Class Polygon

  • All Implemented Interfaces:
    java.io.Serializable, java.lang.Cloneable, IGeometry, IRotatable<Polygon>, IScalable<Polygon>, IShape, ITranslatable<Polygon>

    public class Polygon
    extends java.lang.Object
    implements IShape
    Represents the geometric shape of a convex polygon. Note that while all manipulations (e.g. within shrink, expand) within this class are based on double precision, all comparisons (e.g. within contains, intersects, equals, etc.) are based on a limited precision (with an accuracy defined within PrecisionUtils) to compensate for rounding effects.
    See Also:
    Serialized Form
    • Constructor Summary

      Constructors 
      Constructor Description
      Polygon​(double... coordinates)
      Constructs a new Polygon from a even-numbered sequence of coordinates.
      Polygon​(Point... points)
      Constructs a new Polygon from the given sequence of Point s.
    • Method Summary

      All Methods Instance Methods Concrete Methods 
      Modifier and Type Method Description
      java.lang.Object clone()
      Overridden with public visibility as recommended within Cloneable .
      boolean contains​(double x, double y)
      Checks whether the point that is represented by its x- and y-coordinates is contained within this Polygon.
      boolean contains​(IGeometry g)
      Tests whether the given IGeometry is fully contained by this IShape.
      boolean contains​(Line line)
      Checks whether the given Line is fully contained within this Polygon.
      boolean contains​(Point p)
      Returns whether the given Point is contained within this IGeometry.
      boolean contains​(Polygon p)
      Checks whether the given Polygon is fully contained within this Polygon.
      boolean contains​(Polyline p)
      Tests if the given Polyline p is contained in this Polygon.
      boolean contains​(Rectangle r)
      Checks whether the given Rectangle is fully contained within this Polygon.
      boolean equals​(java.lang.Object o)  
      boolean equals​(Point[] points)
      Checks whether this Polygon and the one that is indirectly given via the given array of points are regarded to be equal.
      double getArea()
      Computes the area of this Polygon.
      Rectangle getBounds()
      Returns the smallest Rectangle fully enclosing this IGeometry.
      Point getCentroid()
      Computes the centroid of this AbstractPointListBasedGeometry.
      double[] getCoordinates()
      Returns a double array which represents the sequence of coordinates of the Points that make up this AbstractPointListBasedGeometry.
      Polygon getCopy()
      Returns a copy of this Polygon, which is made up by the same points.
      Polyline getOutline()
      Returns an ICurve representing the outline of this IShape .
      Line[] getOutlineSegments()
      Returns a sequence of Lines, representing the segments that are obtained by linking each two successive point of this Polygon (including the last and the first one).
      Point[] getPoints()
      Returns a copy of the Points that make up this AbstractPointListBasedGeometry.
      T getRotatedCCW​(Angle alpha)
      Rotates the calling object by specified Angle counter-clock-wise (CCW) around its center Point.
      T getRotatedCCW​(Angle angle, double cx, double cy)
      Rotates the calling object by the specified Angle counter-clock-wise (CCW) around the specified center Point (cx, cy).
      T getRotatedCCW​(Angle alpha, Point center)
      Rotates the calling object by the specified Angle counter-clock-wise (CCW) around the specified center Point.
      T getRotatedCW​(Angle alpha)
      Rotates the calling object by specified Angle clock-wise (CW) around its center Point.
      T getRotatedCW​(Angle angle, double cx, double cy)
      Rotates the calling object by the specified Angle clock-wise (CW) around the specified center Point (cx, cy).
      T getRotatedCW​(Angle alpha, Point center)
      Rotates the calling object by the specified Angle clock-wise (CW) around the specified center Point.
      T getScaled​(double factor)
      Scales a copy of the calling object by the given factor relative to its center Point.
      T getScaled​(double factorX, double factorY)
      Scales a copy of the calling object by the given factors relative to its center Point.
      T getScaled​(double factor, double cx, double cy)
