Beginner's problems with generics

[flex567]

public static <T> T[] fill(T[] a, Generator<T> gen) {
     for(int i = 0; i < a.length; i++)
           a[i] = gen.next();
     return a;
}

What does the bolded symbol mean?

[flex567]

And why do I get error at runtime for this program:

import java.util.*

public class BasicHolder<T> {
	T element;
	void set(T arg) { element = arg; }
	T get() { return element; }
	void f() {
		System.out.println(element.getClass().getSimpleName());
	}
	public static void main(String[] args){
		BasicHolder<String> s1 = new BasicHolder<String>();
		s1.f();
	}

}

the error is:

Error: Main method not found in class BasicHolder, please define the main method as:
   public static void main(String[] args)
or a JavaFX application class must extend javafx.application.Application

[Norm]

The posted code does not compile without errors. Fix the compiler errors before trying to execute the code.

[flex567]

I fixed it now I get

Exception in thread "main" java.lang.NullPointerException
	at BasicHolder.f(BasicHolder.java:8)
	at BasicHolder.main(BasicHolder.java:12)

[Norm]

Exception in thread "main" java.lang.NullPointerException
at BasicHolder.f(BasicHolder.java:8)
at BasicHolder.main(BasicHolder.java:12)

There is a variable on line 8 that has a null value when that statement is executed. Look at that line, find the variable with the null value and then backtrack in the code to see why it does not have a valid value.

[flex567]

I dont know, the entire line = 0?

System.out.println(element.getClass().getSimpleName());

or class String doesn't have getClass() method

[Norm]

the entire line = 0?

What does that mean?

There are two values on that line to test for null: element and the value returned by getClass()
Print each of them separately.

[flex567]

I thought that println() can return 0, but even if it did, the compiler wouldn't throw an error?

now it runs as expected:

import java.util.*;

public class BasicHolder<T> {
	T element;
	void set(T arg) { element = arg; }
	T get() { return element; }
	void f() {
		System.out.println(element.getClass().getSimpleName());
	}
	public static void main(String[] args){
		BasicHolder<String> s1 = new BasicHolder<String>();
		s1.element = new String();
		s1.f();
	}

}

thanx

getClass() is not method inside a String class but it is inherited from Object?

I thought I could have problems with the compiler because the name of the class is BasicHolder<T> and the name of the file is just BasicHolder.java