ColorSpace

public abstract class ColorSpace
extends Object

java.lang.Object
   ↳ android.graphics.ColorSpace


A ColorSpace is used to identify a specific organization of colors. Each color space is characterized by a color model that defines how a color value is represented (for instance the RGB color model defines a color value as a triplet of numbers).

Each component of a color must fall within a valid range, specific to each color space, defined by getMinValue(int) and getMaxValue(int) This range is commonly \([0..1]\). While it is recommended to use values in the valid range, a color space always clamps input and output values when performing operations such as converting to a different color space.

Using color spaces

This implementation provides a pre-defined set of common color spaces described in the Named enum. To obtain an instance of one of the pre-defined color spaces, simply invoke get(android.graphics.ColorSpace.Named):

 ColorSpace sRgb = ColorSpace.get(ColorSpace.Named.SRGB);
 

The get(android.graphics.ColorSpace.Named) method always returns the same instance for a given name. Color spaces with an RGB color model can be safely cast to Rgb. Doing so gives you access to more APIs to query various properties of RGB color models: color gamut primaries, transfer functions, conversions to and from linear space, etc. Please refer to Rgb for more information.

The documentation of Named provides a detailed description of the various characteristics of each available color space.

Color space conversions

To allow conversion between color spaces, this implementation uses the CIE XYZ profile connection space (PCS). Color values can be converted to and from this PCS using toXyz(float[]) and fromXyz(float[]).

For color space with a non-RGB color model, the white point of the PCS must be the CIE standard illuminant D50. RGB color spaces use their native white point (D65 for sRGB for instance and must undergo chromatic adaptation as necessary.

Since the white point of the PCS is not defined for RGB color space, it is highly recommended to use the variants of the connect(android.graphics.ColorSpace, android.graphics.ColorSpace) method to perform conversions between color spaces. A color space can be manually adapted to a specific white point using adapt(android.graphics.ColorSpace, float[]). Please refer to the documentation of RGB color spaces for more information. Several common CIE standard illuminants are provided in this class as reference (see ILLUMINANT_D65 or ILLUMINANT_D50 for instance).

Here is an example of how to convert from a color space to another:

 // Convert from DCI-P3 to Rec.2020
 ColorSpace.Connector connector = ColorSpace.connect(
         ColorSpace.get(ColorSpace.Named.DCI_P3),
         ColorSpace.get(ColorSpace.Named.BT2020));

 float[] bt2020 = connector.transform(p3r, p3g, p3b);
 

You can easily convert to sRGB by omitting the second parameter:

 // Convert from DCI-P3 to sRGB
 ColorSpace.Connector connector = ColorSpace.connect(ColorSpace.get(ColorSpace.Named.DCI_P3));

 float[] sRGB = connector.transform(p3r, p3g, p3b);
 

Conversions also work between color spaces with different color models:

 // Convert from CIE L*a*b* (color model Lab) to Rec.709 (color model RGB)
 ColorSpace.Connector connector = ColorSpace.connect(
         ColorSpace.get(ColorSpace.Named.CIE_LAB),
         ColorSpace.get(ColorSpace.Named.BT709));
 

Color spaces and multi-threading

Color spaces and other related classes (Connector for instance) are immutable and stateless. They can be safely used from multiple concurrent threads.

Public static methods provided by this class, such as get(android.graphics.ColorSpace.Named) and connect(android.graphics.ColorSpace, android.graphics.ColorSpace), are also guaranteed to be thread-safe.

Summary

Nested classes

class ColorSpace.Connector

A connector transforms colors from a source color space to a destination color space. 

class ColorSpace.Rgb

An RGB color space is an additive color space using the RGB color model (a color is therefore represented by a tuple of 3 numbers). 

Constants

int MAX_ID

The maximum ID value a color space can have.

int MIN_ID

The minimum ID value a color space can have.

