Java 如果文件存在，则 PrintWriter 附加数据
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PrintWriter to append data if file exist
提问by TheWaveLad
I have a savegame file called mysave.savand I want to add data to this file if the file already exists. If the file doesn't exists, I want to create the file and then add the data.
我有一个游戏存档文件mysave.sav，如果该文件已经存在，我想将数据添加到该文件中。如果文件不存在，我想创建文件然后添加数据。
Adding data works fine. But appending data overwrites existing data. I followed the instructions of axtavt here (PrintWriter append method not appending).
添加数据工作正常。但是附加数据会覆盖现有数据。我按照 axtavt 的说明进行操作（PrintWriter append method not append）。
String savestr = "mysave.sav";
File f = new File(savestr);
PrintWriter out = new PrintWriter(savestr);
if ( f.exists() && !f.isDirectory() ) {
out = new PrintWriter(new FileOutputStream(new File(savestr), true));
out.append(mapstring);
out.close();
}
else {
out.println(mapstring);
out.close();
}
where mapstringis the string I want to append. Can you help me? Thank you!
mapstring我要附加的字符串在哪里。你能帮助我吗？谢谢！
采纳答案by Braj
Once you call PrintWriter out = new PrintWriter(savestr);the file is created if doesn't exist hence first check for file existence then initialize it.
一旦您调用PrintWriter out = new PrintWriter(savestr);该文件，如果不存在则创建该文件，因此首先检查文件是否存在，然后对其进行初始化。
As mentioned in it's Constructor Docmentationas well that says:
正如它的构造函数文档中提到的那样：
If the file exists then it will be truncated to zero size; otherwise, a new file will be created.
如果文件存在，那么它将被截断为零大小；否则，将创建一个新文件。
Since file is created before calling f.exists()hence it will return truealways and ìfblock is never executed at all.
由于文件是在调用之前创建的，f.exists()因此它将true始终返回并且ìf根本不会执行块。
Sample code:
示例代码：
String savestr = "mysave.sav";
File f = new File(savestr);
PrintWriter out = null;
if ( f.exists() && !f.isDirectory() ) {
out = new PrintWriter(new FileOutputStream(new File(savestr), true));
}
else {
out = new PrintWriter(savestr);
}
out.append(mapstring);
out.close();
For better resource handling use Java 7 - The try-with-resources Statement
为了更好的资源处理使用 Java 7 - The try-with-resources Statement
