Instantiation error

Question

I am following a beginner's java book and one of the projects is to pick an error from commonly thrown errors and try to do what you can to induce the error.

I picked InstantiationError and am up to the level of knowing you can't instantiate abstract classes or interfaces. The docs says the error is usually caught by the compiler but can be thrown at runtime if the definition of a class has incompatibly changed.

I don't know how a class definition could be changed while the program is running and am looking for some tips on where I should read further. Thanks!

Answer 1

I don't know how a class definition could be changed while the program is running

It can't change while it's running but it can change after you've compiled.

For example, try this:

// In Test.java
public class Test {
    public static void main(String[] args){
        System.out.println(new Foo());
    }
}

// In Foo.java
public class Foo {
}

Compile:

javac Foo.java Test.java

Run:

java Test

// Some output like this...
Foo@1d6535bf

Now change Foo.java like this:

// No parameterless constructor any more!
public class Foo {
    public Foo(int x) {
    }
}

Recompile just Foo.java:

javac Foo.java

And rerun Test:

Exception in thread "main" java.lang.NoSuchMethodError: Foo: method <init>()V 
    not found
    at Test.main(Test.java:3)

This is not what I'd call a "commonly thrown error" though.

Note that that's not InstantiationError - but you can change Foo.java again, to:

public interface Foo {
}

Again, just recompile Foo.java and this time you'll get:

Exception in thread "main" java.lang.InstantiationError: Foo
        at Test.main(Test.java:3)

Answer 2

Maybe when you instantiate an abstract class at runtime using reflection.

Code sample:

public abstract class MyAbstractClass{
}

public class MyMainClass() {
    public void test(String[] args) {
             this.getClass().getClassLoader().loadClass("MyAbstractClass").getConstructors()[0].newInstance();
    }
}

Answer 3

Have a look at the Reflection API, which offers a way to instantiate a class by name.

public void throwError() {
    AbstractType type = this.getClassLoader().newInstance("my.abstract.Type");
}