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Handling Data Returned from Java Methods

Introduction

In many cases, data returned from a Java® method is incompatible with the types operated on in the MATLAB® platform. When this is the case, MATLAB converts the returned value to a type native to the MATLAB language. This section describes the conversion performed on the various types that can be returned from a call to a Java method.

Conversion of Java Return Types

The following table lists Java return types and the resulting MATLAB types. For some Java base return types, MATLAB treats scalar and array returns differently. For information about java.lang.Object type, see Conversion of Java Object Return Types.

Conversion of Java Types to MATLAB Types

Java Return TypeIf Scalar Return, Resulting MATLAB TypeIf Array Return, Resulting MATLAB Type

boolean

logical

logical

byte

double

int8

short

double

int16

int

double

int32

long

double

int64

float

double

single

double

double

double

char

char

char

    Note:   MATLAB converts rectangular Java arrays to arrays of the resulting type. When Java returns a nonrectangular (jagged) array, MATLAB converts it to a cell array. For more information, see How MATLAB Represents the Java Array.

Conversion of Java Object Return Types

When a method call returns data of type java.lang.Object, MATLAB converts its value, depending on its actual type, according to the following table. Other object types, including Object arrays, are not converted; they remain Java objects. However, if you index into a returned Object array, the value will be converted according to the table. For more information, see Elements of java.lang.Object Arrays Are Converted to MATLAB Types.

Conversion of java.lang.Object Type to MATLAB Types

Actual Java TypeReturn Scalar MATLAB Type

java.lang.Boolean

logical

java.lang.Byte

double

java.lang.Short

double

java.lang.Integer

double

java.lang.Long

double

java.lang.Float

double

java.lang.Double

double

java.lang.Character

char

java.lang.String

char

Built-In Types

Java built-in types are described in Passing Built-In Types. This type includes boolean, byte, short, long, int, double, float, and char. When the value returned from a method call is one of these types, MATLAB converts it according to the table Conversion of Java Types to MATLAB Types.

A single numeric or boolean value converts to a 1-by-1 matrix of double, which is convenient for use in MATLAB. An array of a numeric or boolean return values converts each member of the array to the closest base type to minimize the required storage space. Array member conversions are listed in the right-hand column of the table.

A return value of Java type char converts to a 1-by-1 matrix of char. An array of Java char converts to a MATLAB array of that type.

Converting Objects to MATLAB Types

Except for objects of class Object, MATLAB does not convert Java objects returned from method calls to a native MATLAB type. If you want to convert Java object data to a form more readily usable in MATLAB, there are a few MATLAB functions that enable you to do this. These are described in the following sections.

Converting to the MATLAB double Type

Using the double function in MATLAB, you can convert any Java object or array of objects to the MATLAB double type. The action taken by the double function depends on the class of the object you specify:

  • If the object is an instance of a numeric class (java.lang.Number or one of the classes that inherit from that class), MATLAB uses a preset conversion algorithm to convert the object to a MATLAB double.

  • If the object is not an instance of a numeric class, MATLAB checks the class definition to see if it implements a method called toDouble. MATLAB uses toDouble to perform its conversion of Java objects to the MATLAB double type. If such a method is implemented for this class, MATLAB executes it to perform the conversion.

  • If you are using a class of your own design, you can write your own toDouble method to perform conversions on objects of that class to a MATLAB double. This enables you to specify your own means of type conversion for objects belonging to your own classes.

      Note:   If the class of the specified object is not java.lang.Number, does not inherit from that java.lang.Number, and does not implement a toDouble method, then an attempt to convert the object using the double function results in a MATLAB error message.

The syntax for the double command is as follows, where object is a Java object or Java array of objects:

double(object);

Converting to the MATLAB char Type

With the MATLAB char function, you can convert java.lang.String objects and arrays to MATLAB character arrays.

The syntax for the char command is as follows, where object is a Java object or Java array of objects:

char(object);

If the object specified in the char command is not an instance of the java.lang.String class, MATLAB checks its class to see if it implements a method named toChar. If so, MATLAB executes the toChar method of the class to perform the conversion. If you write your own class definitions, you can use this feature by writing a toChar method that performs the conversion according to your own needs.

    Note:   If the class of the specified object is not java.lang.String and it does not implement a toChar method, an attempt to convert the object using the char function results in a MATLAB error message.

Converting to a MATLAB Structure

Java objects are similar to the MATLAB struct type in that many of an object's characteristics are accessible via field names defined within the object. You might want to convert a Java object into a MATLAB struct to facilitate the handling of its data in MATLAB. Use the MATLAB struct function to do this.

The syntax for the struct command is as follows, where object is a Java object or a Java array of objects:

struct(object);

The following example converts a java.awt.Polygon object into a MATLAB struct. You can access the fields of the object directly using MATLAB struct operations. The last line indexes into the array, pstruct.xpoints, to deposit a new value into the third array element.

polygon = java.awt.Polygon([14 42 98 124], [55 12 -2 62], 4);
pstruct = struct(polygon)
pstruct = 
    npoints: 4
    xpoints: [4x1 int32]
    ypoints: [4x1 int32]
pstruct.xpoints
ans =
    14
    42
    98
   124
pstruct.xpoints(3) = 101;

Converting to a MATLAB Cell Array

Use the cell function to convert a Java array or Java object into a MATLAB cell array. Elements of the resulting cell array are of the MATLAB type (if any) closest to the Java array elements or Java object.

The syntax for the cell command is as follows, where object is a Java object or a Java array of objects.

cell(object);

The following example uses the cell command to create a MATLAB cell array in which each cell holds an array of a different type.

import java.lang.* java.awt.*;

% Create a Java array of double
dblArray = javaArray('java.lang.Double', 1, 10);
for m = 1:10
   dblArray(1, m) = Double(m * 7);
end

% Create a Java array of points
ptArray = javaArray('java.awt.Point', 3);
ptArray(1) = Point(7.1, 22);
ptArray(2) = Point(5.2, 35);
ptArray(3) = Point(3.1, 49);

% Create a Java array of strings
strArray = javaArray('java.lang.String', 2, 2);
strArray(1,1) = String('one');    strArray(1,2) = String('two');
strArray(2,1) = String('three');  strArray(2,2) = String('four');

% Convert each to cell arrays
cellArray = {cell(dblArray), cell(ptArray), cell(strArray)}
cellArray = 
    {1x10 cell}    {3x1 cell}    {2x2 cell}

cellArray{1,1}       % Array of type double
ans = 

   [7]   [14]   [21]   [28]   [35]   [42]   [49]   [56]   [63]   [70]

cellArray{1,2}       % Array of type Java.awt.Point

ans = 
    [1x1 java.awt.Point]
    [1x1 java.awt.Point]
    [1x1 java.awt.Point]

cellArray{1,3}       % Array of type char array

ans = 
    'one'      'two' 
    'three'    'four'
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