Review for Quiz 6

Question 6-1

In your own words, describe what a Java interface is (this is related to the Java keyword interface), and what a class must do in order to claim that it implements the interface.

Question 6-2

The following program simply displays a window containing a button and a field. Show how to edit it so that clicking on the button will clear the text field.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class Question3 extends JFrame implements ActionListener {
  private JButton button;
  private JTextField field;

  public Question3() {
    button = new JButton("Reset");
    field = new JTextField();

    getContentPane().add(field, BorderLayout.CENTER);
    getContentPane().add(button, BorderLayout.SOUTH);
    pack();
  }

  public static void main(String[] args) {
    (new Question3()).show();
  }
}

Question 6-3

Consider the following method.

public class Quadratic {
  public static int main(String[] args) {
    double a = IO.readDouble();
    double b = IO.readDouble();
    double c = IO.readDouble();
    double disc = b * b - 4 * a * c;

    if(disc < 0.0) {
        return Double.NaN;
    }

    if(-b > Math.sqrt(disc)) {
        return (-b - Math.sqrt(disc)) / (2 * a);
    } else {
        return (-b + Math.sqrt(disc)) / (2 * a);
    }
  }
}

List test cases that test each possible path through this method. Do not list any two cases that test the same path.

Question 6-4

Consider the following method, which finds and removes some duplicated nonzero number in the parameter array, returning true if it found one.

public static boolean removeDuplicate(char[] arr) {
    int i, j;
outer:
    for(i = 0; i < arr.length; i++) {
        for(int j = i + 1; j < arr.length; j++) {
            if(arr[i] != 0 && arr[i] == arr[j]) break outer;
        }
    }
    if(i == arr.length { // no duplicate found
        return false;
    } else { // we found a duplicate; shift elements down over j
        for(int k = j + 1; k < arr.length; k++) {
            arr[k - 1] = arr[k];
        }
        arr[arr.length - 1] = 0;
        return true;
    }
}

Let n represent the length of arr. In terms of n, how much time does the above method take? Use Big-O notation to express your answer, and provide the tightest bound that you can.

Solutions

Solution 6-1

An interface in Java is a set of methods. Any class that claims to implement the interface must define bodies for all these methods.

Solution 6-2

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class Question3 extends JFrame implements ActionListener {
  private JButton button;
  private JTextField field;

  public Question3() {
    button = new JButton("Reset");
    field = new JTextField();

    getContentPane().add(field, BorderLayout.CENTER);
    getContentPane().add(button, BorderLayout.SOUTH);
    pack();

    button.addActionListener(this);
  }

  public void actionPerformed(ActionEvent evt) {
    field.setText("");
  }

  public static void main(String[] args) {
    (new Question3()).show();
  }
}

Solution 6-3

Case 1: 1.0 0.0 1.0
Case 2: 1.0 2.0 1.0
Case 3: 1.0 2.0 -1.0

Solution 6-4

O(n²)