Java: getConstructor + newInstance working in main but not outside it - exactly the same code

Question

import java.lang.reflect.*;

public class Test {

    public Test(int x) {
        System.out.println("Constuctor called! x = " + x);
    }

    public static void main(String[] args) throws Exception {
        Class<? extends Thing> clazz = Stuff.class;
        Constructor<? extends Thing> ctor = clazz.getConstructor(int.class);
        Thing instance = ctor.newInstance(5);           
    }
}

public class Thing{
  public Thing(int x){
    System.out.println("Constructor! x = " + x);
  }
}

public class Stuff extends Thing{
  public Stuff(int x){
    super(x*2);
  }
}

The code above works as expected.

import java.lang.reflect.*;

public class Test {

    public Test(int x) {
        System.out.println("Constuctor called! x = " + x);
    }

  static void Create(){
    Class<? extends Thing> clazz = Stuff.class;
    Constructor<? extends Thing> ctor = clazz.getConstructor(int.class);
    Thing instance = ctor.newInstance(5);     
  }


    public static void main(String[] args) throws Exception {
      Create();      
    }
}

public class Thing{
  public Thing(int x){
    System.out.println("Constructor! x = " + x);
  }
}

public class Stuff extends Thing{
  public Stuff(int x){
    super(x*2);
  }
}

This code does not. I get these errors:

/tmp/java_tXpJ5P/Test.java:11: error: unreported exception NoSuchMethodException; must be caught or declared to be thrown
        Constructor<? extends Thing> ctor = clazz.getConstructor(int.class);
                                                                ^
/tmp/java_tXpJ5P/Test.java:12: error: unreported exception InstantiationException; must be caught or declared to be thrown
        Thing instance = ctor.newInstance(5);     
                                         ^
2 errors

Am I missing something very obvious or something very arcane here? It just seems bizarre. This is a simplification of code in a much more complex project that is structured slightly differently (and I'm passing Class as an argument) but produces the same errors.

Thanks very much

Well, the exception meesage just tells it – `newInstance()` declares that it *may* throw a `NoSuchMethodException`. This is a *checked exception*, so the method calling it should either handle it with try/catch or declare it to be thrown. In your first code snippet, `main` declares a possible `Exception` to be thrown, satisfying the compiler. However, your `Create` method does not have a throws clause while it should. `Create()` should be `create() throws NoSuchMethodException, InstantiationException`. — MC Emperor, May 02 '20 at 18:17

score 1 · Answer 1 · answered May 02 '20 at 18:13

Your main method throws Exception, which means whatever happens there it will be throw. Your new code has no throws Exception and your code is throwing 2 potential Exceptions (InstantiationException and NoSuchMethodException). You have to take care of them either throwing (as the main method) or wrapping it on a try-catch statement.

Java: getConstructor + newInstance working in main but not outside it - exactly the same code

1 Answers1