Fields

public static final float[] ILLUMINANT_A

Standard CIE 1931 2° illuminant A, encoded in xyY.

public static final float[] ILLUMINANT_B

Standard CIE 1931 2° illuminant B, encoded in xyY.

public static final float[] ILLUMINANT_C

Standard CIE 1931 2° illuminant C, encoded in xyY.

public static final float[] ILLUMINANT_D50

Standard CIE 1931 2° illuminant D50, encoded in xyY.

public static final float[] ILLUMINANT_D55

Standard CIE 1931 2° illuminant D55, encoded in xyY.

public static final float[] ILLUMINANT_D60

Standard CIE 1931 2° illuminant D60, encoded in xyY.

public static final float[] ILLUMINANT_D65

Standard CIE 1931 2° illuminant D65, encoded in xyY.

public static final float[] ILLUMINANT_D75

Standard CIE 1931 2° illuminant D75, encoded in xyY.

public static final float[] ILLUMINANT_E

Standard CIE 1931 2° illuminant E, encoded in xyY.

Public methods

static ColorSpace adapt(ColorSpace colorSpace, float[] whitePoint, ColorSpace.Adaptation adaptation)

Performs the chromatic adaptation of a color space from its native white point to the specified white point.

static ColorSpace adapt(ColorSpace colorSpace, float[] whitePoint)

Performs the chromatic adaptation of a color space from its native white point to the specified white point.

static float[] cctToXyz(int cct)

Computes the chromaticity coordinates of a specified correlated color temperature (CCT) on the Planckian locus.

static float[] chromaticAdaptation(ColorSpace.Adaptation adaptation, float[] srcWhitePoint, float[] dstWhitePoint)

Computes the chromatic adaptation transform from the specified source white point to the specified destination white point.

static ColorSpace.Connector connect(ColorSpace source, ColorSpace.RenderIntent intent)

Connects the specified color spaces to sRGB.

static ColorSpace.Connector connect(ColorSpace source)

Connects the specified color spaces to sRGB.

static ColorSpace.Connector connect(ColorSpace source, ColorSpace destination, ColorSpace.RenderIntent intent)

Connects two color spaces to allow conversion from the source color space to the destination color space.

static ColorSpace.Connector connect(ColorSpace source, ColorSpace destination)

Connects two color spaces to allow conversion from the source color space to the destination color space.

boolean equals(Object o)

Indicates whether some other object is "equal to" this one.

abstract float[] fromXyz(float[] v)

Converts tristimulus values from the CIE XYZ space to this color space's color model.

float[] fromXyz(float x, float y, float z)

Converts tristimulus values from the CIE XYZ space to this color space's color model.

static ColorSpace get(ColorSpace.Named name)

Returns an instance of ColorSpace identified by the specified name.

int getComponentCount()

Returns the number of components that form a color value according to this color space's color model.

int getDataSpace()

Retrieve the DataSpace value from a ColorSpace object.

static ColorSpace getFromDataSpace(int dataSpace)

Create a ColorSpace object using a DataSpace value.

int getId()

Returns the ID of this color space.

abstract float getMaxValue(int component)

Returns the maximum valid value for the specified component of this color space's color model.

abstract float getMinValue(int component)

Returns the minimum valid value for the specified component of this color space's color model.

ColorSpace.Model getModel()

Return the color model of this color space.

String getName()

Returns the name of this color space.

int hashCode()

Returns a hash code value for the object.

boolean isSrgb()

Indicates whether this color space is the sRGB color space or equivalent to the sRGB color space.

abstract boolean isWideGamut()

Returns whether this color space is a wide-gamut color space.

static ColorSpace match(float[] toXYZD50, ColorSpace.Rgb.TransferParameters function)

Returns a Named instance of ColorSpace that matches the specified RGB to CIE XYZ transform and transfer functions.

String toString()

Returns a string representation of the object.

float[] toXyz(float r, float g, float b)

Converts a color value from this color space's model to tristimulus CIE XYZ values.

abstract float[] toXyz(float[] v)

Converts a color value from this color space's model to tristimulus CIE XYZ values.

Inherited methods

Constants

MAX_ID

Added in API level 26
public static final int MAX_ID

The maximum ID value a color space can have.

See also:

Constant Value: 63 (0x0000003f)

MIN_ID

Added in API level 26
public static final int MIN_ID

The minimum ID value a color space can have.

See also:

Constant Value: -1 (0xffffffff)

Fields

ILLUMINANT_A

Added in API level 26
public static final float[] ILLUMINANT_A

Standard CIE 1931 2° illuminant A, encoded in xyY. This illuminant has a color temperature of 2856K.