      Scales a copy of the calling object by the given factor relative to the given center Point (cx, cy).
      T getScaled​(double fx, double fy, double cx, double cy)
      Scales a copy of the calling object by the given factors relative to the given center Point (cx, cy).
      T getScaled​(double factorX, double factorY, Point center)
      Scales a copy of the calling object by the given factors relative to the given center Point.
      T getScaled​(double factor, Point center)
      Scales a copy of the calling object by the given factor relative to the given center Point.
      double getSignedArea()
      Computes the signed area of this Polygon.
      Polygon getTransformed​(AffineTransform t)
      Default implementation returning a transformed Path representation of this IGeometry.
      T getTranslated​(double dx, double dy)
      Translates a copy of this object by the given values in x and y direction.
      T getTranslated​(Point pt)
      Translates a copy of this object by the given Point.
      Polygon[] getTriangulation()
      Naive, recursive ear-clipping algorithm to triangulate this simple, planar Polygon.
      int hashCode()  
      T rotateCCW​(Angle alpha)
      Rotates this AbstractPointListBasedGeometry counter-clockwise (CCW) by the given Angle around its centroid (see getCentroid()).
      T rotateCCW​(Angle alpha, double cx, double cy)
      Rotates this AbstractPointListBasedGeometry counter-clockwise (CCW) by the given Angle around the Point specified by the passed-in x and y coordinates.
      T rotateCCW​(Angle alpha, Point center)
      Rotates this AbstractPointListBasedGeometry counter-clockwise (CCW) by the given Angle around the given Point.
      T rotateCW​(Angle alpha)
      Rotates this AbstractPointListBasedGeometry clockwise (CW) by the given Angle around its centroid (see getCentroid()).
      T rotateCW​(Angle alpha, double cx, double cy)
      Rotates this AbstractPointListBasedGeometry clockwise (CW) by the given Angle around the Point specified by the passed-in x and y coordinates.
      T rotateCW​(Angle alpha, Point center)
      Rotates this AbstractPointListBasedGeometry clockwise (CW) by the given Angle around the given Point.
      T scale​(double factor)
      Scales the calling object by the given factor relative to its center Point.
      T scale​(double fx, double fy)
      Scales the calling object by the given factors relative to the given center Point.
      T scale​(double factor, double cx, double cy)
      Scales the calling object by the given factor relative to the given center Point (cx, cy).
      T scale​(double fx, double fy, double cx, double cy)
      Scales the calling object by the given factors relative to the given center Point (cx, cy).
      T scale​(double fx, double fy, Point center)
      Scales the calling object by the given factors relative to the given center Point.
      T scale​(double factor, Point center)
      Scales the calling object by the given factor relative to the given center Point.
      Path toPath()
      Converts this IGeometry into a Path representation.
      java.lang.String toString()  
      boolean touches​(IGeometry g)
      Returns true if the input IGeometry touches this IGeometry, i.e. there is at least one common point.
      T translate​(double dx, double dy)
      Translates the object by the given values in x and y direction.
      T translate​(Point p)
      Translates the object by the given Point.
      • Methods inherited from class java.lang.Object

        finalize, getClass, notify, notifyAll, wait, wait, wait
    • Constructor Detail

      • Polygon

        public Polygon​(double... coordinates)
        Constructs a new Polygon from a even-numbered sequence of coordinates.
        Parameters:
        coordinates - an alternating, even-numbered sequence of x and y coordinates, representing the Points from which the Polygon is to be created
        See Also:
        Polygon(Point...)
      • Polygon

        public Polygon​(Point... points)
        Constructs a new Polygon from the given sequence of Point s. The Polygon that is created will be automatically closed, i.e. it will not only contain a segment between succeeding points of the sequence but as well back from the last to the first point.
        Parameters:
        points - a sequence of points, from which the Polygon is to be created.
    • Method Detail

