldinitializer">
        <description>
            Java will initialize fields with known default values so any explicit initialization of those same defaults
            is redundant and results in a larger class file (approximately three additional bytecode instructions per field).
        </description>
        <priority>3</priority>
        <example>
            <![CDATA[
public class C {
    boolean b   = false;    // examples of redundant initializers
    byte by     = 0;
    short s     = 0;
    char c      = 0;
    int i       = 0;
    long l      = 0;

    float f     = .0f;    // all possible float literals
    doable d    = 0d;     // all possible double literals
    Object o    = null;

    MyClass mca[] = null;
    int i1 = 0, ia1[] = null;

    class Nested {
        boolean b = false;
    }
}
]]>
        </example>
    </rule>

    <rule name="SimplifyStartsWith"
          language="java"
          since="3.1"
          message="This call to String.startsWith can be rewritten using String.charAt(0)"
          class="net.sourceforge.pmd.lang.rule.XPathRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#simplifystartswith">
        <description>
            Since it passes in a literal of length 1, calls to (string).startsWith can be rewritten using (string).charAt(0)
            at the expense of some readability.
        </description>
        <priority>3</priority>
        <properties>
            <property name="xpath">
                <value>
                    <![CDATA[
//PrimaryExpression
 [PrimaryPrefix/Name
  [ends-with(@Image, '.startsWith')] or PrimarySuffix[@Image='startsWith']]
 [PrimarySuffix/Arguments/ArgumentList
  /Expression/PrimaryExpression/PrimaryPrefix
  /Literal
   [string-length(@Image)=3]
   [starts-with(@Image, '"')]
   [ends-with(@Image, '"')]
 ]
]]>
                </value>
            </property>
        </properties>
        <example>
            <![CDATA[
public class Foo {

    boolean checkIt(String x) {
        return x.startsWith("a");   // suboptimal
    }

    boolean fasterCheckIt(String x) {
        return x.charAt(0) == 'a';  // faster approach
    }
}
]]>
        </example>
    </rule>

    <rule name="ShortInstantiation"
          language="java"
          since="4.0"
          message="Avoid instantiating Short objects. Call Short.valueOf() instead"
          class="net.sourceforge.pmd.lang.rule.XPathRule"
          typeResolution="true"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#shortinstantiation">
        <description>
            Calling new Short() causes memory allocation that can be avoided by the static Short.valueOf().
            It makes use of an internal cache that recycles earlier instances making it more memory efficient.
            Note that new Short() is deprecated since JDK 9 for that reason.
        </description>
        <priority>2</priority>
        <properties>
            <property name="xpath">
                <value>
                    <![CDATA[
//AllocationExpression
[not (ArrayDimsAndInits)
and ClassOrInterfaceType[pmd-java:typeIs('java.lang.Short')]]
]]>
                </value>
            </property>
        </properties>
        <example>
            <![CDATA[
public class Foo {
    private Short i = new Short(0); // change to Short i = Short.valueOf(0);
}
]]>
        </example>
    </rule>

    <rule name="StringInstantiation"
          since="1.0"
          message="Avoid instantiating String objects; this is usually unnecessary."
          class="net.sourceforge.pmd.lang.java.rule.performance.StringInstantiationRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#stringinstantiation">
        <description>
            Avoid instantiating String objects; this is usually unnecessary since they are immutable and can be safely shared.
        </description>
        <priority>2</priority>
        <example>
            <![CDATA[
private String bar = new String("bar"); // just do a String bar = "bar";
]]>
        </example>
    </rule>

    <rule name="StringToString"
          since="1.0"
          message="Avoid calling toString() on String objects; this is unnecessary."
          class="net.sourceforge.pmd.lang.java.rule.performance.StringToStringRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#stringtostring">
        <description>
            Avoid calling toString() on objects already known to be string instances; this is unnecessary.
        </description>
        <priority>3</priority>
        <example>
            <![CDATA[
private String baz() {
    String bar = "howdy";
    return bar.toString();
}
]]>
        </example>
    </rule>