ILLUMINANT_B

Added in API level 26
public static final float[] ILLUMINANT_B

Standard CIE 1931 2° illuminant B, encoded in xyY. This illuminant has a color temperature of 4874K.

ILLUMINANT_C

Added in API level 26
public static final float[] ILLUMINANT_C

Standard CIE 1931 2° illuminant C, encoded in xyY. This illuminant has a color temperature of 6774K.

ILLUMINANT_D50

Added in API level 26
public static final float[] ILLUMINANT_D50

Standard CIE 1931 2° illuminant D50, encoded in xyY. This illuminant has a color temperature of 5003K. This illuminant is used by the profile connection space in ICC profiles.

ILLUMINANT_D55

Added in API level 26
public static final float[] ILLUMINANT_D55

Standard CIE 1931 2° illuminant D55, encoded in xyY. This illuminant has a color temperature of 5503K.

ILLUMINANT_D60

Added in API level 26
public static final float[] ILLUMINANT_D60

Standard CIE 1931 2° illuminant D60, encoded in xyY. This illuminant has a color temperature of 6004K.

ILLUMINANT_D65

Added in API level 26
public static final float[] ILLUMINANT_D65

Standard CIE 1931 2° illuminant D65, encoded in xyY. This illuminant has a color temperature of 6504K. This illuminant is commonly used in RGB color spaces such as sRGB, BT.709, etc.

ILLUMINANT_D75

Added in API level 26
public static final float[] ILLUMINANT_D75

Standard CIE 1931 2° illuminant D75, encoded in xyY. This illuminant has a color temperature of 7504K.

ILLUMINANT_E

Added in API level 26
public static final float[] ILLUMINANT_E

Standard CIE 1931 2° illuminant E, encoded in xyY. This illuminant has a color temperature of 5454K.

Public methods

adapt

Added in API level 26
public static ColorSpace adapt (ColorSpace colorSpace, 
                float[] whitePoint, 
                ColorSpace.Adaptation adaptation)

Performs the chromatic adaptation of a color space from its native white point to the specified white point. If the specified color space does not have an RGB color model, or if the color space already has the target white point, the color space is returned unmodified.

The chromatic adaptation is performed using the von Kries method described in the documentation of Adaptation.

The color space returned by this method always has an ID of MIN_ID.

Parameters
colorSpace ColorSpace: The color space to chromatically adapt This value cannot be null.

whitePoint float: The new white point This value cannot be null.

adaptation ColorSpace.Adaptation: The adaptation matrix This value cannot be null.

Returns
ColorSpace A new color space if the specified color space has an RGB model and a white point different from the specified white point; the specified color space otherwise This value cannot be null.

adapt

Added in API level 26
public static ColorSpace adapt (ColorSpace colorSpace, 
                float[] whitePoint)

Performs the chromatic adaptation of a color space from its native white point to the specified white point.

The chromatic adaptation is performed using the Adaptation#BRADFORD matrix.

The color space returned by this method always has an ID of MIN_ID.

Parameters
colorSpace ColorSpace: The color space to chromatically adapt This value cannot be null.

whitePoint float: The new white point This value cannot be null.

Returns
ColorSpace A ColorSpace instance with the same name, primaries, transfer functions and range as the specified color space This value cannot be null.

cctToXyz

Added in API level 31
public static float[] cctToXyz (int cct)

Computes the chromaticity coordinates of a specified correlated color temperature (CCT) on the Planckian locus. The specified CCT must be greater than 0. A meaningful CCT range is [1667, 25000].

The transform is computed using the methods in Kang et al., Design of Advanced Color - Temperature Control System for HDTV Applications, Journal of Korean Physical Society 41, 865-871 (2002).

Parameters
cct int: The correlated color temperature, in Kelvin Value is 1 or greater

Returns
float[] Corresponding XYZ values This value cannot be null.

Throws
IllegalArgumentException If cct is invalid

chromaticAdaptation

Added in API level 31
public static float[] chromaticAdaptation (ColorSpace.Adaptation adaptation, 
                float[] srcWhitePoint, 
                float[] dstWhitePoint)

Computes the chromatic adaptation transform from the specified source white point to the specified destination white point.