      • contains

        public boolean contains​(double x,
                                double y)
        Checks whether the point that is represented by its x- and y-coordinates is contained within this Polygon.
        Parameters:
        x - the x-coordinate of the point to test
        y - the y-coordinate of the point to test
        Returns:
        true if the point represented by its coordinates if contained within this Polygon, false otherwise
      • contains

        public boolean contains​(IGeometry g)
        Description copied from interface: IShape
        Tests whether the given IGeometry is fully contained by this IShape.
        Specified by:
        contains in interface IShape
        Parameters:
        g - The IGeometry to test for containment
        Returns:
        true if the given IGeometry is fully contained by this IShape, false otherwise.
      • contains

        public boolean contains​(Line line)
        Checks whether the given Line is fully contained within this Polygon.
        Parameters:
        line - The Line to test for containment
        Returns:
        true if the given Line is fully contained, false otherwise
      • contains

        public boolean contains​(Polygon p)
        Checks whether the given Polygon is fully contained within this Polygon.
        Parameters:
        p - The Polygon to test for containment
        Returns:
        true if the given Polygon is fully contained, false otherwise.
      • contains

        public boolean contains​(Polyline p)
        Tests if the given Polyline p is contained in this Polygon.
        Parameters:
        p - The Polyline to test for containment.
        Returns:
        true if it is contained, false otherwise
      • contains

        public boolean contains​(Rectangle r)
        Checks whether the given Rectangle is fully contained within this Polygon.
        Parameters:
        r - the Rectangle to test for containment
        Returns:
        true if the given Rectangle is fully contained, false otherwise.
      • equals

        public boolean equals​(java.lang.Object o)
        Overrides:
        equals in class java.lang.Object
      • equals

        public boolean equals​(Point[] points)
        Checks whether this Polygon and the one that is indirectly given via the given array of points are regarded to be equal. The Polygons will be regarded equal, if they are characterized by the same segments. As a Polygon is always regarded to be closed, the list of points may not have to correspond in each index value, they may also be shifted by a certain offset. Moreover, the vertices of two equally Polygons may be reverted in order.
        Parameters:
        points - an array of Point characterizing a Polygon to be checked for equality
        Returns:
        true if the sequence of points that characterize this Polygon and the Polygon indirectly given via the array of points are regarded to form the same segments.
      • getArea

        public double getArea()
        Computes the area of this Polygon.
        Returns:
        the area of this Polygon
      • getCopy

        public Polygon getCopy()
        Returns a copy of this Polygon, which is made up by the same points.
        Specified by:
        getCopy in interface IGeometry
        Returns:
        a new Polygon with an identical set of points.
      • getOutlineSegments

        public Line[] getOutlineSegments()
        Returns a sequence of Lines, representing the segments that are obtained by linking each two successive point of this Polygon (including the last and the first one).
        Specified by:
        getOutlineSegments in interface IShape
        Returns:
        an array of Lines, representing the segments that make up this Polygon
      • getSignedArea

        public double getSignedArea()
        Computes the signed area of this Polygon. The sign of the area is negative for counter clockwise ordered vertices. It is positive for clockwise ordered vertices.
        Returns:
        the signed area of this Polygon
      • getTriangulation

        public Polygon[] getTriangulation()
        Naive, recursive ear-clipping algorithm to triangulate this simple, planar Polygon.
        Returns:
        triangulation Polygons (triangles)
      • toString

        public java.lang.String toString()
        Overrides:
        toString in class java.lang.Object
      • getCentroid

        public Point getCentroid()
        Computes the centroid of this AbstractPointListBasedGeometry. The centroid is the "center of gravity", i.e. assuming a Polygon is spanned by the Points of this AbstractPointListBasedGeometry and it is made of a material of constant density, then it is in a balanced state, if you put it on a pin that is placed exactly on its centroid.
        Returns:
        the center Point (or centroid) of this AbstractPointListBasedGeometry
      • getCoordinates

