Class c = Integer.class
Say I only have c how can I create an Integer object from that?
Note that it doesn't necessarily need to be Integer, I'd like to do this for anything.
3 Answers
You can use newInstance() method.
c.newInstance();
Creates exception.
Output:
Caused by: java.lang.NoSuchMethodException: java.lang.Integer.<init>()
Update:
For the class who do not have default (no parameterized) constructor you can not create instance without knowing its type. For others see the below example and its output.
public static void main(String[] args) throws InstantiationException, IllegalAccessException {
Class stringClass = String.class;
Class integerClass = Integer.class;
try {
System.out.println(stringClass.getConstructor());
Object obj = stringClass.newInstance();
if (obj instanceof String) {
System.out.println("String object created.");
}
System.out.println(integerClass.getConstructor());
obj = integerClass.newInstance();
if (obj instanceof Integer) {
System.out.println("String object created.");
}
} catch (NoSuchMethodException e) {
// You can not create instance as it does not have default constructor.
e.printStackTrace();
} catch (SecurityException e) {
e.printStackTrace();
}
}
Output :
public java.lang.String()
String object created.
java.lang.NoSuchMethodException: java.lang.Integer.<init>()
at java.lang.Class.getConstructor0(Unknown Source)
at java.lang.Class.getConstructor(Unknown Source)
at xyz.Abc.main(Abc.java:15)
5 Comments
Aequitas
how would I assign this to an object of Integer? like
c.class integerObject = c.newInstance();
Robby Cornelissen
It will fail with an
InstantiationException. The Integer class does not have a nullary constructor.
MadProgrammer
Caused by: java.lang.NoSuchMethodException: java.lang.Integer.<init>()
Naman Gala
@RobbyCornelissen yes, you are right.
Naman Gala
I have updated my answer.
Since instances of Integer class are immutable, you need to something like this :
public static void main(String args[]) throws Exception {
Class<?> c = Integer.class;
Constructor<?>[] co = c.getDeclaredConstructors(); // get Integer constructors
System.out.println(Arrays.toString(co));
Integer i = (Integer) co[1].newInstance("5"); //call one of those constructors.
System.out.println(i);
}
O/P :
[public java.lang.Integer(int), public java.lang.Integer(java.lang.String) throws java.lang.NumberFormatException]
5
You need to explicitly do these things because Integer class does not provide mutators / default constructor all we are initializing values by using constructor injection.
12 Comments
Codebender
Just curious, what does this have to do with the class being
immutable?
Andreas
co[1]? Hope all versions of Java has the same constructors in the same sequence.
user2864740
Integer objects are immutable; but such is not relevant to the question.
TheLostMind
@user2864740 - It is. Since instances of
Integer class are immutable, they don't have a default public constructor. So, you need to call another one explicitlyAndreas
@TheLostMind
Integer has 2 public constructors. |
Try this
Class c = Class.forName("package.SomeClass");//If you have any specific class
And then instance :
Object obj = c.newInstance();
int intObj = (Integer) obj
9 Comments
MadProgrammer
java.lang.InstantiationException ... Caused by: java.lang.NoSuchMethodException: java.lang.Integer.<init>()Aequitas
Put back what you edited please, this answer actually helped me, thankyou
Luiggi Mendoza
For upvoter:
Integer class has no default constructor, so current code will raise an exception (noted by @MadProgrammer).Luiggi Mendoza
@Aequitas your question is about
Integer, and this doesn't respond that. Also, this is already covered in the duplicate Q/A, there's no need to repost it here.Luiggi Mendoza
@Aequitas You seriously don't help the community...
|
Object) and/or specialize-cast as required (eg.(Integer)i) and/or use reflection.java.lang.Class.getName()??Class#isAssignableFromto determine if the givenClasscan be assigned to a different class type, for examplec.isAssignableFrom(Integer.class), this will, at least, tell you if the given class or compatible.Class#getConstructor(Class...), becauseIntegerdoes not have a default constructor, something likec.getConstructor(int.class). Use the resultingConstructorto create a new instance of the class, something likeInteger value = (Integer)con.newInstance(42);