The transform is computed using the von Kries method, described in more details in the documentation of Adaptation. The Adaptation enum provides different matrices that can be used to perform the adaptation.

Parameters
adaptation ColorSpace.Adaptation: The adaptation method This value cannot be null.

srcWhitePoint float: The white point to adapt from This value cannot be null.

dstWhitePoint float: The white point to adapt to This value cannot be null.

Returns
float[] A 3x3 matrix as a non-null array of 9 floats

connect

Added in API level 26
public static ColorSpace.Connector connect (ColorSpace source, 
                ColorSpace.RenderIntent intent)

Connects the specified color spaces to sRGB. If the source color space does not use CIE XYZ D65 as its profile connection space, the two spaces are chromatically adapted to use the CIE standard illuminant D50 as needed.

If the source is the sRGB color space, an optimized connector is returned to avoid unnecessary computations and loss of precision.

Parameters
source ColorSpace: The color space to convert colors from This value cannot be null.

intent ColorSpace.RenderIntent: The render intent to map colors from the source to the destination This value cannot be null.

Returns
ColorSpace.Connector A non-null connector between the specified color space and sRGB

connect

Added in API level 26
public static ColorSpace.Connector connect (ColorSpace source)

Connects the specified color spaces to sRGB. If the source color space does not use CIE XYZ D65 as its profile connection space, the two spaces are chromatically adapted to use the CIE standard illuminant D50 as needed.

If the source is the sRGB color space, an optimized connector is returned to avoid unnecessary computations and loss of precision.

Colors are mapped from the source color space to the destination color space using the perceptual render intent.

Parameters
source ColorSpace: The color space to convert colors from This value cannot be null.

Returns
ColorSpace.Connector A non-null connector between the specified color space and sRGB

connect

Added in API level 26
public static ColorSpace.Connector connect (ColorSpace source, 
                ColorSpace destination, 
                ColorSpace.RenderIntent intent)

Connects two color spaces to allow conversion from the source color space to the destination color space. If the source and destination color spaces do not have the same profile connection space (CIE XYZ with the same white point), they are chromatically adapted to use the CIE standard illuminant D50 as needed.

If the source and destination are the same, an optimized connector is returned to avoid unnecessary computations and loss of precision.

Parameters
source ColorSpace: The color space to convert colors from This value cannot be null.

destination ColorSpace: The color space to convert colors to This value cannot be null.

intent ColorSpace.RenderIntent: The render intent to map colors from the source to the destination This value cannot be null.

Returns
ColorSpace.Connector A non-null connector between the two specified color spaces

connect

Added in API level 26
public static ColorSpace.Connector connect (ColorSpace source, 
                ColorSpace destination)

Connects two color spaces to allow conversion from the source color space to the destination color space. If the source and destination color spaces do not have the same profile connection space (CIE XYZ with the same white point), they are chromatically adapted to use the CIE standard illuminant D50 as needed.

If the source and destination are the same, an optimized connector is returned to avoid unnecessary computations and loss of precision.

Colors are mapped from the source color space to the destination color space using the perceptual render intent.

Parameters
source ColorSpace: The color space to convert colors from This value cannot be null.

destination ColorSpace: The color space to convert colors to This value cannot be null.

Returns
ColorSpace.Connector A non-null connector between the two specified color spaces

equals

Added in API level 26
public boolean equals (Object o)

Indicates whether some other object is "equal to" this one.

The equals method implements an equivalence relation on non-null object references:

  • It is reflexive: for any non-null reference value x, x.equals(x) should return true.
  • It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
  • It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
  • It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
  • For any non-null reference value x, x.equals(null) should return false.

An equivalence relation partitions the elements it operates on into equivalence classes; all the members of an equivalence class are equal to each other. Members of an equivalence class are substitutable for each other, at least for some purposes.

Parameters
o Object: the reference object with which to compare.

Returns
boolean true if this object is the same as the obj argument; false otherwise.

fromXyz

Added in API level 26
public abstract float[] fromXyz (float[] v)

Converts tristimulus values from the CIE XYZ space to this color space's color model. The resulting value is passed back in the specified array.