        public final double[] getCoordinates()
        Returns a double array which represents the sequence of coordinates of the Points that make up this AbstractPointListBasedGeometry.
        Returns:
        an array that alternately contains the x and y coordinates of this AbstractPointListBasedGeometry's points
      • getPoints

        public final Point[] getPoints()
        Returns a copy of the Points that make up this AbstractPointListBasedGeometry.
        Returns:
        an array of Points representing the Points that make up this AbstractPointListBasedGeometry
      • getRotatedCCW

        public T getRotatedCCW​(Angle alpha)
        Description copied from interface: IRotatable
        Rotates the calling object by specified Angle counter-clock-wise (CCW) around its center Point. Does not necessarily return an object of the same type.
        Specified by:
        getRotatedCCW in interface IRotatable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        alpha - rotation Angle
        Returns:
        an IGeometry representing the result of the rotation
      • getRotatedCCW

        public T getRotatedCCW​(Angle angle,
                               double cx,
                               double cy)
        Description copied from interface: IRotatable
        Rotates the calling object by the specified Angle counter-clock-wise (CCW) around the specified center Point (cx, cy). Does not necessarily return an object of the same type.
        Specified by:
        getRotatedCCW in interface IRotatable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        angle - rotation Angle
        cx - x-coordinate of the relative Point for the rotation
        cy - y-coordinate of the relative Point for the rotation
        Returns:
        an IGeometry representing the result of the rotation
      • getRotatedCCW

        public T getRotatedCCW​(Angle alpha,
                               Point center)
        Description copied from interface: IRotatable
        Rotates the calling object by the specified Angle counter-clock-wise (CCW) around the specified center Point. Does not necessarily return an object of the same type.
        Specified by:
        getRotatedCCW in interface IRotatable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        alpha - rotation Angle
        center - relative Point for the rotation
        Returns:
        an IGeometry representing the result of the rotation
      • getRotatedCW

        public T getRotatedCW​(Angle alpha)
        Description copied from interface: IRotatable
        Rotates the calling object by specified Angle clock-wise (CW) around its center Point. Does not necessarily return an object of the same type.
        Specified by:
        getRotatedCW in interface IRotatable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        alpha - rotation Angle
        Returns:
        an IGeometry representing the result of the rotation
      • getRotatedCW

        public T getRotatedCW​(Angle angle,
                              double cx,
                              double cy)
        Description copied from interface: IRotatable
        Rotates the calling object by the specified Angle clock-wise (CW) around the specified center Point (cx, cy). Does not necessarily return an object of the same type.
        Specified by:
        getRotatedCW in interface IRotatable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        angle - rotation Angle
        cx - x-coordinate of the relative Point for the rotation
        cy - y-coordinate of the relative Point for the rotation
        Returns:
        an IGeometry representing the result of the rotation
      • getRotatedCW

        public T getRotatedCW​(Angle alpha,
                              Point center)
        Description copied from interface: IRotatable
        Rotates the calling object by the specified Angle clock-wise (CW) around the specified center Point. Does not necessarily return an object of the same type.
        Specified by:
        getRotatedCW in interface IRotatable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        alpha - rotation Angle
        center - relative Point for the rotation
        Returns:
        an IGeometry representing the result of the rotation
      • getScaled

        public T getScaled​(double factor)
        Description copied from interface: IScalable
        Scales a copy of the calling object by the given factor relative to its center Point.
        Specified by:
        getScaled in interface IScalable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        factor - scale-factor
        Returns:
        the new, scaled object
      • getScaled

        public T getScaled​(double factorX,
                           double factorY)
        Description copied from interface: IScalable
        Scales a copy of the calling object by the given factors relative to its center Point.
        Specified by:
        getScaled in interface IScalable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        factorX - x-scale-factor
        factorY - y-scale-factor
        Returns:
        the new, scaled object
      • getScaled

