第十二章 异常处理与文本I/O

课本笔记 第十二章

12.1 引言

程序清单 12-1 Quotient.java

import java.util.Scanner;

public class Quotient {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        //Prompt the user to enter two integers
        System.out.print("Enter two integers: ");
        int number1 = input.nextInt();
        int number2 = input.nextInt();

        System.out.println(number1 + " / " + number2 + " is " +
                (number1 / number2));
    }
}

程序清单 12-2 QuotientWithIf.java

import java.util.Scanner;
public class  {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        //Prompt the user to enter two integers
        int number1 = input.nextInt();
        int number2 = input.nextInt();

        if (number2 != 0){
            System.out.println(number1 + " / " + number2
            + " is " + (number1 / number2));
        }else {
            System.out.println("Divisor cannot be zero ");
        }
    }
}

程序清单 12-3 QuotientWithMethod.java

import java.util.Scanner;
public class QuotientWithMethod {
    public static int quotient(int number1, int number2) {
        if (number2 == 0) {
            System.out.println("Divisor cannot be zero");
            System.exit(1);
        }

        return number1 / number2;
    }

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        //Prompt the user to enter two integers
        System.out.print("Enter two integers: ");
        int number1 = input.nextInt();
        int number2 = input.nextInt();

        int result = quotient(number1, number2);
        System.out.println(number1 + " / " + number2 + " is "
                + result);
    }
}

程序清单 12-4 QuotientWithException.java

import java.util.Scanner;
public class QuotientWithException {
    public static int quotient(int number1, int number2) {
        if (number2 == 0) {
            throw new ArithmeticException("Divisor cannot be zero");
        }
        return number1 / number2;
    }

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        //Prompt the user to enter two integers
        System.out.print("Enter two integers: ");
        int number1 = input.nextInt();
        int number2 = input.nextInt();

        try {
            int result = quotient(number1, number2);
            System.out.println(number1 + " / " + number1 + " is " +
                    result);
        } catch (ArithmeticException ex) {
            System.out.println("Exception: an integer " + " cannot bo divided by zero");
        }
        System.out.println("Execution continues ...");
    }
}

程序清单 12-5 InputMismatchExceptionDemo.java

import java.util.InputMismatchException;
import java.util.Scanner;

public class InputMismatchExceptionDemo {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        boolean continueInput = true;

        do {
            try {
                System.out.print("Enetr an integer: ");
                int number = input.nextInt();

                //Display the result
                System.out.println("The number entered is " + number);

                continueInput = false;
            } catch (InputMismatchException ex) {
                System.out.println("Tru again. (" + "Incorrect input: an integer is required)");
                //Discard input
                input.nextLine();
            }
        } while (continueInput);
    }
}

程序清单 12-6 TestException.java

public class TestException {
    public static void main(String[] args) {
        try {
            System.out.println(sum(new int[]{1, 2, 3, 4, 5}));
        } catch (Exception ex) {
            ex.printStackTrace();
            System.out.println("\n" + ex.getMessage());
            System.out.println("\n" + ex.toString());
            System.out.println("\nTrace Info Obtained from getStackTrace");
            StackTraceElement[] traceElements = ex.getStackTrace();
            for (int i = 0; i < traceElements.length; i++) {
                System.out.print("method " + traceElements[i].getMethodName());
                System.out.print("(" + traceElements[i].getClassName() + ":");
                System.out.print(traceElements[i].getLineNumber() + ")");
            }
        }
    }

    private static int sum(int[] list) {
        int result = 0;
        for (int i = 0; i <= list.length; i++) {
            result += list[i];
        }
        return result;
    }
}

程序清单 12-7 CircleWithException.java

public class CircleWithException {
    /**
     * The radius of the circle
     */
    private double radius;

    /**
     * The number of the objects created
     */
    private static int numberOfObjects = 0;

    /**
     * Construct a circle with radius 1
     */
    public CircleWithException() {
        this(1.0);
    }

    /**
     * Construct a circle with a specified radius
     */
    public CircleWithException(double newRadius) {
        setRadius(newRadius);
        numberOfObjects++;
    }

    /**
     * Return radius
     */
    public double getRadius() {
        return radius;
    }

    /**
     * Set a new radius
     */
    public void setRadius(double newRadius) throws IllegalArgumentException {
        if (newRadius >= 0) {
            radius = newRadius;
        } else {
            throw new IllegalArgumentException("Radius cannot be negative");
        }

    }

    /**
     * Return numberOfObjects
     */
    public static int getNumberOfObjects() {
        return numberOfObjects;
    }

