https://openjdk.java.net/ 
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package java.lang.reflect;

import java.lang.annotation.*;
import java.util.HashMap;
import java.util.Map;
import java.util.Objects;
import sun.reflect.annotation.AnnotationSupport;


Information about method parameters. A Parameter provides information about method parameters, including its name and modifiers. It also provides an alternate means of obtaining attributes for the parameter. 
Since:1.8
/**
 * Information about method parameters.
 *
 * A {@code Parameter} provides information about method parameters,
 * including its name and modifiers.  It also provides an alternate
 * means of obtaining attributes for the parameter.
 *
 * @since 1.8
 */
public final class Parameter implements AnnotatedElement {

    private final String name;
    private final int modifiers;
    private final Executable executable;
    private final int index;

    
Package-private constructor for Parameter. If method parameter data is present in the classfile, then the JVM creates Parameter objects directly. If it is absent, however, then Executable uses this constructor to synthesize them. 
Params:
  • name – The name of the parameter.
  • modifiers – The modifier flags for the parameter.
  • executable – The executable which defines this parameter.
  • index – The index of the parameter.
/**
     * Package-private constructor for {@code Parameter}.
     *
     * If method parameter data is present in the classfile, then the
     * JVM creates {@code Parameter} objects directly.  If it is
     * absent, however, then {@code Executable} uses this constructor
     * to synthesize them.
     *
     * @param name The name of the parameter.
     * @param modifiers The modifier flags for the parameter.
     * @param executable The executable which defines this parameter.
     * @param index The index of the parameter.
     */
    Parameter(String name,
              int modifiers,
              Executable executable,
              int index) {
        this.name = name;
        this.modifiers = modifiers;
        this.executable = executable;
        this.index = index;
    }

    
Compares based on the executable and the index.

Params:
  • obj – The object to compare.
Returns:Whether or not this is equal to the argument.
/**
     * Compares based on the executable and the index.
     *
     * @param obj The object to compare.
     * @return Whether or not this is equal to the argument.
     */
    @Override
    public boolean equals(Object obj) {
        if(obj instanceof Parameter) {
            Parameter other = (Parameter)obj;
            return (other.executable.equals(executable) &&
                    other.index == index);
        }
        return false;
    }

    
Returns a hash code based on the executable's hash code and the
index.

Returns:A hash code based on the executable's hash code.
/**
     * Returns a hash code based on the executable's hash code and the
     * index.
     *
     * @return A hash code based on the executable's hash code.
     */
    @Override
    public int hashCode() {
        return executable.hashCode() ^ index;
    }

    
Returns true if the parameter has a name according to the class file; returns false otherwise. Whether a parameter has a name is determined by the MethodParameters attribute of the method which declares the parameter. 
Returns:true if and only if the parameter has a name according
to the class file.
/**
     * Returns true if the parameter has a name according to the class
     * file; returns false otherwise. Whether a parameter has a name
     * is determined by the {@literal MethodParameters} attribute of
     * the method which declares the parameter.
     *
     * @return true if and only if the parameter has a name according
     * to the class file.
     */
    public boolean isNamePresent() {
        return executable.hasRealParameterData() && name != null;
    }

    
Returns a string describing this parameter.  The format is the
modifiers for the parameter, if any, in canonical order as
recommended by The Java Language
Specification, followed by the fully-qualified type of
the parameter (excluding the last [] if the parameter is
variable arity), followed by "..." if the parameter is variable
arity, followed by a space, followed by the name of the
parameter.

Returns:A string representation of the parameter and associated
information.
/**
     * Returns a string describing this parameter.  The format is the
     * modifiers for the parameter, if any, in canonical order as
     * recommended by <cite>The Java Language
     * Specification</cite>, followed by the fully-qualified type of
     * the parameter (excluding the last [] if the parameter is
     * variable arity), followed by "..." if the parameter is variable
     * arity, followed by a space, followed by the name of the
     * parameter.
     *
     * @return A string representation of the parameter and associated
     * information.
     */
    @Override
    public String toString() {
        final StringBuilder sb = new StringBuilder();
        final Type type = getParameterizedType();
        final String typename = type.getTypeName();

        sb.append(Modifier.toString(getModifiers()));

        if(0 != modifiers)
            sb.append(' ');

        if(isVarArgs())
            sb.append(typename.replaceFirst("\\[\\]$", "..."));
        else
            sb.append(typename);

        sb.append(' ');
        sb.append(getName());

        return sb.toString();
    }

    
Return the Executable which declares this parameter. 
Returns:The Executable declaring this parameter.
/**
     * Return the {@code Executable} which declares this parameter.
     *
     * @return The {@code Executable} declaring this parameter.
     */
    public Executable getDeclaringExecutable() {
        return executable;
    }

