17.5

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

import javax.swing.*;

public class Test_17_5 extends JFrame{
    private JPanel jpHold = new JPanel();
    private JLabel jlb = new JLabel("Text file");
    private JTextField jtf = new JTextField(10);
    private JP jp = new JP();
    private File file;
    StringBuffer str = new StringBuffer();
    
    public Test_17_5(){
        jpHold.setLayout(new FlowLayout());
        jpHold.add(jlb);
        jpHold.add(jtf);
        
        setLayout(new BorderLayout(5,5));
        add(jpHold,BorderLayout.SOUTH);
        add(jp,BorderLayout.CENTER);    
        
        jtf.addKeyListener(new KeyAdapter(){

            @Override
            public void keyPressed(KeyEvent e) {
                // TODO Auto-generated method stub
                if(e.getKeyCode() == KeyEvent.VK_ENTER)
                {                    
                    try {
                        jp.showHistogram(countLetters());
                    } catch (FileNotFoundException e1) {
                        // TODO Auto-generated catch block
                        e1.printStackTrace();
                    }
                }
            }            
        });    
    }
    private String getTextContent() throws FileNotFoundException{
        
        file = new File(jtf.getText());
        Scanner input;
        if(file.exists() && file.canRead()){
            input = new Scanner(file);
            while(input.hasNext()){
//                string= input.nextLine();
                str.append(input.nextLine());
            }
        }
        return str.toString();
    }
    private int[] countLetters() throws FileNotFoundException{            
        int[] count = new int[26];
        String text = getTextContent();
        for(int i = 0; i < text.length(); i++){
            char character = text.charAt(i);
            
            if((character >= 'A') && (character <= 'Z')){
                count[character - 'A']++;
            }
            else if((character >= 'a') && (character <= 'z')){
                count[character - 'a']++;
            }
        }    
        return count;
    }
    
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Test_17_5 frame = new Test_17_5();
        frame.setTitle("Test_17_5");
        frame.setSize(400,500);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);        
    }
    
    
    class JP extends JPanel{
        private int[] count;
        
        public void showHistogram(int[] count){
            this.count = count;
            repaint();
        }        
        
        protected void paintComponent(Graphics g){
            if(count == null) return;
            
            super.paintComponent(g);
            
            int width = getWidth();
            int height = getHeight();
            int interval = (width - 40)/ count.length;
            int individualWidth = (int)(((width - 40) / 24) * 0.6);
            
            int maxCount = 0;
            for(int i = 0; i < count.length; i++){
                if(maxCount < count[i])
                    maxCount = count[i];
            }
            
            int x = 30;
            
            g.drawLine(10, height - 45, width - 10, height - 45);
            for(int i = 0; i < count.length; i++){
                int barHeight = (int)(((double)count[i] / (double)maxCount) * (height - 55));
                
                g.drawRect(x, height - 45 - barHeight, individualWidth, barHeight);
                g.drawString((char)(65 + i) + "", x, height - 30);
                
                x += interval;
            }
        }
    }
}

效果图：                                                                                  需要注意的地方：之前错误的使用了StringBuffer StringBuilder，导致程序一运行即错误，如下图：