    /**
     * Return the area of this circle
     */
    public double findArea() {
        return radius * radius * 3.14159;
    }
}

程序清单 12-8 TestCircleWithException.java

public class TestCircleWithException {
    public static void main(String[] args) {
        try {
            CircleWithException c1 = new CircleWithException(5);
            CircleWithException c2 = new CircleWithException(-5);
            CircleWithException c3 = new CircleWithException(0);
        } catch (IllegalArgumentException ex) {
            System.out.println(ex);
        }
        System.out.println("Number of objects created: " + CircleWithException.getNumberOfObjects());
    }
}

程序清单 12-9 ChainedExceptionDemo.java

public class ChainedExceptionDemo {
    public static void main(String[] args) {
        try {
            method1();
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }

    public static void method1() throws Exception {
        try {
            method2();
        } catch (Exception ex) {
            throw new Exception("New info from method1", ex);
        }
    }

    public static void method2() throws Exception {
        throw new Exception("New info from method2");
    }
}

程序清单 12-10 InvalidRadiusException.java

public class InvalidRadiusException extends Exception {
    private double radius;

    /**
     * Construct an exception
     */
    public InvalidRadiusException(double radius) {
        super("Invalid radius " + radius);
        this.radius = radius;
    }

    /**
     * Return the radius
     */
    public double getRadius() {
        return radius;
    }
}

程序清单 12-11 TestCircleWithCustomException.java

public class TestCircleWithCustomException {
    public static void main(String[] args) {
        try {
            new CircleWithCustomException(5);
            new CircleWithCustomException(-5);
            new CircleWithCustomException(0);
        } catch (InvalidRadiusException ex) {
            System.out.println(ex);
        }
        System.out.println("Number of objects created: " +
                CircleWithCustomException.getNumberOfObjects());
    }
}

class CircleWithCustomException {
    /**
     * The radius of the circle
     */
    private double radius;

    /**
     * The number of objects created
     */
    private static int numberOfObjects = 0;

    /**
     * Construct a circle with radius 1
     */
    public CircleWithCustomException() throws InvalidRadiusException {
        this(1.0);
    }

    /**
     * Construct a circle with a specified radius
     */
    public CircleWithCustomException(double newRadius) throws InvalidRadiusException {
        setRadius(newRadius);
        numberOfObjects++;
    }

    /**
     * Return radius
     */
    public double getRadius() {
        return radius;
    }

    /**
     * Set a new radius
     */
    public void setRadius(double newRadius) throws InvalidRadiusException {
        if (newRadius >= 0) {
            radius = newRadius;
        } else {
            throw new InvalidRadiusException(newRadius);
        }
    }

    /**
     * Return numberOfObjects
     */
    public static int getNumberOfObjects() {
        return numberOfObjects;
    }

    /**
     * Return the area of this circle
     */
    public double findArea() {
        return radius * radius * 3.14159;
    }
}

程序清单 12-12 TestFileClass.java

public class TestFileClass {
    public static void main(String[] args) {
        java.io.File file = new java.io.File("image/us.gif");
        System.out.println("Does it exist? " + file.exists());
        System.out.println("The file has " + file.length() + " bytes");
        System.out.println("Can it be read? " + file.canRead());
        System.out.println("Can it be written? " + file.canWrite());
        System.out.println("Is it a directory? " + file.isDirectory());
        System.out.println("Is it a file? " + file.isFile());
        System.out.println("Is it absolute? " + file.isAbsolute());
        System.out.println("Is it hidden? " + file.isHidden());
        System.out.println("Absolute path is " + file.getAbsolutePath());
        System.out.println("Last modified on " + new java.util.Date(file.lastModified()));
    }
}

程序清单 12-13 WriteData.java

public class WriteData {
    public static void main(String[] args) throws java.io.IOException {
        java.io.File file = new java.io.File("scores.txt");
        if (file.exists()) {
            System.out.println("File already exists");
            System.exit(1);
        }

        //Create a file
        java.io.PrintWriter output = new java.io.PrintWriter(file);

        //Write formatted output to the file
        output.print("John T Smith ");
        output.print(90);
        output.print("Eric K Jones ");
        output.println(85);

