[英]Attempting to break a while loop in Java
我花了最后幾個小時在互聯網和Java書籍中搜索答案,但最終我還是在這里發布了一個問題。 一旦用戶輸入“ DONE”,我試圖打破while循環,但是當我輸入完成時,它並沒有脫離循環。 我該如何解決這個問題。
這是我的代碼:
import java.io.*;
import java.util.Scanner;
public class Murray_A04Q2 {
public static void main(String[] args) throws IOException {
// Name of the file
String fileName = "userStrings.txt";
Scanner scan = new Scanner(System.in);
try {
// FileReader reading the text files in the default encoding.
FileWriter fileWriter = new FileWriter("userStrings.txt");
// Wrapping FileReader in BufferedReader.
BufferedWriter bufferedWriter = new BufferedWriter(fileWriter);
PrintWriter outFile = new PrintWriter (bufferedWriter);
// InputStreamReader & BufferedReader for user input
InputStreamReader isr = new InputStreamReader(System.in);
BufferedReader br = new BufferedReader(isr);
String input = "";
System.out.print("Enter something (DONE to quit): ");
input = scan.nextLine();
while (true) {
System.out.println("Enter something else or DONE to quit: ");
input = scan.nextLine();
input = br.readLine();
System.out.println(input);
if ((input = scan.nextLine()).equalsIgnoreCase("DONE")){
break;
}
}
bufferedWriter.write(input);
// Closing file
bufferedWriter.close();
}
catch (IOException ex){
System.out.println("Error writing to file " + "userStrings.txt" + "");
}
} // End of method header
} // End of class header
謝謝您的幫助!
該問題似乎來自於同時從兩個讀取器讀取相同的輸入流(以及有相當多的readLine()調用)
這是您的代碼中出現的錯誤:
import java.io.*;
import java.util.Scanner;
public class Murray_A04Q2 {
public static void main(String[] args) throws IOException {
String fileName = "userStrings.txt";
// Here you create a scanner using Standard Input as the input
// stream. (This is fine)
Scanner scan = new Scanner(System.in);
try {
// You say this is a FileREADER though it is clearly not...
// fix this comment!
// Either have remotely correct comments or just remove them.
FileWriter fileWriter = new FileWriter("userStrings.txt");
// Once again these are WRITERS not READERS.
BufferedWriter bufferedWriter = new BufferedWriter(fileWriter);
PrintWriter outFile = new PrintWriter(bufferedWriter);
// Despite bad comments, the above code is NOT a problem.
// You just created a scanner, using it for input.
// Why make a second object to read from the SAME input stream?
// Not only does this cause confusion in how the stream is handled,
// but its useless code when you already have a Scanner.
// Remove these two objects and all references to them and
// replace them with the Scanner reference (or vice versa).
InputStreamReader isr = new InputStreamReader(System.in);
BufferedReader br = new BufferedReader(isr);
// Setting to emptyquotes is useless unless you are using the
// variable before its next write. Still, not the error.
String input = "";
System.out.print("Enter something (DONE to quit): ");
// Begin problems. You are reading in a line here,
// and once again, YOU ARE DOING >NOTHING< with it
// before the next write. In your print you say
// "DONE to quit" but you are NOT checking if
// input.equals("DONE")
input = scan.nextLine();
while (true) {
System.out.println("Enter something else or DONE to quit: ");
// Stream Read number 2, variable write number 3, still you
// have not READ from the input variable.
input = scan.nextLine();
// Stream Read number 3, variable write number 4, still you
// have not READ from the input variable.
// Also, despite you using a different reader, this is still
// accessing and pulling data out of the same input stream.
// Whatever the Scanner has pulled out is no longer in the
// input stream.
input = br.readLine();
// Finally you are reading the string in the variable
// 'input'. On the first iteration of the while loop you
// will have written to input 3 times already. You will
// only be printing the 3RD LINE of input from the user.
// Still however, you are not checking if
// input.equals("DONE") which is supposed to be the
// condition terminating your loop.
System.out.println(input);
// Stream read number 4, variable write number 5, FINALLY
// checking the user's input against the string "DONE" and
// terminating the loop. On the first iteration of the loop,
// this stream read will only see the 4TH line of input from
// the user.
if ((input = scan.nextLine()).equalsIgnoreCase("DONE")) {
break;
}
}
bufferedWriter.write(input);
// Closing file
bufferedWriter.close();
}
catch (IOException ex) {
System.out.println("Error writing to file " + "userStrings.txt"
+ "");
}
} // End of method header
} // End of class header
給定以下示例用戶數據:
完成
你好,世界!
