import java.io.FileReader;
 
　・・・
 
String filename = "c:\\hoge.txt"
FileReader reader = new FileReader(filename);
 
char c = (char)reader.read();  // 1文字読み込み
 
reader.close();

import java.io.FileWriter;
 
　・・・
 
String filename="c:\\hoge.txt"
FileWriter writer = new FileWriter(filename);
 
writer.write('あ');
 
writer.close();

import java.io.InputStream;
import java.net.URL;
 
　・・・
 
URL url = new URL("https://s-soto.mydns.jp/wiki");
InputStream in = url.openStream();
 
int i = in.read();
while (i != -1) {
    System.out.print((char)i);
    i = in.read();
}
 
in.close();