Inheritance and extending classes - homework

Question

Here ive got a problem, firstly i don't know why this code won't work, ive got to use the keywords abstract + extend.
Ive put this all into one .java file, when should I put a class into a different file and not place it as an inner like i have done a number of times here ?
public abstract class shape {
int pi;
int radius;
public class twoD extends shape {
twoD(int r, int p) {   //constructor for circle
radius = r;
pi = p;
public void setAreaCircle() {   // area of circle method
int circleArea = pi * radius^radius;
System.out.println(circleArea);
public void setCircumCircle() {   // circumfreanceof circle method
int circleCircum = 2 * pi^radius;
System.out.println(circleCircum);
class circle extends twoD {
public static void main(String args[]){
twoD ob = new twoD(5,10);
ob.setAreaCircle();
ob.setCircumCircle();

Answer 1

K ive done as suggested and added a constructor that calls its superclass contrustor but its still not working. :(
Any clues ?
public abstract class shape {
int pi;
int radius;
class twoD extends shape {
twoD(int r, int p) {   //constructor for circle
radius = r;
pi = p;
public void setAreaCircle() {   // area of circle method
int circleArea = pi * radius^radius;
System.out.println(circleArea);
public void setCircumCircle() {   // circumfreanceof circle method
int circleCircum = 2 * pi^radius;
System.out.println(circleCircum);
public class circle extends twoD {
public static void main(String args[]) {
public circle(int r, int p) {     // call to superclass constructor
super(r,p);
twoD ob = new twoD(5,10);
ob.setAreaCircle();
ob.setCircumCircle();