《Java编程入门官方教程》第十六章练习答案

【练习16-1(SwingFC.java)】基于Swing的文件比较实用程序

虽然读者只学习了一点有关Swing的知识，但是已经可以使用这些知识来创建一个实用的小程序了。练习10-1中创建了一个基于控制台的文件比较实用程序，本练习将创建该程序的一个基于Swing的版本。你将会看到，为该程序提供基于Swing的用户界面能够显著改善程序的外观，使它易于使用。图16-6为该程序的输出。

由于Swing简化了基于GUI程序的创建，你会对创建该程序的简单程度感到惊奇。

package javaone.a.beginners.guide.chaptersixteen;

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.FileInputStream;
import java.io.IOException;

public class ChapterSixteenProgramOne implements ActionListener {

    JTextField jtfFirst;    // holds the first file name
    JTextField jtfSecond;   // holds the second file name

    JButton jbtnComp;    // button to compare the files

    JLabel jlabFirst, jlabSecond;   // display prompts
    JLabel jlabResult;  // display results and error messages

    ChapterSixteenProgramOne(){
        // Create a new JFrame container.
        JFrame jfrm = new JFrame("Compare Files");
        // Specify FlowLayout for the layout manager.
        jfrm.setLayout(new FlowLayout());
        // Give the frame an initial size.
        jfrm.setSize(200, 190);
        // Terminate the program when the user closes the application.
        jfrm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        // Create the text fields for the file names.
        jtfFirst = new JTextField(14);
        jtfSecond = new JTextField(14);
        // Set the action commands for the text fields.
        jtfFirst.setActionCommand("fileA");
        jtfSecond.setActionCommand("fileB");
        // Create the Compare button.
        jbtnComp = new JButton("Compare");
        // Add action listener for the Compare button.
        jbtnComp.addActionListener(this);
        // Create the labels.
        jlabFirst = new JLabel("First file: ");
        jlabSecond = new JLabel("Second file: ");
        jlabResult = new JLabel("");
        // Add the compontents to the content pane.
        jfrm.add(jlabFirst);
        jfrm.add(jtfFirst);
        jfrm.add(jlabSecond);
        jfrm.add(jtfSecond);
        jfrm.add(jbtnComp);
        jfrm.add(jlabResult);
        // Display the frame.
        jfrm.setVisible(true);
    }
    // Compare the files when the Compare button is pressed.
    @Override
    public void actionPerformed(ActionEvent ae) {
        int i=0, j=0;
        // First, confirm that both file names have
        // been entered.
        if(jtfFirst.getText().equals("")){
            jlabFirst.setText("First file name missing.");
        }
        if(jtfSecond.getText().equals("")){
            jlabSecond.setText("Second file name missing.");
        }
        // Compare files. Use try-with-resources to manage the files.
        try(FileInputStream fOne = new FileInputStream(jtfFirst.getText());
            FileInputStream fTwo = new FileInputStream(jtfSecond.getText())) {
            // Check the contents of each file.
            do{
                i = fOne.read();
                j = fTwo.read();
                if(i != j){
                    break;
                }
            }while (i != -1 && j != -1);
            if(i != j){
                jlabResult.setText("Files are not the same.");
            }else{
                jlabResult.setText("Files compare equal.");
            }
        }catch (IOException exc){
            jlabResult.setText("File Error");
        }
    }
    public static void main(String[] args) {
        // Create the frame on the event dispatching thread.
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                new ChapterSixteenProgramOne();
            }
        });
    }
}

16.11 自测题

1. 一般而言，AWT组件是重量级的，而Swing组件是________。

答案：轻量级。

2. Swing组件的外观和行为可以修改吗?如果可以，是哪一种功能提供的支持?

答案：可以，Swing的可插入式外观支持该操作。

3. 应用程序最常用的顶级容器是什么?

答案：JFrame。

4. 顶级容器有多个窗格，哪一个窗格用来添加组件?

答案：内容窗格。

5. 说明如何创建一个包含消息"Select an entry from the list"的标签?

答案：JLabel("Select an entry from the list");

6. 与GUI组件的所有交互都必须发生在哪个线程之上?

答案：事件分派线程。

7. 与JButton关联的默认动作命令是什么?如何修改该动作命令?

答案：默认的动作命令字符串是按钮中显示的文本，可以通过调用setActionCommand()方法修改。

8. 单击按钮时生成什么事件?

答案： ActionEvent。

9. 如何创建一个包含32列的文本域?

答案：JTextField(32);

10. JTextField有动作命令集吗?如果有，它是如何起作用的?

答案：可以，通过调用setActionCommand()。

11. 哪种Swing组件可以创建复选框，选中和取消选中复选框时将发生哪一种事件?

答案：JCheckBox创建复选框。ItemEvent事件在选中或取消选中复选框时生成。

12. JList显示一个条目列表，用户可以从中进行选择，这种说法是否正确?

答案：正确。

13. 用户在JList中选择或取消选择条目时将生成什么事件?

答案：ListSelectionEvent。

14. 什么方法用来设置JList的选择模式?什么方法获得第一个选中的条目的索引?

答案：setSelectionMode()设置选择模式。getSelectionIndex()获取第一个选中条目的索引。

15. 为练习15-1中开发的文件比较器添加一个复选框，包含以下文本:Show position of mismatch(显示未匹配的位置)。当复选框选中时，让程序显示文件中遇到的第一处未匹配的位置。

package javaone.a.beginners.guide.chaptersixteen;

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.FileInputStream;
import java.io.IOException;

/*
    Try This 16-1

    A Swing-based file comparsion utility.