The specified array's length must be at least equal to to the number of color components as returned by Model#getComponentCount(), and its first 3 values must be the XYZ components to convert from.

Parameters
v float: An array of color components containing the XYZ values to convert from, and large enough to hold the number of components of this color space's model This value cannot be null.

Returns
float[] The array passed in parameter This value cannot be null.

fromXyz

Added in API level 26
public float[] fromXyz (float x, 
                float y, 
                float z)

Converts tristimulus values from the CIE XYZ space to this color space's color model.

Parameters
x float: The X component of the color value

y float: The Y component of the color value

z float: The Z component of the color value

Returns
float[] A new array whose size is equal to the number of color components as returned by Model#getComponentCount() This value cannot be null.

get

Added in API level 26
public static ColorSpace get (ColorSpace.Named name)

Returns an instance of ColorSpace identified by the specified name. The list of names provided in the Named enum gives access to a variety of common RGB color spaces.

This method always returns the same instance for a given name.

This method is thread-safe.

Parameters
name ColorSpace.Named: The name of the color space to get an instance of This value cannot be null.

Returns
ColorSpace A non-null ColorSpace instance

getComponentCount

Added in API level 26
public int getComponentCount ()

Returns the number of components that form a color value according to this color space's color model.

Returns
int An integer between 1 and 4 Value is between 1 and 4 inclusive

getFromDataSpace

Added in API level 33
public static ColorSpace getFromDataSpace (int dataSpace)

Create a ColorSpace object using a DataSpace value.

This function maps from a dataspace to a Named ColorSpace. If no Named ColorSpace object matching the dataSpace value can be created, null will return.

Parameters
dataSpace int: The dataspace value Value is either 0 or a combination of DataSpace.DATASPACE_UNKNOWN, DataSpace.DATASPACE_SCRGB_LINEAR, DataSpace.DATASPACE_SRGB, DataSpace.DATASPACE_SCRGB, DataSpace.DATASPACE_DISPLAY_P3, DataSpace.DATASPACE_BT2020_HLG, DataSpace.DATASPACE_BT2020_PQ, DataSpace.DATASPACE_ADOBE_RGB, DataSpace.DATASPACE_JFIF, DataSpace.DATASPACE_BT601_625, DataSpace.DATASPACE_BT601_525, DataSpace.DATASPACE_BT2020, DataSpace.DATASPACE_BT709, DataSpace.DATASPACE_DCI_P3, and DataSpace.DATASPACE_SRGB_LINEAR

Returns
ColorSpace the ColorSpace object or null if no matching colorspace can be found.

getId

Added in API level 26
public int getId ()

Returns the ID of this color space. Positive IDs match the color spaces enumerated in Named. A negative ID indicates a color space created by calling one of the public constructors.

Returns
int An integer between MIN_ID and MAX_ID Value is between MIN_ID and MAX_ID inclusive

getMaxValue

Added in API level 26
public abstract float getMaxValue (int component)

Returns the maximum valid value for the specified component of this color space's color model.

Parameters
component int: The index of the component Value is between 0 and 3 inclusive

Returns
float A floating point value greater than getMinValue(int)

getMinValue

Added in API level 26
public abstract float getMinValue (int component)

Returns the minimum valid value for the specified component of this color space's color model.

Parameters
component int: The index of the component Value is between 0 and 3 inclusive

Returns
float A floating point value less than getMaxValue(int)

getModel

Added in API level 26
public ColorSpace.Model getModel ()

Return the color model of this color space.

Returns
ColorSpace.Model A non-null Model

getName

Added in API level 26
public String getName ()

Returns the name of this color space. The name is never null and contains always at least 1 character.

Color space names are recommended to be unique but are not guaranteed to be. There is no defined format but the name usually falls in one of the following categories:

  • Generic names used to identify color spaces in non-RGB color models. For instance: Generic L*a*b*.
  • Names tied to a particular specification. For instance: sRGB IEC61966-2.1 or SMPTE ST 2065-1:2012 ACES.
  • Ad-hoc names, often generated procedurally or by the user during a calibration workflow. These names often contain the make and model of the display.

