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java-to-js.md

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Java to JavaScript
Java to JavaScript
NativeScript Android Runtime Data Conversion (Marshalling)
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Java to JavaScript Conversion

The article lists the available types in Java and how they are projected to Java.

String & Character

Both java.lang.String and java.lang.Character types are projected as JavaScript String:

var file = new java.io.File("/path/to/file");
var path = file.getPath(); // returns java.lang.String, converted to JS String

Boolean & Primitive boolean

Both the primitive boolean and reference java.lang.Boolean types are projected as JavaScript Boolean:

var context = ...
var button = new android.widget.Button(context);
var enabled = button.isEnabled(); // returns primitive boolean, converted to JS Boolean

Byte & Primitive byte

Both the primitive byte and reference java.lang.Byte types are projected as JavaScript Number:

var byte = new java.lang.Byte("1");
var jsByteValue = byte.byteValue(); // returns primitive byte, converted to Number

Short & Primitive short

Both the primitive short and reference java.lang.Short types are projected as JavaScript Number:

var short = new java.lang.Short("1");
var jsShortValue = short.shortValue(); // returns primitive short, converted to Number

Integer & Primitive int

Both the primitive int and reference java.lang.Integer types are projected as JavaScript Number:

var int = new java.lang.Integer("1");
var jsIntValue = int.intValue(); // returns primitive int, converted to Number

Float & Primitive float

Both the primitive float and reference java.lang.Float types are projected as JavaScript Number:

var float = new java.lang.Float("1.5");
var jsFloatValue = float.floatValue(); // returns primitive float, converted to Number

Double & Primitive double

Both the primitive double and reference java.lang.Double types are projected as JavaScript Number:

var double = new java.lang.Double("1.5");
var jsDoubleValue = double.doubleValue(); // returns primitive double, converted to Number

Long & Primitive long

java.lang.Long and its primitive equivalent are special types which are projected to JavaScript by applying the following rules:

  • If the value is in the interval (-2^53, 2^53) then it is converted to Number
  • Else a special object with the following characteristics is created:
    • Has Number.NaN set as a prototype
    • Has value property set to the string representation of the Java long value
    • Its valueOf() method returns NaN
    • Its toString() method returns the string representation of the Java long value
public class TestClass {
	public long getLongNumber54Bits(){
		return 1 << 54;
	}
	public long getLongNumber53Bits(){
		return 1 << 53;
	}
}
var testClass = new TestClass();
var jsNumber = testClass.getLongNumber53Bits(); // result is JavaScript Number
var specialObject = testClass.getLongNumber54Bits(); // result is the special object described above

Array

Array in Java is a special java.lang.Object that have an implicit Class associated. A Java Array is projected to JavaScript as a special JavaScript proxy object with the following characteristics:

  • Has length property
  • Has registered indexed getter and setter callbacks, which:
    • If the array contains elements of type convertible to a JavaScript type, then accessing the i-th element will return a converted type
    • If the array contains elements of type non-convertible to JavaScript, then accessing the i-th element will return a proxy object over the Java/Android type (see Accessing APIs)
var directory = new java.io.File("path/to/myDir");
var files = directory.listFiles(); // files is a special object as described above
var singleFile = files[0]; // the indexed getter callback is triggered and a proxy object over the java.io.File is returned

Note: A Java Array is intentionally not converted to a JavaScript Array for the sake of performance, especially when it comes to large arrays.

Null

The Java null literal (or null pointer) is projected to JavaScript Null:

var context = ...
var button = new android.widget.Button(context);
var background = button.getBackground(); // if there is no background drawable method will return JS null

Android Types

All Android-declared types are projected to JavaScript using the Package and Class proxies as described in Accessing APIs

See Also