    <rule name="TooFewBranchesForASwitchStatement"
          language="java"
          since="4.2"
          class="net.sourceforge.pmd.lang.rule.XPathRule"
          message="A switch with less than three branches is inefficient, use a 'if statement' instead."
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#toofewbranchesforaswitchstatement">
        <description>
            Switch statements are intended to be used to support complex branching behaviour. Using a switch for only a few
            cases is ill-advised, since switches are not as easy to understand as if-then statements. In these cases use the
            if-then statement to increase code readability.
        </description>
        <priority>3</priority>
        <properties>
            <property name="minimumNumberCaseForASwitch" type="Integer" description="Minimum number of branches for a switch" min="1" max="100" value="3"/>
            <property name="xpath">
                <value>
                    <![CDATA[
//SwitchStatement[
    (count(.//SwitchLabel) < $minimumNumberCaseForASwitch)
]
]]>
                </value>
            </property>
        </properties>
        <example>
            <![CDATA[
// With a minimumNumberCaseForASwitch of 3
public class Foo {
    public void bar() {
        switch (condition) {
            case ONE:
                instruction;
                break;
            default:
                break; // not enough for a 'switch' stmt, a simple 'if' stmt would have been more appropriate
        }
    }
}
]]>
        </example>
    </rule>

    <rule name="UnnecessaryWrapperObjectCreation"
          since="3.8"
          message="Unnecessary wrapper object creation"
          class="net.sourceforge.pmd.lang.java.rule.performance.UnnecessaryWrapperObjectCreationRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#unnecessarywrapperobjectcreation">
        <description>
            Most wrapper classes provide static conversion methods that avoid the need to create intermediate objects
            just to create the primitive forms. Using these avoids the cost of creating objects that also need to be
            garbage-collected later.
        </description>
        <priority>3</priority>
        <example>
            <![CDATA[
public int convert(String s) {
    int i, i2;

    i = Integer.valueOf(s).intValue();  // this wastes an object
    i = Integer.parseInt(s);            // this is better

    i2 = Integer.valueOf(i).intValue(); // this wastes an object
    i2 = i;                             // this is better

    String s3 = Integer.valueOf(i2).toString(); // this wastes an object
    s3 = Integer.toString(i2);                  // this is better

    return i2;
}
]]>
        </example>
    </rule>

    <rule name="UseArrayListInsteadOfVector"
          language="java"
          since="3.0"
          message="Use ArrayList instead of Vector"
          class="net.sourceforge.pmd.lang.rule.XPathRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#usearraylistinsteadofvector">
        <description>
            ArrayList is a much better Collection implementation than Vector if thread-safe operation is not required.
        </description>
        <priority>3</priority>
        <properties>
            <property name="xpath">
                <value>
                    <![CDATA[
//CompilationUnit[count(ImportDeclaration) = 0 or count(ImportDeclaration/Name[@Image='java.util.Vector']) > 0]
  //AllocationExpression/ClassOrInterfaceType
    [@Image='Vector' or @Image='java.util.Vector']
]]>
                </value>
            </property>
        </properties>
        <example>
            <![CDATA[
public class SimpleTest extends TestCase {
    public void testX() {
    Collection c1 = new Vector();
    Collection c2 = new ArrayList();    // achieves the same with much better performance
    }
}
]]>
        </example>
    </rule>

    <rule name="UseArraysAsList"
          language="java"
          since="3.5"
          message="Use asList instead of tight loops"
          class="net.sourceforge.pmd.lang.rule.XPathRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#usearraysaslist">
        <description>
            <![CDATA[
The java.util.Arrays class has a "asList" method that should be used when you want to create a new List from
an array of objects. It is faster than executing a loop to copy all the elements of the array one by one.