Because the format of color space names is not defined, it is not recommended to programmatically identify a color space by its name alone. Names can be used as a first approximation.

It is however perfectly acceptable to display color space names to users in a UI, or in debuggers and logs. When displaying a color space name to the user, it is recommended to add extra information to avoid ambiguities: color model, a representation of the color space's gamut, white point, etc.

Returns
String A non-null String of length >= 1

hashCode

Added in API level 26
public int hashCode ()

Returns a hash code value for the object. This method is supported for the benefit of hash tables such as those provided by HashMap.

The general contract of hashCode is:

  • Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
  • If two objects are equal according to the equals method, then calling the hashCode method on each of the two objects must produce the same integer result.
  • It is not required that if two objects are unequal according to the equals method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables.

Returns
int a hash code value for this object.

isSrgb

Added in API level 26
public boolean isSrgb ()

Indicates whether this color space is the sRGB color space or equivalent to the sRGB color space.

A color space is considered sRGB if it meets all the following conditions:

  • Its color model is Model#RGB.
  • Its primaries are within 1e-3 of the true sRGB primaries.
  • Its white point is within 1e-3 of the CIE standard illuminant D65.
  • Its opto-electronic transfer function is not linear.
  • Its electro-optical transfer function is not linear.
  • Its transfer functions yield values within 1e-3 of Named#SRGB.
  • Its range is \([0..1]\).

This method always returns true for Named#SRGB.

Returns
boolean True if this color space is the sRGB color space (or a close approximation), false otherwise

isWideGamut

Added in API level 26
public abstract boolean isWideGamut ()

Returns whether this color space is a wide-gamut color space. An RGB color space is wide-gamut if its gamut entirely contains the sRGB gamut and if the area of its gamut is 90% of greater than the area of the NTSC gamut.

Returns
boolean True if this color space is a wide-gamut color space, false otherwise

match

Added in API level 26
public static ColorSpace match (float[] toXYZD50, 
                ColorSpace.Rgb.TransferParameters function)

Returns a Named instance of ColorSpace that matches the specified RGB to CIE XYZ transform and transfer functions. If no instance can be found, this method returns null.

The color transform matrix is assumed to target the CIE XYZ space a D50 standard illuminant.

Parameters
toXYZD50 float: 3x3 column-major transform matrix from RGB to the profile connection space CIE XYZ as an array of 9 floats, cannot be null

function ColorSpace.Rgb.TransferParameters: Parameters for the transfer functions This value cannot be null.

Returns
ColorSpace A non-null ColorSpace if a match is found, null otherwise

toString

Added in API level 26
public String toString ()

Returns a string representation of the object. This method returns a string equal to the value of:

 getName() + "(id=" + getId() + ", model=" + getModel() + ")"
 

For instance, the string representation of the sRGB color space is equal to the following value:

 sRGB IEC61966-2.1 (id=0, model=RGB)
 

Returns
String A string representation of the object This value cannot be null.

toXyz

Added in API level 26
public float[] toXyz (float r, 
                float g, 
                float b)

Converts a color value from this color space's model to tristimulus CIE XYZ values. If the color model of this color space is not RGB, it is assumed that the target CIE XYZ space uses a D50 standard illuminant.

This method is a convenience for color spaces with a model of 3 components (RGB or Model#LAB for instance). With color spaces using fewer or more components, use toXyz(float[]) instead

.

Parameters
r float: The first component of the value to convert from (typically R in RGB)

g float: The second component of the value to convert from (typically G in RGB)

b float: The third component of the value to convert from (typically B in RGB)

Returns
float[] A new array of 3 floats, containing tristimulus XYZ values This value cannot be null.

toXyz

Added in API level 26
public abstract float[] toXyz (float[] v)

Converts a color value from this color space's model to tristimulus CIE XYZ values. If the color model of this color space is not RGB, it is assumed that the target CIE XYZ space uses a D50 standard illuminant.

The specified array's length must be at least equal to to the number of color components as returned by Model#getComponentCount().

Parameters
v float: An array of color components containing the color space's color value to convert to XYZ, and large enough to hold the resulting tristimulus XYZ values This value cannot be null.

Returns
float[] The array passed in parameter This value cannot be null.