        //Close the file
        output.close();
    }
}

程序清单 12-14 WriteDataWithAutoClose.java

public class WriteDataWithAutoClose {
    public static void main(String[] args) throws Exception {
        java.io.File file = new java.io.File("scores.txt");
        if (file.exists()) {
            System.out.println("File already exists");
            System.exit(0);
        }
        try (
                //Create a file
                java.io.PrintWriter output = new java.io.PrintWriter(file);
        ) {
            //Write formatted output to the file
            output.print("John T Smith ");
            output.println(90);
            output.print("Eric K Jones ");
            output.print(85);
        }
    }
}

程序清单 12-15 ReadData.java

import java.util.Scanner;
public class ReadData {
    public static void main(String[] args) throws Exception {
        //Create a File instance
        java.io.File file = new java.io.File("scores.txt");

        //Create a Scanner for the file
        Scanner input = new Scanner(System.in);

        //Read data from a file
        while (input.hasNext()) {
            String firstName = input.next();
            String mi = input.next();
            String lastName = input.next();
            int score = input.nextInt();
            System.out.println(firstName + " " + mi + " " + lastName + " " + score);
        }

        //Close the file
        input.close();
    }
}

程序清单 12-16 ReplaceText.java

import java.io.File;
import java.io.PrintWriter;
import java.util.Scanner;

public class ReplaceText {
    public static void main(String[] args) throws Exception {
        //Check command line parameter usage
        if (args.length != 4) {
            System.out.println("Usage: java ReplaceText sourceFile targetFile oldStr newStr");
            System.exit(1);
        }

        //Check if source file exists
        File sourceFile = new File(args[0]);
        if (!sourceFile.exists()) {
            System.out.println("Source file " + args[0] + " does not exist");
            System.exit(2);
        }

        //Check if target file exists
        File targetFile = new File(args[1]);
        if (targetFile.exists()) {
            System.out.println("Target file " + args[1] + " already exists");
            System.exit(3);
        }

        try (
                //Create input and ouput files
                Scanner input = new Scanner(System.in);
                PrintWriter output = new PrintWriter(targetFile);
        ) {
            while (input.hasNext()) {
                String s1 = input.nextLine();
                String s2 = s1.replaceAll(args[2], args[3]);
                output.println(s2);
            }
        }
    }
}

程序清单 12-17 ReadFileFromURL.java

import java.util.Scanner;
public class ReadFileFromURL {
    public static void main(String[] args) {
        System.out.print("Enter a URL: ");
        String URLString = new Scanner(System.in).next();

        try {
            java.net.URL url = new java.net.URL(URLString);
            int count = 0;
            Scanner input = new Scanner(url.openStream());
            while (input.hasNext()) {
                String line = input.nextLine();
                count += line.length();
            }
            System.out.println("The file size is " + count + " characters");
        } catch (java.net.MalformedURLException ex) {
            System.out.println("Invalid URL");
        } catch (java.io.IOException ex) {
            System.out.println("I/O Errors: no such file");
        }
    }
}

程序清单 12-18 WebCrawler.java

import java.util.ArrayList;
import java.util.Scanner;

public class WebCrawler {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        System.out.print("Enter a URL: ");
        String url = input.nextLine();
        //Traverse the Web from a starting url
        crawler(url);
    }

    public static void crawler(String startingURL) {
        ArrayList<String> listOfPendingURLs = new ArrayList<>();
        ArrayList<String> listOfTraversedURLs = new ArrayList<>();

        listOfPendingURLs.add(startingURL);
        while (!listOfPendingURLs.isEmpty() && listOfTraversedURLs.size() <= 100) {
            String urlString = listOfPendingURLs.remove(0);
            if (!listOfTraversedURLs.contains(urlString)) {
                listOfTraversedURLs.add(urlString);
                System.out.println("Crawl " + urlString);

                for (String s : getSubURLs(urlString)) {
                    if (!listOfTraversedURLs.contains(s)) {
                        listOfPendingURLs.add(s);
                    }
                }
            }
        }
    }

    public static ArrayList<String> getSubURLs(String urlString) {
        ArrayList<String> list = new ArrayList<>();

        try {
            java.net.URL url = new java.net.URL(urlString);
            Scanner input = new Scanner(System.in);
            int current = 0;
            while (input.hasNext()) {
                String line = input.nextLine();
                current = line.indexOf("https:", current);
                while (current > 0) {
                    int endIndex = line.indexOf("\"", current);
                    if (endIndex > 0) {
                        //Ensure that a correct URL is found
                        list.add(line.substring(current, endIndex));
                        current = line.indexOf("https:", endIndex);
                    } else {
                        current = -1;
                    }
                }
            }
        } catch (Exception ex) {
            System.out.println("Error: " + ex.getMessage());
        }
        return list;
    }
}