Java是愚蠢的。
是時候來壓縮漏洞了。
完成
堂
哈哈nope
這是您的代碼從第一個nextLine()調用逐行執行的操作。
首先從流中讀取(在循環之前, input = scan.nextLine();
)
發生了什么: input = "DONE";
從輸入流中刪除第一行“ DONE”,並將其分配給input
。 input
的先前值(空字符串)丟失。
從流中進行第二次讀取(循環中, input = scan.nextLine();
)
發生了什么: input = "Hello World!";
第二行,“ Hello World!” 已從輸入流中刪除並分配給input
。 input
的先前值DONE
丟失。
從流中進行第三次讀取(循環中, input = br.readLine();
)
發生了什么: input = "Java is kool.";
第三行,“ Java是愚人”。 已從輸入流中刪除並分配給input
。 input
的先前值“ Hello World!” 迷路了。
輸出到標准輸出流(循環, System.out.println(input);
)
發生了什么: System.out.println("Java is kool.");
第三行,“ Java是愚人”。 打印到標准輸出流(System.out)。
從流中第四次讀取(循環中, input = scan.nextLine();
)
發生了什么: input = "Time to squash bugs.";
第四行,“壓縮錯誤的時間”。 已從輸入流中刪除並分配給input
。 input
的先前值“ Java is kool”丟失。
對賦值結果調用equalsIgnoreCase(String)(循環中, (input = scan.nextLine()).equalsIgnoreCase("DONE")
)
發生了什么事: "Time to squash bugs.".equalsIgnoreCase("DONE")
輸入的第四行是“壓縮錯誤的時間”。 被測試以查看字符串是否相等,而忽略所有大寫字母。 它們不是,因此跳過if
塊。 沒有else
因此代碼執行在if
塊之后繼續。
循環結束。 在這一點上,沒有任何東西可以強制終止循環,因此執行可以返回到循環的條件。 條件( true
)允許循環執行,因此它再次從頂部開始。
從流中第五次讀取(循環中, input = scan.nextLine();
)
發生了什么: input = "Done";
第五行“完成”已從輸入流中刪除,並分配給input
。 input
的先前值“壓縮錯誤的時間”。 迷路了。
從流中第六次讀取(循環中, input = br.readLine();
)
發生了什么: input = "dONe";
第六行“ dONe”從輸入流中刪除,並分配給input
。 input
的先前值“ Done”丟失。
輸出到標准輸出流(循環, System.out.println(input);
)
發生了什么: System.out.println("dONe");
第六行“ dONe”被打印到標准輸出流(System.out)。
從流中讀取第七次(循環中, input = scan.nextLine();
)
發生了什么: input = "lol nope";
第七行“哈哈nope”從輸入流中刪除,並分配給input
。 input
的先前值“ dONe”丟失。
對賦值結果調用equalsIgnoreCase(String)(循環中, (input = scan.nextLine()).equalsIgnoreCase("DONE")
)
發生了什么: "lol nope".equalsIgnoreCase("DONE")
輸入的第七行“ lol nope”被測試以查看字符串是否相等,而忽略所有大寫字母。 它們不是,因此跳過if
塊。 沒有else
因此代碼執行在if
塊之后繼續。
循環結束。 在這一點上,沒有任何東西可以強制終止循環,因此執行可以返回到循環的條件。 條件(true)允許循環執行,因此它再次從頂部開始。
在示例輸入中,“ lol nope”之后沒有更多輸入,因此對scan.nextLine()
的下一次調用將阻塞,程序將凍結,直到有輸入才無法執行任何操作。 通過當前設置代碼的方式,僅會讀入用戶輸入的第四行(包括第四行)之后的第三行,並檢查與“ DONE”是否相等。
完成
你好,世界!
