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To create an object with a constructor that has arguments, you invoke thenewInstance
method on aConstructor
object, not aClass
object. This technique involves several steps:
- Create a
Class
object for the object you want to create.- Create a
Constructor
object by invokinggetConstructor
on theClass
object. ThegetConstructor
method has one parameter: an array ofClass
objects that correspond to the constructor's parameters.- Create the object by invoking
newInstance
on theConstructor
object. ThenewInstance
method has one parameter: anObject
array whose elements are the argument values being passed to the constructor.The sample program that follows creates a
Rectangle
with the constructor that accepts two integers as parameters. InvokingnewInstance
on this constructor is analogous to this statement:This constructor's arguments are primitive types, but the argument values passed toRectangle rectangle = new Rectangle(12, 34);newInstance
must be objects. Therefore, each of the primitiveint
types is wrapped in anInteger
object.The sample program hardcodes the argument passed to the
getConstructor
method. In a real-life application such as a debugger, you would probably let the user select the constructor. To verify the user's selection, you could use the methods described in the section Discovering Class Constructors.The source code for the sample program follows:
The sample program prints a description of the constructor and the object that it creates:import java.lang.reflect.*; import java.awt.*; class SampleInstance { public static void main(String[] args) { Rectangle rectangle; Class rectangleDefinition; Class[] intArgsClass = new Class[] {int.class, int.class}; Integer height = new Integer(12); Integer width = new Integer(34); Object[] intArgs = new Object[] {height, width}; Constructor intArgsConstructor; try { rectangleDefinition = Class.forName("java.awt.Rectangle"); intArgsConstructor = rectangleDefinition.getConstructor(intArgsClass); rectangle = (Rectangle) createObject(intArgsConstructor, intArgs); } catch (ClassNotFoundException e) { System.out.println(e); } catch (NoSuchMethodException e) { System.out.println(e); } } public static Object createObject(Constructor constructor, Object[] arguments) { System.out.println ("Constructor: " + constructor.toString()); Object object = null; try { object = constructor.newInstance(arguments); System.out.println ("Object: " + object.toString()); return object; } catch (InstantiationException e) { System.out.println(e); } catch (IllegalAccessException e) { System.out.println(e); } catch (IllegalArgumentException e) { System.out.println(e); } catch (InvocationTargetException e) { System.out.println(e); } return object; } }Constructor: public java.awt.Rectangle(int,int) Object: java.awt.Rectangle[x=0,y=0,width=12,height=34]
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