    
Get the modifier flags for this the parameter represented by this Parameter object. 
Returns:The modifier flags for this parameter.
/**
     * Get the modifier flags for this the parameter represented by
     * this {@code Parameter} object.
     *
     * @return The modifier flags for this parameter.
     */
    public int getModifiers() {
        return modifiers;
    }

    
Returns the name of the parameter. If the parameter's name is present, then this method returns the name provided by the class file. Otherwise, this method synthesizes a name of the form argN, where N is the index of the parameter in the descriptor of the method which declares the parameter. 
Returns:The name of the parameter, either provided by the class
        file or synthesized if the class file does not provide
        a name.
/**
     * Returns the name of the parameter.  If the parameter's name is
     * {@linkplain #isNamePresent() present}, then this method returns
     * the name provided by the class file. Otherwise, this method
     * synthesizes a name of the form argN, where N is the index of
     * the parameter in the descriptor of the method which declares
     * the parameter.
     *
     * @return The name of the parameter, either provided by the class
     *         file or synthesized if the class file does not provide
     *         a name.
     */
    public String getName() {
        // Note: empty strings as parameter names are now outlawed.
        // The .isEmpty() is for compatibility with current JVM
        // behavior.  It may be removed at some point.
        if(name == null || name.isEmpty())
            return "arg" + index;
        else
            return name;
    }

    // Package-private accessor to the real name field.
    String getRealName() {
        return name;
    }

    
Returns a Type object that identifies the parameterized type for the parameter represented by this Parameter object. 
Returns:a Type object identifying the parameterized type of the parameter represented by this object
/**
     * Returns a {@code Type} object that identifies the parameterized
     * type for the parameter represented by this {@code Parameter}
     * object.
     *
     * @return a {@code Type} object identifying the parameterized
     * type of the parameter represented by this object
     */
    public Type getParameterizedType() {
        Type tmp = parameterTypeCache;
        if (null == tmp) {
            tmp = executable.getAllGenericParameterTypes()[index];
            parameterTypeCache = tmp;
        }

        return tmp;
    }

    private transient volatile Type parameterTypeCache;

    
Returns a Class object that identifies the declared type for the parameter represented by this Parameter object. 
Returns:a Class object identifying the declared type of the parameter represented by this object
/**
     * Returns a {@code Class} object that identifies the
     * declared type for the parameter represented by this
     * {@code Parameter} object.
     *
     * @return a {@code Class} object identifying the declared
     * type of the parameter represented by this object
     */
    public Class<?> getType() {
        Class<?> tmp = parameterClassCache;
        if (null == tmp) {
            tmp = executable.getParameterTypes()[index];
            parameterClassCache = tmp;
        }
        return tmp;
    }

    
Returns an AnnotatedType object that represents the use of a type to
specify the type of the formal parameter represented by this Parameter.

Returns:an AnnotatedType object representing the use of a type to specify the type of the formal parameter represented by this Parameter
/**
     * Returns an AnnotatedType object that represents the use of a type to
     * specify the type of the formal parameter represented by this Parameter.
     *
     * @return an {@code AnnotatedType} object representing the use of a type
     *         to specify the type of the formal parameter represented by this
     *         Parameter
     */
    public AnnotatedType getAnnotatedType() {
        // no caching for now
        return executable.getAnnotatedParameterTypes()[index];
    }

    private transient volatile Class<?> parameterClassCache;

    
Returns true if this parameter is implicitly declared in source code; returns false otherwise. 
Returns:true if and only if this parameter is implicitly
declared as defined by The Java Language
Specification.
/**
     * Returns {@code true} if this parameter is implicitly declared
     * in source code; returns {@code false} otherwise.
     *
     * @return true if and only if this parameter is implicitly
     * declared as defined by <cite>The Java Language
     * Specification</cite>.
     */
    public boolean isImplicit() {
        return Modifier.isMandated(getModifiers());
    }

