Question

I’ve just approached to the Java language, so I usually get even into banal errors. The following should be one of them.
Es1.java:93: error: variable f might not have been initialized
Things to know before looking into the code – I’m using some custom class: ConsoleInputManager and ConsoleOutputManager work substantially like Scanner and System.out; Figure is an abstract class; Circle, Square and Rectangle extend Figure

import prog.io.*;
//ConsoleInputManager, ConsoleOutputManager
import prog.utili.*;
//Figure, Circle, Squade, Rectangle
class Es1{
public static void main(String[] args){

//Variables
ConsoleInputManager in = new ConsoleInputManager();
ConsoleOutputManager out = new ConsoleOutputManager();
int com1,com2,com3;
com1 = com2 = com3 = 0;
double r,s,l,w;
r = s = l = w = 0.0;
Figure f;

do{
    //Menu
    out.println("1. Insert a new figure");
    out.println("2. Calculate the area of the current figure");
    out.println("3. Examine the current figure");
    out.println("4. Exit the program\n");

    //Input
    com1 = in.readInt("Command: ");

    //New figure
    if (com1 == 1){
        do{
            out.println("1. Circle");
            out.println("2. Square");
            out.println("3. Rectangle");
            out.println("4. Back to main menu\n");
            com2 = in.readInt("Command: ");
            //Circle
            if (com2 == 1){
                s = l = w = 0.0;
                do{
                    r = in.readDouble("Radius: ");
                } while (r <= 0);
            }
            else
            //Square
            if (com2 == 2){
                r = l = w = 0.0;
                do{
                    s = in.readDouble("Side: ");
                } while (s <= 0);
            }
            else
            //Rectangle
            if (com2 == 3){
                r = s = 0.0;
                do{
                    l = in.readDouble("Length: ");
                } while (l <= 0);
                do{
                    w = in.readDouble("Width: ");
                } while (w <= 0);
            }
        } while (com2 != 4);
    }
    else

    //Area
    if (com1 == 2){
        if (r != 0.0){
            f = new Circle(r);
            out.println("Area = " + f.getArea());
        }
        else
        if (l != 0.0){
            f = new Square(s);
            out.println("Area = " + f.getArea());
        }
        else
        if ( b != 0.0){
            f = new Rectangle(l,w);
            out.println("Area = " + f.getArea());
        }
        else
            out.println("It hasn't been inserted any figure");
    }
    else

    //Examination
    if (com1 == 3){
        do{
            out.println("1. Examine the current figure");
            out.println("2. Back to main menu");
            com3 = in.readInt("Command: ");
            if (com3 == 1){
                if (f instanceof Circle)
                    out.println("Circle");
                else
                if (f instanceof Square)
                    out.println("Square");
                else
                if (f instanceof Rectangle)
                    out.println("Rectangle");
                else
                    out.println("It hasn't been inserted any figure");
            }   
        } while (com3 != 2);
    }

} while (com1 != 4);
}
}

Answer 1

Figure f;

Above statement is inside main method. There is a possibility that if/else conditions not satisfied, which leaves f unassigned.

Change it to something like below:

Figure f=null;

All local variables need to have some value assigned before being used.

Along with that, make sure to perform null check before performing any operations on f.