I am very new to Java so this may be a dumb question, but I am trying to understand how to create a new instance of a class by getting the class from an existing instance (I think this is called reflection).
Currently I have a super class and several subclasses of it.
public abstract class SuperClazz {...}
public class SubClazz1 extends SuperClazz {...}
public class SubClazz2 extends SuperClazz {...}
I have an existing instance of one of these subclasses (declared only as a member of the super class, as I do not yet know which subclass it will belong to). I am trying to get whichever subclass this existing instance belongs to and make a new instance of that same subclass.
This is my setup:
private SuperClazz oldSubInstance;
private SuperClazz newSubInstance;
newSubInstance = oldSubInstance.getClass().getConstructor(String.class, char.class, int.class).newInstance("abc", 'e', 6);
Which throws NoSuchMethodException.
I am confused because I know SuperClazz has a constructor that takes in three parameters, a String, a char and an int. I have viewed answers here and here but have found that implementing the suggested fixes does not work, or that their issues do not apply to my situation.
Am I completely misunderstanding how getConstructor works?
You need to make sure that the all-args constructor exists in the children class(es).
The following example creates a newSubInstance
:
// using Lombok annotations for brevity
@Getter
@Setter
@AllArgsConstructor
@NoArgsConstructor
@ToString
abstract class SuperClazz {
private String str;
private char chr;
private int x;
// getters/setters/default and all-args constructors provided by Lombok
}
class SubClazz1 extends SuperClazz {
public SubClazz1(String str, char c, int i) { // providing constructor in the child class
super(str, c, i);
}
}
// test
SuperClazz oldSubInstance = new SubClazz1("def", 'g', 10);
SuperClazz newSubInstance;
newSubInstance = oldSubInstance.getClass()
.getConstructor(String.class, char.class, int.class)
.newInstance("abc", 'e', 6);
System.out.println(newSubInstance);
output ( toString
overridden at the base class level):
SuperClazz(str=abc, chr=e, x=6)
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