    
Returns true if this parameter is neither implicitly nor explicitly declared in source code; returns false otherwise. 
@jls13.1 The Form of a Binary
Returns:true if and only if this parameter is a synthetic
construct as defined by
The Java Language Specification.
/**
     * Returns {@code true} if this parameter is neither implicitly
     * nor explicitly declared in source code; returns {@code false}
     * otherwise.
     *
     * @jls 13.1 The Form of a Binary
     * @return true if and only if this parameter is a synthetic
     * construct as defined by
     * <cite>The Java Language Specification</cite>.
     */
    public boolean isSynthetic() {
        return Modifier.isSynthetic(getModifiers());
    }

    
Returns true if this parameter represents a variable argument list; returns false otherwise. 
Returns:true if an only if this parameter represents a variable argument list.
/**
     * Returns {@code true} if this parameter represents a variable
     * argument list; returns {@code false} otherwise.
     *
     * @return {@code true} if an only if this parameter represents a
     * variable argument list.
     */
    public boolean isVarArgs() {
        return executable.isVarArgs() &&
            index == executable.getParameterCount() - 1;
    }


    
{@inheritDoc}

Note that any annotation returned by this method is a
declaration annotation.

Throws:
/**
     * {@inheritDoc}
     * <p>Note that any annotation returned by this method is a
     * declaration annotation.
     * @throws NullPointerException {@inheritDoc}
     */
    @Override
    public <T extends Annotation> T getAnnotation(Class<T> annotationClass) {
        Objects.requireNonNull(annotationClass);
        return annotationClass.cast(declaredAnnotations().get(annotationClass));
    }

    
{@inheritDoc}

Note that any annotations returned by this method are
declaration annotations.

Throws:
/**
     * {@inheritDoc}
     * <p>Note that any annotations returned by this method are
     * declaration annotations.
     *
     * @throws NullPointerException {@inheritDoc}
     */
    @Override
    public <T extends Annotation> T[] getAnnotationsByType(Class<T> annotationClass) {
        Objects.requireNonNull(annotationClass);

        return AnnotationSupport.getDirectlyAndIndirectlyPresent(declaredAnnotations(), annotationClass);
    }

    
{@inheritDoc}

Note that any annotations returned by this method are
declaration annotations.

/**
     * {@inheritDoc}
     * <p>Note that any annotations returned by this method are
     * declaration annotations.
     */
    @Override
    public Annotation[] getDeclaredAnnotations() {
        return executable.getParameterAnnotations()[index];
    }

    
{@inheritDoc}

Note that any annotation returned by this method is a
declaration annotation.

Throws:
/**
     * {@inheritDoc}
     * <p>Note that any annotation returned by this method is a
     * declaration annotation.
     *
     * @throws NullPointerException {@inheritDoc}
     */
    @Override
    public <T extends Annotation> T getDeclaredAnnotation(Class<T> annotationClass) {
        // Only annotations on classes are inherited, for all other
        // objects getDeclaredAnnotation is the same as
        // getAnnotation.
        return getAnnotation(annotationClass);
    }

    
{@inheritDoc}

Note that any annotations returned by this method are
declaration annotations.

Throws:
/**
     * {@inheritDoc}
     * <p>Note that any annotations returned by this method are
     * declaration annotations.
     *
     * @throws NullPointerException {@inheritDoc}
     */
    @Override
    public <T extends Annotation> T[] getDeclaredAnnotationsByType(Class<T> annotationClass) {
        // Only annotations on classes are inherited, for all other
        // objects getDeclaredAnnotations is the same as
        // getAnnotations.
        return getAnnotationsByType(annotationClass);
    }

    
{@inheritDoc}

Note that any annotations returned by this method are
declaration annotations.

/**
     * {@inheritDoc}
     * <p>Note that any annotations returned by this method are
     * declaration annotations.
     */
    @Override
    public Annotation[] getAnnotations() {
        return getDeclaredAnnotations();
    }

    private transient Map<Class<? extends Annotation>, Annotation> declaredAnnotations;

    private synchronized Map<Class<? extends Annotation>, Annotation> declaredAnnotations() {
        if(null == declaredAnnotations) {
            declaredAnnotations = new HashMap<>();
            for (Annotation a : getDeclaredAnnotations())
                declaredAnnotations.put(a.annotationType(), a);
        }
        return declaredAnnotations;
   }

}