In this example we shall show you how to handle exceptions in constructors. To handle exceptions in constructors we have performed the following steps:
- We have created a class,
InputFile, that has a BufferedReader field. - In its constructor, it gets a String and it creates a new FileReader with the given String name of path to file to read from. A FileNotFoundException might be thrown here, that needs to be caught, but since the file is not found the BufferedReader is not opened so does not need to close.
- If any other exception occurs after that, the FileReader has opened so it has to close.
- The
InputFileclass also has two methods,getLine(), that gets the line of the text in the BufferedReader, anddispose()that closes the BufferedReader. The methods throw RuntimeException that has to be caught. - We create a new instance of
InputFile, with a given String. Since the constructor of the class is called if the file is not found the exception will be thrown,
as described in the code snippet below.
001 002 003 004 005 006 007 008 009 010 011 012 013 014 015 016 017 018 019 020 021 022 023 024 025 026 027 028 029 030 031 032 033 034 035 036 037 038 039 040 041 042 043 044 045 046 047 048 049 050 051 052 053 054 055 056 057 058 059 060 061 062 063 064 065 066 067 068 069 070 071 072 073 074 075 076 077 078 079 080 081 082 083 084 085 086 087 088 089 090 091 092 093 094 095 096 097 098 099 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 | package com.javacodegeeks.snippets.core;import java.io.BufferedReader;import java.io.FileNotFoundException;import java.io.FileReader;import java.io.IOException;public class ExceptionInConstructor { public static void main(String[] args) { try {InputFile inputFile = new InputFile("Cleanup.java");String string;int i = 1;while ((string = inputFile.getLine()) != null) ; // Perform line-by-line processing here...inputFile.dispose(); } catch (Exception e) {System.err.println("Caught Exception in main");e.printStackTrace(); } }}class InputFile { private BufferedReader input; public InputFile(String fileName) throws Exception { try {input = new BufferedReader(new FileReader(fileName));// Other code that might throw exceptions } catch (FileNotFoundException e) {System.err.println("Could not open " + fileName);// Wasn't open, so don't close itthrow e; } catch (Exception e) {// All other exceptions must close ittry { input.close();} catch (IOException e2) { System.err.println("in.close() unsuccessful");}throw e; // Rethrow } finally {// Don't close it here!!! } } public String getLine() { String s; try {s = input.readLine(); } catch (IOException e) {throw new RuntimeException("readLine() failed"); } return s; } public void dispose() { try {input.close();System.out.println("dispose() successful"); } catch (IOException e2) {throw new RuntimeException("in.close() failed"); } }} |
Output:
dispose() successful
This was an example of how to handle exceptions in constructors in Java.
