Exercises

Exercise 09.1 Determine whether the following program will generate (i) compilation errors, (ii) runtime errors. If the program does not generate errors, say what it will print out; if the program generates errors, correct them and say what it will print out after the correction. Motivate your answers.

public class Exercise1 {
  public static void main(String[] args) {
    for (int i = 0, j = 0; i < 10, j < 10; i++, j++) {
      System.out.println(i + " + " + j + " = " + (i+j));
    }
    System.out.println("I've printed out the sums of i and j up to "
                       + i + "," + j);
  }
}

Exercise 09.2 Determine whether the following program will generate (i) compilation errors, (ii) runtime errors. If the program does not generate errors, say what it will print out; if the program generates errors, correct them and say what it will print out after the correction. Motivate your answers.

public class Exercise2 {

  private int x = 101;

  private void f(int x) {
    x++;
    g();
  }

  private void g() {
    System.out.println(x);
  }

  public static void main(String[] args) {
    Exercise2 e = new Exercise2();
    int x = 200;
    e.f(x);
  }
}

Exercise 09.3 Determine whether the following classes will generate (i) compilation errors, (ii) runtime errors. If the program does not generate errors, say what it will print out; if the program generates errors, correct them and say what it will print out after the correction. Motivate your answers.

public class Base {
  public Base() {
    infob = "I am an object of the Base class";
  }
  public String getInfo() {
    return infob;
  }
  private String infob;
}

public class Derived extends Base {
  public Derived() {
    super();
    infod = "I am an object of the Derived class";
  }
  public String getInfo() {
    return infod + ", " + super.getInfo();
  }
  private String infod;
}

public class Exercise3 {
  public static void main(String[] args) {
    Base b = new Base();
    Derived d = new Derived();
    System.out.println(b.getInfo());
    System.out.println(d.getInfo());
    b = d;
    System.out.println(b.getInfo());
  }
}

Exercise 09.4 Capture all exceptions in the following program, printing out error messages that describe the type of error that occurred.

import java.io.*;

public class Exercise4 {

  public static void main(String[] args) {
    int n=10;
    int[] v = new int[n];
    FileReader f = new FileReader("dati.txt");
    BufferedReader in = new BufferedReader(f);
    int i=0;
    String linea = in.readLine();
    while (linea!=null) {
      v[i] = Integer.parseInt(linea);
      linea = in.readLine();
      i++;
    }
    f.close();
  }
}

Exercise 09.5 Solve Exercise 8.6 by handling explicitly all exceptions by printing out suitable error messages.

Exercise 09.6 Define a new exception, called ExceptionLineTooLong, that prints out the error message "The strings is too long". Write a program that reads all lines of a file and throws an exception of type ExceptionLineTooLong in the case where a string of the file is longer than 80 characters. Handle also all exceptions that could be thrown by the program.

Exercise 09.7 Write a class containing the following static methods:

• fileExists, that takes as a parameter a string and returns a boolean value that is true, if the file whose name is passed as parameter exists, and false otherwise;
• isInt, that takes as parameter a string and returns a boolean value that is true if the string represents an integer, and false otherwise.
• isDouble, that takes as parameter a string and returns a boolean value that is true if the string represents a real number, and false otherwise.

Solve the exercise by suitably catching exceptions.

Exercise 09.8 Define the exceptions that are necessary to catch the possible errors that can occur in the class Matrix of Exercise 8.9.

• ExceptionWrongMatrixValues that is thrown in the method read if the data on the file does not correspond to numeric values, or if the data are not consistent with the form of a matrix (e.g., the rows have different length);
• ExceptionWrongMatrixDimension that is thrown in the method read if the data on the file do not correspond to the dimension of the matrix.

Modify the class Matrix in such a way that it generates the new exceptions when necessary.