    This version has a check box that causes the
    location of the first mismatch to be shown.
 */
public class ChapterSixteenExerciseFifteen implements ActionListener {

    JTextField jtfFirst;    // holds the first file name
    JTextField jtfSecond;   // holds the second file name

    JButton jbtnComp;    // button to compare the files

    JLabel jlabFirst, jlabSecond;   // display prompts
    JLabel jlabResult;  // display results and error messages

    JCheckBox jcbLoc;   // check to display location of mismatch

    ChapterSixteenExerciseFifteen(){
        // Create a new JFrame container.
        JFrame jfrm = new JFrame("Compare Files");
        // Specify FlowLayout for the layout manager.
        jfrm.setLayout(new FlowLayout());
        // Give the frame an initial size.
        jfrm.setSize(200, 220);
        // Terminate the program when the user closes the application.
        jfrm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        // Create the text fields for the file names.
        jtfFirst = new JTextField(14);
        jtfSecond = new JTextField(14);
        // Set the action commands for the text fields.
        jtfFirst.setActionCommand("fileA");
        jtfSecond.setActionCommand("fileB");
        // Create the Compare button.
        jbtnComp = new JButton("Compare");
        // Add action listener for the Compare button.
        jbtnComp.addActionListener(this);
        // Create the labels.
        jlabFirst = new JLabel("First file: ");
        jlabSecond = new JLabel("Second file: ");
        jlabResult = new JLabel("");
        // Create check box.
        jcbLoc = new JCheckBox("Show position of mismatch");
        // Add the compontents to the content pane.
        jfrm.add(jlabFirst);
        jfrm.add(jtfFirst);
        jfrm.add(jlabSecond);
        jfrm.add(jtfSecond);
        jfrm.add(jcbLoc);
        jfrm.add(jbtnComp);
        jfrm.add(jlabResult);
        // Display the frame.
        jfrm.setVisible(true);
    }
    // Compare the files when the Compare button is pressed.
    @Override
    public void actionPerformed(ActionEvent e) {
        int i=0, j=0;
        int count = 0;
        // First, confirm that both file names have
        // been entered.
        if(jtfFirst.getText().equals("")){
            jlabFirst.setText("First file name missing.");
        }
        if(jtfSecond.getText().equals("")){
            jlabSecond.setText("Second file name missing.");
        }
        // Compare files. Use try-with-resources to manage the files.
        try(FileInputStream fOne = new FileInputStream(jtfFirst.getText());
            FileInputStream fTwo = new FileInputStream(jtfSecond.getText())) {
            // Check the contents of each file.
            do{
                i = fOne.read();
                j = fTwo.read();
                if(i != j){
                    break;
                }
                count++;
            }while (i != -1 && j != -1);
            if(i != j){
                if(jcbLoc.isSelected()){
                    jlabResult.setText("Files differ at location " + count);
                }else{
                    jlabResult.setText("Files are not the same.");
                }
            }else{
                jlabResult.setText("Files compare equal.");
            }
        }catch (IOException exc){
            jlabResult.setText("File Error");
        }
    }
    public static void main(String[] args) {
        // Create the frame on the event dispatching thread.
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                new ChapterSixteenExerciseFifteen();
            }
        });
    }
}

16. 修改ListDemo程序，使其允许在列表中选中多个条目。

package javaone.a.beginners.guide.chaptersixteen;

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

// Demonstarte multiple selection in a JList.
public class ChapterSixteenExerciseSixteen implements ListSelectionListener {

    JList<String> jlst;
    JLabel jlab;
    JScrollPane jscrlp;

    // Create an array of names.
    String names[]= { "Sherry", "Jon", "Rachel",
            "Saaha", "Josselyn", "Randy",
            "Tom", "Mary", "Ken",
            "Andrew", "Matt", "Todd"
    };

    ChapterSixteenExerciseSixteen(){
        // Create a new JFrame container.
        JFrame jfrm =  new JFrame("JList Demo");
        // Specify a flow layout.
        jfrm.setLayout(new FlowLayout());
        // Given the frame an initial size.
        jfrm.setSize(200, 160);
        // Terminate the program when the user closes the application.
        jfrm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        // Create a JList.
        jlst = new JList<String>(names);
        // By removing following line, multiple selection (which
        // is the default behavior of a JList) will be used.
//        jlst.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
        // Add list to a scroll pane.
        jscrlp = new JScrollPane(jlst);
        // Set the preferred size of the scroll pane.
        jscrlp.setPreferredSize(new Dimension(120, 90));
        // Make a label that displays the selection.
        jlab = new JLabel("Please choose a name");
        // Add list selection handler.
        jlst.addListSelectionListener(this);
        // Add the list and label to the content pane.
        jfrm.add(jscrlp);
        jfrm.add(jlab);
        // Display the frame.
        jfrm.setVisible(true);
    }
    // Handle list selection events.
    @Override
    public void valueChanged(ListSelectionEvent le) {
        // Get the indices of the changed item.
        int indices[] = jlst.getSelectedIndices();
        // Display the selection, if one or more items
        // were selected.
        if(indices.length != 0){
            String who = "";
            // Contruct a string of the names:
            for(int i: indices){
                who += names[i] + " ";
            }
            jlab.setText("Current selection: " + who);
        }else{  // Otherwise, reprompt.
            jlab.setText("Please choose a name");
        }
    }
    public static void main(String[] args) {
        // Create the frame on the event dispatching thread.
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                new ChapterSixteenExerciseSixteen();
            }
        });
    }
}