如何在泛型类中使用getConstructor？

Question

This is my first brush with reflection and generics in Java so please pardon my ignorance. I am trying to instantiate a class using reflection and generics but getting error in my toy program. The goal is to instantiate with constructor of inst class.

Code:

/*
*This is the builder class to build an instance
*/
package generics.expClassT;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public class builder {
    public static<E> E createInst(Class<E> c) throws InstantiationException, IllegalAccessException {
        //Class<?>[] type = new Class<?>[1];
        //type[0] = inst.class;
        try {
            Constructor<E> ctor = c.getConstructor(c); //c.getConstructor(type);
            return (ctor.newInstance("Testing"));
        } catch (NoSuchMethodException | SecurityException | IllegalArgumentException | InvocationTargetException e) {
            e.printStackTrace();
            return null;
        }
    }
}


/*
* This is the class for which we need an instance
*/
package generics.expClassT;
public class inst {
private String var;
public inst(String s) { 
    this.var = s;
}
public String getVar() {
    return var;
}

public static void main(String args[]){
    inst instObj;
    try {
        instObj = builder.createInst(inst.class);
        System.out.println(instObj.getVar());
    } catch (InstantiationException | IllegalAccessException e) {
        //e.printStackTrace();
    }

}
}

Exception:

java.lang.NoSuchMethodException: generics.expClassT.inst.<init>(generics.expClassT.inst)
at java.lang.Class.getConstructor0(Class.java:2800)
at java.lang.Class.getConstructor(Class.java:1708)
at generics.expClassT.builder.createInst(builder.java:15)
at generics.expClassT.inst.main(inst.java:19)
Exception in thread "main" java.lang.NullPointerException
at generics.expClassT.inst.main(inst.java:20)

Thank you in advance for your time and assistance!!

Answer 1

Constructor<E> ctor = c.getConstructor(c); should be Constructor<E> ctor = c.getConstructor(String.class);

From the JavaDocs

Returns a Constructor object that reflects the specified public constructor of the class represented by this Class object. The parameterTypes parameter is an array of Class objects that identify the constructor's formal parameter types, in declared order . If this Class object represents an inner class declared in a non-static context, the formal parameter types include the explicit enclosing instance as the first parameter.

The constructor to reflect is the public constructor of the class represented by this Class object whose formal parameter types match those specified by parameterTypes.

Parameters:
parameterTypes - the parameter array

This, basically, means, that getConstructor(Class...) expects you to pass the class types that have been defined by the classes constructor, in your case public Inst(String s)

Builder

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public class Builder {

    public static <E> E createInst(Class<E> c) throws InstantiationException, IllegalAccessException {
        //Class<?>[] type = new Class<?>[1];
        //type[0] = inst.class;
        try {
            Constructor<E> ctor = c.getConstructor(String.class); //c.getConstructor(type);
            return (ctor.newInstance("Testing"));
        } catch (NoSuchMethodException | SecurityException | IllegalArgumentException | InvocationTargetException e) {
            e.printStackTrace();
            return null;
        }
    }

}

Inst

public class Inst {

    private String var;

    public Inst(String s) {
        this.var = s;
    }

    public String getVar() {
        return var;
    }
}

Main

public class Test {

    public static void main(String[] args) {
        Inst instObj;
        try {
            instObj = Builder.createInst(Inst.class);
            System.out.println(instObj.getVar());
        } catch (InstantiationException | IllegalAccessException e) {
            //e.printStackTrace();
        }
    }

}

You might like to have a read through Code Conventions for the Java TM Programming Language , it will make it easier for people to read your code and for you to read others