        public T getScaled​(double factor,
                           double cx,
                           double cy)
        Description copied from interface: IScalable
        Scales a copy of the calling object by the given factor relative to the given center Point (cx, cy).
        Specified by:
        getScaled in interface IScalable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        factor - scale-factor
        cx - x-coordinate of the relative Point for the scaling
        cy - y-coordinate of the relative Point for the scaling
        Returns:
        the new, scaled object
      • getScaled

        public T getScaled​(double fx,
                           double fy,
                           double cx,
                           double cy)
        Description copied from interface: IScalable
        Scales a copy of the calling object by the given factors relative to the given center Point (cx, cy).
        Specified by:
        getScaled in interface IScalable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        fx - x-scale-factor
        fy - y-scale-factor
        cx - x-coordinate of the relative Point for the scaling
        cy - y-coordinate of the relative Point for the scaling
        Returns:
        the new, scaled object
      • getScaled

        public T getScaled​(double factorX,
                           double factorY,
                           Point center)
        Description copied from interface: IScalable
        Scales a copy of the calling object by the given factors relative to the given center Point.
        Specified by:
        getScaled in interface IScalable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        factorX - x-scale-factor
        factorY - y-scale-factor
        center - relative Point for the scaling
        Returns:
        the new, scaled object
      • getScaled

        public T getScaled​(double factor,
                           Point center)
        Description copied from interface: IScalable
        Scales a copy of the calling object by the given factor relative to the given center Point.
        Specified by:
        getScaled in interface IScalable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        factor - scale-factor
        center - relative Point for the scaling
        Returns:
        the new, scaled object
      • getTranslated

        public T getTranslated​(double dx,
                               double dy)
        Description copied from interface: ITranslatable
        Translates a copy of this object by the given values in x and y direction.
        Specified by:
        getTranslated in interface ITranslatable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        dx - x-translation
        dy - y-translation
        Returns:
        a new, translated object
      • getTranslated

        public T getTranslated​(Point pt)
        Description copied from interface: ITranslatable
        Translates a copy of this object by the given Point.
        Specified by:
        getTranslated in interface ITranslatable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        pt - translation Point
        Returns:
        a new, translated object
      • rotateCCW

        public T rotateCCW​(Angle alpha)
        Rotates this AbstractPointListBasedGeometry counter-clockwise (CCW) by the given Angle around its centroid (see getCentroid()).
        Parameters:
        alpha - the rotation Angle
        Returns:
        this for convenience
        See Also:
        rotateCCW(Angle, Point)
      • rotateCCW

        public T rotateCCW​(Angle alpha,
                           double cx,
                           double cy)
        Rotates this AbstractPointListBasedGeometry counter-clockwise (CCW) by the given Angle around the Point specified by the passed-in x and y coordinates.
        Parameters:
        alpha - the rotation Angle
        cx - the x coordinate of the Point to rotate around
        cy - the y coordinate of the Point to rotate around
        Returns:
        this for convenience
        See Also:
        rotateCCW(Angle, Point)
      • rotateCCW

        public T rotateCCW​(Angle alpha,
                           Point center)
        Rotates this AbstractPointListBasedGeometry counter-clockwise (CCW) by the given Angle around the given Point. The rotation is done by
        1. translating this AbstractPointListBasedGeometry by the negated Point center
        2. rotating each Point of this AbstractPointListBasedGeometry counter-clockwise by the given Angle
        3. translating this AbstractPointListBasedGeometry back by the Point center
        Parameters:
        alpha - the rotation Angle
        center - the Point to rotate around
        Returns:
        this for convenience
      • rotateCW

        public T rotateCW​(Angle alpha)
        Rotates this AbstractPointListBasedGeometry clockwise (CW) by the given Angle around its centroid (see getCentroid()).
        Parameters:
        alpha - the rotation Angle
        Returns:
        this for convenience
        See Also:
        rotateCW(Angle, Point)
      • rotateCW