Java是愚蠢的。
是時候來壓縮漏洞了。
完成
堂
哈哈nope
首先修復:擺脫變量isr
和br
干脆只使用scan
(或相反,擺脫了scan
,並使用isr
和br
)。 由於他們從相同的輸入流中提取數據,因此它們以幾乎相同的方式完成相同的工作,從而創建了不必要的且令人困惑的代碼。
第二種解決方法:如果您在兩次調用readLine()或nextLine()之間沒有使用輸入變量做任何事情,並且沒有在使用這些調用來消除垃圾輸入數據,請清除它們。 您只需調用一次readLine()或nextLine()並將其存儲到input
,該值將一直保留到下一次分配給input
為止。
如果您實際上使用了多次調用readLine()或nextLine()來檢索用戶數據,但該數據可能包含“ DONE”,則您需要用if塊替換對readLine()或nextLine()的調用,因為它會讀取輸入並立即檢查是否等於“ DONE”。
根據您的需求,對代碼進行一些可能的修復(我假設您修剪了一些您認為不相關的代碼,或者只是尚未添加代碼而已。)
import java.io.*;
import java.util.Scanner;
public class Murray_A04Q2 {
public static void main(String[] args) throws IOException {
String fileName = "userStrings.txt";
Scanner scan = new Scanner(System.in);
try {
FileWriter fileWriter = new FileWriter("userStrings.txt");
BufferedWriter bufferedWriter = new BufferedWriter(fileWriter);
PrintWriter outFile = new PrintWriter(bufferedWriter);
String input;
System.out.print("Enter something (DONE to quit): ");
if (!(input = scan.nextLine()).equalsIgnoreCase("DONE")) { // Added "!" (not)
// Some one time use code goes here.
System.out.println("Enter something else or DONE to quit: ");
while (true) {
if ((input = scan.nextLine()).equalsIgnoreCase("DONE")) {
break;
}
// Some code goes here
if ((input = scan.nextLine()).equalsIgnoreCase("DONE")) {
break;
}
// Some DIFFERENT code goes here.
System.out.println(input);
if ((input = scan.nextLine()).equalsIgnoreCase("DONE")) {
break;
}
// Some code that didn't belong above can go here.
}
}
bufferedWriter.write(input);
bufferedWriter.close();
}
catch (IOException ex) {
System.out.println("Error writing to file " + "userStrings.txt"
+ "");
}
}
}
也:
import java.io.*;
import java.util.Scanner;
public class Murray_A04Q2 {
public static void main(String[] args) throws IOException {
String fileName = "userStrings.txt";
Scanner scan = new Scanner(System.in);
try {
FileWriter fileWriter = new FileWriter("userStrings.txt");
BufferedWriter bufferedWriter = new BufferedWriter(fileWriter);
PrintWriter outFile = new PrintWriter(bufferedWriter);
String input;
System.out.print("Enter something (DONE to quit): ");
if (!(input = scan.nextLine()).equalsIgnoreCase("DONE")) { // Added "!" (not)
// Some one time use code goes here.
System.out.println("Enter something else or DONE to quit: ");
while ((input = scan.nextLine()).equalsIgnoreCase("DONE")) {
System.out.println(input);
// Some code can go here.
}
}
bufferedWriter.write(input);
bufferedWriter.close();
}
catch (IOException ex) {
System.out.println("Error writing to file " + "userStrings.txt"
+ "");
}
}
}
或者,如果您希望所有代碼在循環中運行:
import java.io.*;
import java.util.Scanner;
public class Murray_A04Q2 {
public static void main(String[] args) throws IOException {
String fileName = "userStrings.txt";
Scanner scan = new Scanner(System.in);
try {
FileWriter fileWriter = new FileWriter("userStrings.txt");
BufferedWriter bufferedWriter = new BufferedWriter(fileWriter);
PrintWriter outFile = new PrintWriter(bufferedWriter);
String input;
System.out.print("Enter something (DONE to quit): ");
while ((input = scan.nextLine()).equalsIgnoreCase("DONE")) {
System.out.println(input);
// Some code can go here.
System.out.println("Enter something else or DONE to quit: ");
}
bufferedWriter.write(input);
bufferedWriter.close();
}
catch (IOException ex) {
System.out.println("Error writing to file " + "userStrings.txt"
+ "");
}
}
}
您正在將掃描儀包裹在閱讀器上,然后嘗試同時讀取它們,這使它們在可用輸入之間進行爭奪。 直接消除閱讀器中的讀物。
我建議您首先嘗試使用實施的簡化版本,然后再嘗試將所需的所有內容添加到文件中。
import java.util.Scanner;
public class Murray_A04Q2 {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
String input = "";
System.out.print("Enter something (DONE to quit): ");
input = scan.nextLine();
System.out.println(input);
while (!input.equalsIgnoreCase("DONE")) {
System.out.println("Enter something else or DONE to quit: ");
input = scan.nextLine();
System.out.println(input);
}
scan.close();
System.out.println("DONE!!");
}
}
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