Passing Built-In Data Types :: Calling Java from MATLAB (External Interfaces)

External Interfaces

Passing Built-In Data Types

Java has eight data types that are intrinsic to the language and are not represented as Java objects. These are often referred to as built-in, or elemental, data types and they include boolean, byte, short, long, int, double, float, and char. MATLAB converts its own data types to these Java built-in types according to the table, Conversion of MATLAB Types to Java Types. Built-in types are in the first 10 rows of the table.

When a Java method you are calling expects one of these data types, you can pass it the type of MATLAB argument shown in the left-most column of the table. If the method takes an array of one of these types, you can pass a MATLAB array of the data type. MATLAB converts the data type of the argument to the type assigned in the method declaration.

The MATLAB code shown below creates a top-level window frame and sets its dimensions. The call to setBounds passes four MATLAB scalars of the double type to the inherited Java Frame method, setBounds, that takes four arguments of the int type. MATLAB converts each 64-bit double data type to a 32-bit integer prior to making the call. Shown here is the setBounds method declaration followed by the MATLAB code that calls the method.

public void setBounds(int x, int y, int width, int height)

frame=java.awt.Frame;
frame.setBounds(200,200,800,400);
frame.setVisible(1);

Passing Built-In Types in an Array

To call a Java method with an argument defined as an array of a built-in type, you can create and pass a MATLAB matrix with a compatible base type. The following code defines a polygon by sending four x and y coordinates to the Polygon constructor. Two 1-by-4 MATLAB arrays of double are passed to java.awt.Polygon, which expects integer arrays in the first two arguments. Shown here is the Java method declaration followed by MATLAB code that calls the method, and then verifies the set coordinates.

public Polygon(int xpoints[], int ypoints[], int npoints)

poly = java.awt.Polygon([14 42 98 124], [55 12 -2 62], 4);
[poly.xpoints poly.ypoints]       % Verify the coordinates
ans =
14     55
42     12
98    -2
124    62

MATLAB Arrays Are Passed by Value

Since MATLAB arrays are passed by value, any changes that a Java method makes to them will not be visible to your MATLAB code. If you need to access changes that a Java method makes to an array, then, rather than passing a MATLAB array, you should create and pass a Java array, which is a reference. For a description of using Java arrays in MATLAB, see Working with Java Arrays.

Note Generally, it is preferable to have methods return data that has been modified using the return argument mechanism as opposed to passing a reference to that data in an argument list.

Passing Data to a Java Method Passing String Arguments