        public T rotateCW​(Angle alpha,
                          double cx,
                          double cy)
        Rotates this AbstractPointListBasedGeometry clockwise (CW) by the given Angle around the Point specified by the passed-in x and y coordinates.
        Parameters:
        alpha - the rotation Angle
        cx - the x coordinate of the Point to rotate around
        cy - the y coordinate of the Point to rotate around
        Returns:
        this for convenience
        See Also:
        rotateCW(Angle, Point)
      • rotateCW

        public T rotateCW​(Angle alpha,
                          Point center)
        Rotates this AbstractPointListBasedGeometry clockwise (CW) by the given Angle around the given Point. The rotation is done by
        1. translating this AbstractPointListBasedGeometry by the negated Point center
        2. rotating each Point of this AbstractPointListBasedGeometry clockwise by the given Angle
        3. translating this AbstractPointListBasedGeometry back by the Point center
        Parameters:
        alpha - the rotation Angle
        center - the Point to rotate around
        Returns:
        this for convenience
      • scale

        public T scale​(double factor)
        Description copied from interface: IScalable
        Scales the calling object by the given factor relative to its center Point.
        Specified by:
        scale in interface IScalable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        factor - scale-factor
        Returns:
        this for convenience
      • scale

        public T scale​(double fx,
                       double fy)
        Description copied from interface: IScalable
        Scales the calling object by the given factors relative to the given center Point.
        Specified by:
        scale in interface IScalable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        fx - x-scale-factor
        fy - y-scale-factor
        Returns:
        this for convenience
      • scale

        public T scale​(double factor,
                       double cx,
                       double cy)
        Description copied from interface: IScalable
        Scales the calling object by the given factor relative to the given center Point (cx, cy).
        Specified by:
        scale in interface IScalable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        factor - scale-factor
        cx - x-coordinate of the relative Point for the scaling
        cy - y-coordinate of the relative Point for the scaling
        Returns:
        this for convenience
      • scale

        public T scale​(double fx,
                       double fy,
                       double cx,
                       double cy)
        Description copied from interface: IScalable
        Scales the calling object by the given factors relative to the given center Point (cx, cy).
        Specified by:
        scale in interface IScalable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        fx - x-scale-factor
        fy - y-scale-factor
        cx - x-coordinate of the relative Point for the scaling
        cy - y-coordinate of the relative Point for the scaling
        Returns:
        this for convenience
      • scale

        public T scale​(double fx,
                       double fy,
                       Point center)
        Description copied from interface: IScalable
        Scales the calling object by the given factors relative to the given center Point.
        Specified by:
        scale in interface IScalable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        fx - x-scale-factor
        fy - y-scale-factor
        center - relative Point for the scaling
        Returns:
        this for convenience
      • scale

        public T scale​(double factor,
                       Point center)
        Description copied from interface: IScalable
        Scales the calling object by the given factor relative to the given center Point.
        Specified by:
        scale in interface IScalable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        factor - scale-factor
        center - relative Point for the scaling
        Returns:
        this for convenience
      • translate

        public T translate​(double dx,
                           double dy)
        Description copied from interface: ITranslatable
        Translates the object by the given values in x and y direction.
        Specified by:
        translate in interface ITranslatable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        dx - x-translation
        dy - y-translation
        Returns:
        this for convenience
      • translate

        public T translate​(Point p)
        Description copied from interface: ITranslatable
        Translates the object by the given Point.
        Specified by:
        translate in interface ITranslatable<T extends org.eclipse.gef.geometry.planar.AbstractPointListBasedGeometry<?>>
        Parameters:
        p - translation Point
        Returns:
        this for convenience
      • clone

        public java.lang.Object clone()
        Overridden with public visibility as recommended within Cloneable .
        Overrides:
        clone in class java.lang.Object
      • hashCode

        public final int hashCode()
        Overrides:
        hashCode in class java.lang.Object
        See Also:
        Object.hashCode()