This is an example of function I use to replace Java for my work.
var string = '\u5909\u63db\u30c6\u30b9\u30c8';
file.initWithPath('C:\\temp\\temp.txt');
file.create(file.NORMAL_FILE_TYPE, 0666);
var charset = 'EUC-JP';
var fileStream = Components
.classes['@mozilla.org/network/file-output-stream;1']
.createInstance(Components.interfaces.nsIFileOutputStream);
fileStream.init(file, 2, 0x200, false);
var converterStream = Components
.classes['@mozilla.org/intl/converter-output-stream;1']
.createInstance(Components.interfaces.nsIConverterOutputStream);
converterStream.init(fileStream, charset, string.length,
Components.interfaces.nsIConverterInputStream.DEFAULT_REPLACEMENT_CHARACTER);
converterStream.writeString(string);
converterStream.close();
fileStream.close();
On mozilla developer documentation you can find more about this XPCOM that can be used for reading/writing files and similar procedures. So instead of Java I use this.