Android Lua Java API

The mapping of Lua referenced objects to Android Java objects is relatively straightforward. All of the API functionality is accessed via the luajava Lua global variable. This variable provides four functions that can be used to access and manipulate standard Java objects and one variable that provides the Android Activity that is required.

luajava.newInstance(className, ...)

This function creates a new Java object based on the fully qualified class name. Any additional parameters that are provided are passed through to the standard Java constructor.

The return value is a Lua variable that is a proxy to the Java object or nil if the class could not be instantiated.

-- Create an instance of a Java string tokenizer
local strTk = luajava.newInstance("java.util.
StringTokenizer","a,b,c,d", ",")
while strTk:hasMoreTokens() do
    print(strTk:nextToken())
end

-- Create a new Android Intent object (unpopulated)
local intent = luajava.newInstance
("android.content.Intent")
                                     
luajava.bindClass(className)

This function creates a reference to a Java class based on a fully qualified class name. This is different from newInstance() in that a new Java object is not created and the constructor is not invoked, but simply a reference to the class is returned. Use this when you need access to static fields or methods of a Java object.

The return value is a Lua variable that is a proxy to the Java Class object specified or nil if the class could not be found.

-- Get the current system time
local sys = luajava.bindClass("java.lang.System")
print ( sys:currentTimeMillis() )

-- Parse a string into an Android Uri
local uriClass = luajava.bindClass("android.net.Uri")
local phoneURI = uriClass:parse("tel:6135951999")
                                  
luajava.new(classObject, ....)

This function is similar to the newInstance() function but rather than taking a fully qualified class name it takes an existing Class reference, generally obtained from calling bindClass(). Additional parameters can be passed to the Java constructor..

The return value is a Lua variable that is a proxy to the Java object or nil if the class could not be instantiated.

-- Create a new string instance
str = luajava.bindClass("java.lang.String")
strInstance = luajava.new(str)
                                    
luajava.createProxy(interfaceNames, luaObject)

If a Java API requires an interface to be implemented or provided as a set of callbacks, then it is where the createProxy() function can be used. The interfaceNames parameter is a comma separated list of fully qualified Java interfaces that will be implemented by the Lua variable luaObject. The names of the interface methods must be present in the luaObject variable.

The return value is a Lua variable that can be passed to any function or method that requires an implementation of that interface. If the creation of the proxy fails, then nil is returned.

-- Create a Lua variable with the same interface as an ActionListener
local button_cb = {}
function button_cb.actionPerformed(ev)
    -- I would do something interesting here ...
end

-- Map the Lua variable to the Java interface
buttonProxy = luajava.createProxy("java.awt.ActionListener", button_cb)

-- Use the newly created interface instance on a Java object
button = luajava.newInstance("java.awt.Button", "execute")
button:addActionListener(buttonProxy)
                                    
luajava.nativeActivity()

All significant interaction on an Android system involves working with an Activity (see http://developer.android.com/reference/android/app/Activity.html) Storyboard applications that are deployed to Android devices run as native activities which is a special class of the general Activity that allows those applications to interact directly with the graphics context and are generally C/C++ applications rather than pure Java applications.

The return value of this function is a Lua variable that is a proxy for the NativeActivity Java class used by this application or nil if the class could not be instantiated.

-- Start an activity specified by a previously created Intent object
local na = luajava.nativeActivity()
if(na ~= nil) then
    na:startActivity(intent)
else
    print("No Native Activity")
end
                                    
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