Exceptional Class (Page 1 of 4 )
When a method throws an exception, Java requires that it be caught. Some exceptions require action on the programmer’s part and others simply need to be reported to the user. The Java class that we will develop in this article is concerned with the latter type of exception. Instead of rewriting code every time you need to catch an exception, you can create a class to do most of the work for you.
Introduction
In a previous article, Java Help Files, we discussed how to create a class for displaying help files. Here we will take the same approach to handling exceptions. In some cases, exceptions can be handled quietly in the background without notifying the end user. However, we are concerned with the situations where it makes sense to display error messages.
The code to do this is not very complicated but gets a bit more interesting when we try to format the way our error message is displayed. We will show a few different approaches to doing this. We might have some fun along the way -– taking different methods out for a spin and seeing how they drive.
The Code
Briefly described, this class uses a JOptionPane to display a dialogue box showing the exception type in the title bar and the error message in the body. Error messages created by the Java API are fairly uniform, but there are many occasions when you will use classes created by other programmers –- database drivers for instance. Here you won’t be sure how or if the messages have been formatted. Error strings can end up being very long and may require the insertion of newline characters in order to display properly. We will develop three different methods of doing this, but for the moment have a quick look at the code below.
1:////////////////////////////////////////////////////////
2://ErrorDialogue class
3:////////////////////////////////////////////////////////
4://comments for javadoc below
5:/** class ErrorDialogue
6:* This class is for use in catch clauses. It will display
7:* a dialogue box showing the error message.
8:*/
9://put imports here
10:import javax.swing.*;
11:import java.awt.*;
12:import java.util.*;
13:
14:public class ErrorDialogue{
15://data members
16:private Component window;
17:private final int increment = 30;
18:////////////////////////////////////////////////////////
19://constructors
20:////////////////////////////////////////////////////////
21:public ErrorDialogue(Exception e, Component window) {
22: this.window = window;
23: doDialogue(e);
24:}
25://end constructors
26:////////////////////////////////////////////////////////
27://private functions
28:////////////////////////////////////////////////////////
29:private void doDialogue(Exception e) {
30: Class error = e.getClass();
31: String errname = error.getName();
32: String message=e.getMessage();
33: String messagetwo = e.getMessage();
34: //check if contains newline
35: if (e.getMessage().indexOf('\n') == -1) {
36: //find length
37: int length = message.length();
38: //break at intervals
39: if (length > increment){
40: message=quickAndDirty(message);
41: messagetwo=insertNewline(messagetwo);
42: }
43: }
44: JOptionPane.showMessageDialog(window, "Error - " + message,
45: errname, JOptionPane.WARNING_MESSAGE);
46: JOptionPane.showMessageDialog(window, "Error - " + messagetwo,
47: errname, JOptionPane.ERROR_MESSAGE);
48:
49:
50:}
51:////////////////////////////////////////////////////////
52:/**
53:* First method of parsing the string
54:*/
55:private String quickAndDirty(String message){
56: //find space closest to midpoint
57: StringBuffer sb= new StringBuffer(message);
58: Character space=new Character(' ');
59: int strlength = message.length();
60: int midpoint = strlength/2;
61: for(int x= midpoint; x < strlength; x++){
62: if(new Character(sb.charAt(x)).equals(space)){
63: sb.insert(x,"\n");
64: break;
65: }
66: }
67: String newstring = new String(sb);
68: return newstring;
69:}
70://///////////////////////////////////////////////////////
71:/**
72:* second method of parsing the string
73:*/
74:private String insertNewline(String message){
75: String tail = "";
76: String head = "";
77: int newstart=29;
78: int breakpoint=0;
79: tail = message.substring(newstart);
80: int length=message.length();
81: head = message.substring(0,newstart);
82: while(length>increment && tail.indexOf(" ")!=-1){
83: //find next space, insert break and concatenate
84: breakpoint = tail.indexOf(" ")+1;
85: head += tail.substring(0,breakpoint);
86: head += "\n";
87: tail=tail.substring(breakpoint);
88: length=tail.length();
89: if (length > increment && tail.indexOf(" ",newstart)!=-1){
90: head+=tail.substring(0,newstart);
91: tail=tail.substring(newstart);
92: }else{
93: head+=tail;
94: message = head;
95: break;
96: }
97: }
98: return message;
99:}
100:////////////////////////////////////////////////////////
101:/**
102:* Third method of parsing the string
103:*/
104:private String insertNewlineToken(String message){
105: StringTokenizer stk = new StringTokenizer(message, " ", true);
106: String temp = "";
107: String newstring = "";
108: int maxlength = increment;
109: while(stk.hasMoreTokens()){
110: temp = stk.nextToken();
111: newstring += temp;
112: //add newline if longer and don't start with a space
113: if (newstring.length() > maxlength && temp.equals(" ")){
114: newstring += "\n";
115: maxlength = newstring.length() + increment;
116: }
117: }
118: return newstring;
119:}
120:public static void main (String [] args){
121: String message = "Let's have a really long message here";
122: message+=" so that we can test our class. ";
123: message+="Blah blah blah blah ...";
124: Exception e = new Exception(message);
125: new ErrorDialogue(e, null);
126: System.exit(0);
127:}
128:}//end class
Next: General Comments >>
More Java Articles
More By Peter Lavin
/ 19 