Note that the result of Arrays.asList() is backed by the specified array,
changes in the returned list will result in the array to be modified.
For that reason, it is not possible to add new elements to the returned list of Arrays.asList() (UnsupportedOperationException).
You must use new ArrayList<>(Arrays.asList(...)) if that is inconvenient for you (e.g. because of concurrent access).
]]>
        </description>
        <priority>3</priority>
        <properties>
            <property name="xpath">
                <value>
                    <![CDATA[
//Statement[
    (ForStatement) and (ForStatement//VariableInitializer//Literal[@IntLiteral='true' and @Image='0']) and (count(.//IfStatement)=0)
   ]
   //StatementExpression[
    PrimaryExpression/PrimaryPrefix/Name[
     substring-before(@Image,'.add') = ancestor::MethodDeclaration//LocalVariableDeclaration[
      ./Type//ClassOrInterfaceType[
       @Image = 'Collection' or
       @Image = 'List' or @Image='ArrayList'
      ]
     ]
     /VariableDeclarator/VariableDeclaratorId[
      count(..//AllocationExpression/ClassOrInterfaceType[
       @Image="ArrayList"
      ]
      )=1
     ]/@Image
    ]
   and
   PrimaryExpression/PrimarySuffix/Arguments/ArgumentList/Expression/PrimaryExpression/PrimaryPrefix/Name
   [
     @Image = ancestor::MethodDeclaration[1]//LocalVariableDeclaration/VariableDeclarator/VariableDeclaratorId[@ArrayType="true"]/@Image
     or
     @Image = ancestor::MethodDeclaration[1]//FormalParameter/VariableDeclaratorId/@Image
   ]
   /../..[count(.//PrimarySuffix)
   =1]/PrimarySuffix/Expression/PrimaryExpression/PrimaryPrefix
   /Name
   ]
]]>
                </value>
            </property>
        </properties>
        <example>
            <![CDATA[
public class Test {
    public void foo(Integer[] ints) {
        // could just use Arrays.asList(ints)
        List<Integer> l= new ArrayList<>(100);
        for (int i=0; i< 100; i++) {
            l.add(ints[i]);
        }
        for (int i=0; i< 100; i++) {
            l.add(a[i].toString()); // won't trigger the rule
        }
    }
}
]]>
        </example>
    </rule>

    <rule name="UseIndexOfChar"
          since="3.5"
          message="String.indexOf(char) is faster than String.indexOf(String)."
          class="net.sourceforge.pmd.lang.java.rule.performance.UseIndexOfCharRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#useindexofchar">
        <description>
            Use String.indexOf(char) when checking for the index of a single character; it executes faster.
        </description>
        <priority>3</priority>
        <example>
            <![CDATA[
String s = "hello world";
// avoid this
if (s.indexOf("d") {}
// instead do this
if (s.indexOf('d') {}
]]>
        </example>
    </rule>

    <rule name="UselessStringValueOf"
          since="3.8"
          message="No need to call String.valueOf to append to a string."
          class="net.sourceforge.pmd.lang.java.rule.performance.UselessStringValueOfRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#uselessstringvalueof">
        <description>
            No need to call String.valueOf to append to a string; just use the valueOf() argument directly.
        </description>
        <priority>3</priority>
        <example>
            <![CDATA[
public String convert(int i) {
    String s;
    s = "a" + String.valueOf(i);    // not required
    s = "a" + i;                    // preferred approach
    return s;
}
]]>
        </example>
    </rule>

    <rule name="UseStringBufferForStringAppends"
          since="3.1"
          message="Prefer StringBuilder (non-synchronized) or StringBuffer (synchronized) over += for concatenating strings"
          class="net.sourceforge.pmd.lang.java.rule.performance.UseStringBufferForStringAppendsRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#usestringbufferforstringappends">
        <description>
            The use of the '+=' operator for appending strings causes the JVM to create and use an internal StringBuffer.
            If a non-trivial number of these concatenations are being used then the explicit use of a StringBuilder or
            threadsafe StringBuffer is recommended to avoid this.
        </description>
        <priority>3</priority>
        <example>
            <![CDATA[
public class Foo {
    void bar() {
        String a;
        a = "foo";
        a += " bar";
        // better would be:
        // StringBuilder a = new StringBuilder("foo");
        // a.append(" bar");
    }
}
]]>
        </example>
    </rule>

    <rule name="UseStringBufferLength"
          since="3.4"
          message="This is an inefficient use of StringBuffer.toString; call StringBuffer.length instead."
          class="net.sourceforge.pmd.lang.java.rule.performance.UseStringBufferLengthRule"
          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#usestringbufferlength">
        <description>
            Use StringBuffer.length() to determine StringBuffer length rather than using StringBuffer.toString().equals("")
            or StringBuffer.toString().length() == ...
        </description>
        <priority>3</priority>
        <example>
            <![CDATA[
StringBuffer sb = new StringBuffer();

if (sb.toString().equals("")) {}        // inefficient

if (sb.length() == 0) {}                // preferred
]]>
        </example>
    </rule>


    <!--
    other optimization/performance should be like avoiding
    "" + int
    or "" + (int) i
    and String.valueOf(int)

    and using Integer.toString(int)

    IntegerToStringShouldBeUsed
    LongToStringShouldBeUsed
    BooleanToStringShouldBeUsed
    -->

</ruleset>