I would like to use Class.newInstance() but the class I am instantiating does not have a nullary constructor. Therefore I need to be able to pass in constructor arguments. Is there a way to do this?
This question is related to
java
constructor
myObject.getClass().getDeclaredConstructors(types list).newInstance(args list);
Edit: according to the comments seems like pointing class and method names is not enough for some users. For more info take a look at the documentation for getting constuctor and invoking it.
Follow below steps to call parameterized consturctor.
Constructor with parameter types by passing types in Class[]
for getDeclaredConstructor method of ClassObject[] fornewInstance method of ConstructorExample code:
import java.lang.reflect.*;
class NewInstanceWithReflection{
public NewInstanceWithReflection(){
System.out.println("Default constructor");
}
public NewInstanceWithReflection( String a){
System.out.println("Constructor :String => "+a);
}
public static void main(String args[]) throws Exception {
NewInstanceWithReflection object = (NewInstanceWithReflection)Class.forName("NewInstanceWithReflection").newInstance();
Constructor constructor = NewInstanceWithReflection.class.getDeclaredConstructor( new Class[] {String.class});
NewInstanceWithReflection object1 = (NewInstanceWithReflection)constructor.newInstance(new Object[]{"StackOverFlow"});
}
}
output:
java NewInstanceWithReflection
Default constructor
Constructor :String => StackOverFlow
I think this is exactly what you want http://da2i.univ-lille1.fr/doc/tutorial-java/reflect/object/arg.html
Although it seems a dead thread, someone might find it useful
Do not use Class.newInstance(); see this thread: Why is Class.newInstance() evil?
Like other answers say, use Constructor.newInstance() instead.
myObject.getClass().getDeclaredConstructors(types list).newInstance(args list);
Edit: according to the comments seems like pointing class and method names is not enough for some users. For more info take a look at the documentation for getting constuctor and invoking it.
You can use the getDeclaredConstructor method of Class. It expects an array of classes. Here is a tested and working example:
public static JFrame createJFrame(Class c, String name, Component parentComponent)
{
try
{
JFrame frame = (JFrame)c.getDeclaredConstructor(new Class[] {String.class}).newInstance("name");
if (parentComponent != null)
{
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
else
{
frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
}
frame.setLocationRelativeTo(parentComponent);
frame.pack();
frame.setVisible(true);
}
catch (InstantiationException instantiationException)
{
ExceptionHandler.handleException(instantiationException, parentComponent, Language.messages.get(Language.InstantiationExceptionKey), c.getName());
}
catch(NoSuchMethodException noSuchMethodException)
{
//ExceptionHandler.handleException(noSuchMethodException, parentComponent, Language.NoSuchMethodExceptionKey, "NamedConstructor");
ExceptionHandler.handleException(noSuchMethodException, parentComponent, Language.messages.get(Language.NoSuchMethodExceptionKey), "(Constructor or a JFrame method)");
}
catch (IllegalAccessException illegalAccessException)
{
ExceptionHandler.handleException(illegalAccessException, parentComponent, Language.messages.get(Language.IllegalAccessExceptionKey));
}
catch (InvocationTargetException invocationTargetException)
{
ExceptionHandler.handleException(invocationTargetException, parentComponent, Language.messages.get(Language.InvocationTargetExceptionKey));
}
finally
{
return null;
}
}
MyClass.class.getDeclaredConstructor(String.class).newInstance("HERESMYARG");
or
obj.getClass().getDeclaredConstructor(String.class).newInstance("HERESMYARG");
Do not use Class.newInstance(); see this thread: Why is Class.newInstance() evil?
Like other answers say, use Constructor.newInstance() instead.
You can get other constructors with getConstructor(...).
Assuming you have the following constructor
class MyClass {
public MyClass(Long l, String s, int i) {
}
}
You will need to show you intend to use this constructor like so:
Class classToLoad = MyClass.class;
Class[] cArg = new Class[3]; //Our constructor has 3 arguments
cArg[0] = Long.class; //First argument is of *object* type Long
cArg[1] = String.class; //Second argument is of *object* type String
cArg[2] = int.class; //Third argument is of *primitive* type int
Long l = new Long(88);
String s = "text";
int i = 5;
classToLoad.getDeclaredConstructor(cArg).newInstance(l, s, i);
You can use the getDeclaredConstructor method of Class. It expects an array of classes. Here is a tested and working example:
public static JFrame createJFrame(Class c, String name, Component parentComponent)
{
try
{
JFrame frame = (JFrame)c.getDeclaredConstructor(new Class[] {String.class}).newInstance("name");
if (parentComponent != null)
{
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
else
{
frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
}
frame.setLocationRelativeTo(parentComponent);
frame.pack();
frame.setVisible(true);
}
catch (InstantiationException instantiationException)
{
ExceptionHandler.handleException(instantiationException, parentComponent, Language.messages.get(Language.InstantiationExceptionKey), c.getName());
}
catch(NoSuchMethodException noSuchMethodException)
{
//ExceptionHandler.handleException(noSuchMethodException, parentComponent, Language.NoSuchMethodExceptionKey, "NamedConstructor");
ExceptionHandler.handleException(noSuchMethodException, parentComponent, Language.messages.get(Language.NoSuchMethodExceptionKey), "(Constructor or a JFrame method)");
}
catch (IllegalAccessException illegalAccessException)
{
ExceptionHandler.handleException(illegalAccessException, parentComponent, Language.messages.get(Language.IllegalAccessExceptionKey));
}
catch (InvocationTargetException invocationTargetException)
{
ExceptionHandler.handleException(invocationTargetException, parentComponent, Language.messages.get(Language.InvocationTargetExceptionKey));
}
finally
{
return null;
}
}
MyClass.class.getDeclaredConstructor(String.class).newInstance("HERESMYARG");
or
obj.getClass().getDeclaredConstructor(String.class).newInstance("HERESMYARG");
You can get other constructors with getConstructor(...).
Assuming you have the following constructor
class MyClass {
public MyClass(Long l, String s, int i) {
}
}
You will need to show you intend to use this constructor like so:
Class classToLoad = MyClass.class;
Class[] cArg = new Class[3]; //Our constructor has 3 arguments
cArg[0] = Long.class; //First argument is of *object* type Long
cArg[1] = String.class; //Second argument is of *object* type String
cArg[2] = int.class; //Third argument is of *primitive* type int
Long l = new Long(88);
String s = "text";
int i = 5;
classToLoad.getDeclaredConstructor(cArg).newInstance(l, s, i);
myObject.getClass().getDeclaredConstructors(types list).newInstance(args list);
Edit: according to the comments seems like pointing class and method names is not enough for some users. For more info take a look at the documentation for getting constuctor and invoking it.
You can get other constructors with getConstructor(...).
Do not use Class.newInstance(); see this thread: Why is Class.newInstance() evil?
Like other answers say, use Constructor.newInstance() instead.
I think this is exactly what you want http://da2i.univ-lille1.fr/doc/tutorial-java/reflect/object/arg.html
Although it seems a dead thread, someone might find it useful
You can get other constructors with getConstructor(...).
Source: Stackoverflow.com