JAVA getConstructor throws NoSuchMethodException

Question

I'm new in JAVA and I'm trying to learn reflection. I want to get specific constructor (picking the example form here) from my class :

public class Example1 {
    public Example1() {
    }

    public Example1(int i) {
    }

    public Example1(String s) {
        System.out.println("using param = " + s);
    }

    public static void main(String[] args) throws Exception {  
        Class<?>[] paramTypes = String.class.getClasses();
        Constructor<Example1> ctor = Example1.class.getConstructor(paramTypes);
        ctor.newInstance("test");
    }
}

I get NoSuchMethodException when trying to instantiate ctor

What am I missing here?

Your code doesn't actually throw any exception. It also doesn't compile as you don't check for exceptions NoSuchMethodException and SecurityException in main(). — tzima
– tzima, Commented Mar 22, 2015 at 13:43
@TomášZíma I cant say what it is not compiling in my station , by the way why the "-1" ? — LordTitiKaka
– LordTitiKaka, Commented Mar 22, 2015 at 13:55
it's not compiling because of the reason I told you -- there are exceptions for which you need to check. Example in answer I posted should be working on your station, isn't it? BTW, I didn't downvote your question... — tzima
– tzima, Commented Mar 22, 2015 at 13:58
Please add throws Exception to your main() method and add attempt to actually instantiate the class (ctor.newInstance("foo")). Question will then meet the site's criteria and I'll upvote it (as it is, it actually doesn't compile/run). — tzima
– tzima, Commented Mar 22, 2015 at 14:08
@LordTitiKaka I suspect that -1 may have something to do with your code example, since because it doesn't compile it prevents us from reproducing your problem. This means that (1) answering your question is harder (2) future readers may not be able to determine if they are facing same problem as you. Try to improving your example for instance by adding throws Exception in your main method (like @TomášZíma suggested). — Pshemo
– Pshemo, Commented Mar 22, 2015 at 14:11

tzima · Accepted Answer · 2015-03-22 14:38:52Z

Working example:

import java.lang.reflect.Constructor;

public class Test {
    public Test(String str) {
        System.out.println(str);
    }

    public Test(int a, int b) {
        System.out.println("Sum is " + (a + b));
    }

    public static void main(String[] args) throws Exception {
        Constructor<Test> constructorStr = Test.class.getConstructor(String.class);
        constructorStr.newInstance("Hello, world!");

        Constructor<Test> constructorInts = Test.class.getConstructor(int.class, int.class);
        constructorInts.newInstance(2, 3);
    }
}

Note that method getConstructor actually doesn't take an array. It's defined like:

public Constructor<T> getConstructor(Class<?>... parameterTypes) {

... meaning that it accepts variable amount of arguments which should have been passed as I did. Passing an array is possible too, but it's not necessary.

What you've done originally was equivalent to:

    Constructor<Test> constructor = Test.class.getConstructor(String.class.getClasses());
    constructor.newInstance("Hello");

But, what does String.class.getClasses() return? Good question! Lets go debug:

    Class<?>[] classes = String.class.getClasses();
    System.out.println(classes.length); // prints 0

There's a documentation about getClasses(): https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html#getClasses. Check it and you'll find out the reason why it's so.

For the sake of completeness. The super-original-question (before edits) contained one more constructor - a non-parametric one:

import java.lang.reflect.Constructor;

public class Example1 {
    public Example1() {
    }

    public Example1(String s) {
        System.out.println("using param = " + s);
    }

    public static void main(String[] args) throws Exception {  
        Constructor<Example1> ctor = Example1.class.getConstructor(String.class.getClasses());
        ctor.newInstance("test");
    }
}

The problem which occurs here is IllegalArgumentException being thrown. It's because even though String.class.getClasses() returns an empty array, there actually is constructor which matches the criteria - a non-parametric constructor! It doesn't have any arguments and the array returned by String.class.getClasses() doesn't contain anything too. This means that constructor is successfully found, but when trying to instantiate it using ctor.newInstance("test"), it fails because the found constructor doesn't accept any arguments.

This was happening to me because I had removed "public" from public MainCameraRollXML(Context context)

flogy · Accepted Answer · 2015-03-22 13:42:54Z

-1

Try it using

Constructor<example1> ctor = examTesting.getConstructor(new Class[]{String.class});

This will return the constructor with one parameter of type String. Your method might return multiple values and therefore a constructor with multiple parameter is searched but not found -> NoSuchMethodException is thrown.

More information about getClasses(): http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Class.html#getClasses%28%29

answered Mar 22, 2015 at 13:42

flogy

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