[JAVA] Ereditarietà e main()

[keratox]

public class Employee{
	int age;
	String firstname;
	String lastname;
	
	public Employee(){ // noargs
	}
	
	public Employee(int age, String firstname, String lastname){
		this.age=age;
		this.firstname=firstname;
		this.lastname=lastname;
	}
	
	public void showDetails(){
		System.out.println("Dipendente: " + firstname + " " + lastname + "\nEta: " + age);
	}
	
	public class Teacher extends Employee{
		String school;
		double salary;
		
		public Teacher(int age, String firstname, String lastname, String school, double salary){
			super(age,firstname,lastname);
			this.school=school;
			this.salary=salary;
		}
		
		public void showMoreDetails(){
			System.out.println("He/She is teaching in " + school + " school.\nHis/Her salary is " + salary+ " €");
		}
		
		public void teach(String subject){
			System.out.println("Teacher is teaching "+subject);
		}
		
		public static void main(String[] args){
			Employee e1 = new Employee(23,"Mr.","Rossi");
			e1.showDetails();
			Teacher t1 = new Teacher(60,"Mrs.","Bianchi","ITIS",1030.5);
			t1.showDetails();
			t1.showMoreDetails();
		}
		
	}
	
}

Come mai c'è questo errore?

Employee.java:40: non-static variable this cannot be referenced from a static context
Teacher t1 = new Teacher(60,"Mrs.","Bianchi","ITIS",1030.5);
             ^                                     
Employee.java:37: inner classes cannot have static declarations
public static void main(String[] args){
                   ^

Inoltre, se levo 'static' dal main compila correttamente, ma all'esecuzione genera questa eccezione:
Exception in thread "main" java.lang.NoSuchMethodError: main

Per la istanziazione/visualizzazione dovrei ricorrere a una classe ausiliaria?

[Andrea1979]

Teacher.java

class Employee{
	int age;
	String firstname;
	String lastname;
	
	public Employee(){ // noargs
	}
	
	public Employee(int age, String firstname, String lastname){
		this.age=age;
		this.firstname=firstname;
		this.lastname=lastname;
	}
	
	public void showDetails(){
		System.out.println("Dipendente: " + firstname + " " + lastname + "\nEta: " + age);
	}		
}
public class Teacher extends Employee{
		String school;
		double salary;
		
		public Teacher(int age, String firstname, String lastname, String school, double salary){
			super(age,firstname,lastname);
			this.school=school;
			this.salary=salary;
		}
		
		public void showMoreDetails(){
			System.out.println("He/She is teaching in " + school + " school.\nHis/Her salary is " + salary+ " €");
		}
		
		public void teach(String subject){
			System.out.println("Teacher is teaching "+subject);
		}
		
		public static void main(String[] args){
			Employee e1 = new Employee(23,"Mr.","Rossi");
			e1.showDetails();
			Teacher t1 = new Teacher(60,"Mrs.","Bianchi","ITIS",1030.5);
			t1.showDetails();
			t1.showMoreDetails();
		}
		
	}
}

Oppure
Employee.java

public class Employee{
	int age;
	String firstname;
	String lastname;
	
	public Employee(){ // noargs
	}
	
	public Employee(int age, String firstname, String lastname){
		this.age=age;
		this.firstname=firstname;
		this.lastname=lastname;
	}
	
	public void showDetails(){
		System.out.println("Dipendente: " + firstname + " " + lastname + "\nEta: " + age);
	}		
}
 Teacher.java
public class Teacher extends Employee{
		String school;
		double salary;
		
		public Teacher(int age, String firstname, String lastname, String school, double salary){
			super(age,firstname,lastname);
			this.school=school;
			this.salary=salary;
		}
		
		public void showMoreDetails(){
			System.out.println("He/She is teaching in " + school + " school.\nHis/Her salary is " + salary+ " €");
		}
		
		public void teach(String subject){
			System.out.println("Teacher is teaching "+subject);
		}
		
		public static void main(String[] args){
			Employee e1 = new Employee(23,"Mr.","Rossi");
			e1.showDetails();
			Teacher t1 = new Teacher(60,"Mrs.","Bianchi","ITIS",1030.5);
			t1.showDetails();
			t1.showMoreDetails();
		}
		
	}
}

[keratox]

Allora... devo separare le classi in 2 file separati?
Dove sta la correzione?

Se comunque li separo mi restituisce la stessa eccezione del main() e se provo a compilare Teacher.java mi dice che 'Employee' non esiste/è indefinito.

A me comunque interessava di più una risposta teorica.