using Java enums or public static fields in MATLAB

Question

I'm wondering how in MATLAB you can get a reference to a Java enum or static public field. In MATLAB, if you are trying to use Java objects/methods, there are equivalents to Java object creation / method call / etc.:

Java: new com.example.test.Foo();

MATLAB: javaObject('com.example.test.Foo');

Java: com.example.test.Foo.staticMethod();

MATLAB: javaMethod('staticMethod', 'com.example.test.Foo');

Java: SomeEnum e = com.example.test.SomeEnum.MY_FAVORITE_ENUM;

MATLAB: ?????

Java: int n = com.example.test.Foo.MAX_FOO;

MATLAB: ?????

Answer 1

Inner classes require conversion of '.' to '$' in Matlab.

This may actually due to the way the Java compiler stores internal class objects. It behaves similarly for internal classes (e.g. javax.swing.plaf.basic.BasicTextUI$UpdateHandler). Matlab is not as smart as the JVM to automatically convert these internal '$' into '.'. Therefore, we can't use the regular simple dot-notation in these cases in Matlab, and since '$' is an invalid character in Matlab syntax, we must resort to using the '$' within javaObject, javaMethod, awtinvoke and their relatives. For example:

Java: InnerClass c = new com.example.test.SomeEnum.InnerClass;
MATLAB: c = javaObject('com.example.test.SomeEnum$InnerClass')

Enums require similar conversion of '.' to '$' in Matlab. But MATLAB's javaObject function calls the class constructor, and since enums have no constructor, we get the following error:

No constructor with appropriate signature exists in Java class

Luckily enum has the builtin method valueOf() that we can use with javaMethod:

Java: SomeEnum e = com.example.test.SomeEnum.MY_FAVORITE_ENUM;
MATLAB: e = javaMethod('valueOf','com.example.test$SomeEnum','MY_FAVORITE_ENUM');

Similarly:

Java: int n = com.example.test.Foo.MAX_FOO;
MATLAB: n = javaMethod('com.example.test.Foo$MAX_FOO')

Static fields can be gotten directly in Matlab using simple dot notation:

redColor = java.awt.Color.red;

The full list of static fields can be gotten using Matlab's built-in struct function:

>> staticFields = struct(java.awt.Color.red)
staticFields = 
      white: [1x1 java.awt.Color]
      WHITE: [1x1 java.awt.Color]
  lightGray: [1x1 java.awt.Color]
 LIGHT_GRAY: [1x1 java.awt.Color]
       gray: [1x1 java.awt.Color]
       GRAY: [1x1 java.awt.Color]
   darkGray: [1x1 java.awt.Color]
  DARK_GRAY: [1x1 java.awt.Color]
      black: [1x1 java.awt.Color]
      BLACK: [1x1 java.awt.Color]
        red: [1x1 java.awt.Color]
        RED: [1x1 java.awt.Color]
       pink: [1x1 java.awt.Color]
       PINK: [1x1 java.awt.Color]
     orange: [1x1 java.awt.Color]
     ORANGE: [1x1 java.awt.Color]
     yellow: [1x1 java.awt.Color]
     YELLOW: [1x1 java.awt.Color]
      green: [1x1 java.awt.Color]
      GREEN: [1x1 java.awt.Color]
    magenta: [1x1 java.awt.Color]
    MAGENTA: [1x1 java.awt.Color]
       cyan: [1x1 java.awt.Color]
       CYAN: [1x1 java.awt.Color]
       blue: [1x1 java.awt.Color]
       BLUE: [1x1 java.awt.Color]
     OPAQUE: 1
    BITMASK: 2
TRANSLUCENT: 3

MATLAB's function javaObject may not work if the default constructor is private (hidden), and javaMethod probably won't work either. If the class with the static methods is nested you may be out of luck. For my systray utility on the File Exchange, I used the reflection approach, as described in this post: http://UndocumentedMatlab.com/blog/setting-system-tray-popup-messages/

Credit: edited by Mark Mikofski

Answer 2

You can reference Java enum constants from Matlab using the package.class.FIELD syntax, as with any other static Java field. Let's say you have an enum.

package com.example;
public enum MyEnum {
    FOO, BAR, BAZ
}

You can get at the enum constants in Matlab using a direct reference. (The Java classes must be in Matlab's javaclasspath, of course.)

% Static reference
foo = com.example.MyEnum.FOO

% Import it if you want to omit the package name
import com.example.MyEnum;
foo = MyEnum.FOO
bar = MyEnum.BAR

If you want a "dynamic" reference determined at runtime, you can just build a string containing the equivalent static reference and pass it to eval(). This works on almost any Matlab code.

% Dynamic reference
foo = eval('com.example.MyEnum.FOO')

And if you want to get really fancy, you can use Java reflection to get at all the enumerated constants at run time. Make a thin wrapper to put with your other custom classes to get around quirks with Matlab's classloader. (There's no Matlab javaClass() equivalent; IMHO this is a Matlab oversight.)

//In Java
package com.example;
public class Reflector {
    public static Class forName(String className) throws Exception {
        return Class.forName(className);
    }
}

Then you can enumerate the constants in Matlab.

% Constant enumeration using reflection
klass = com.example.Reflector.forName('com.example.MyEnum');
enums = klass.getEnumConstants();