Java读取文件乱码,请帮忙

类名:Test.java ,作用:读取一个文本文件中的繁体字,然后转为unicode.

aa.txt中的内容为:舉辦!

当我把类的Text file encoding设置gbk时,一切都能正常显示,可是我现在要把Text file encoding设为utf-8,此时
文字变成了乱码,unicode也不是文本中文字对应的编码了,请问怎么解决.我试着在程序中转码也不行,现在必须用utf-8.

Java code:

import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.io.UnsupportedEncodingException;

public class Test {
public String readTxt(int off, int leng) {
RandomAccessFile r = null;
String str = "";
try {
r = new RandomAccessFile(new File("D:/aa.txt"), "r");
byte[] c = new byte[leng];
r.seek(off);
r.read(c);
str = new String(c);
} catch (Exception e) {
try {
r.close();
} catch (IOException e1) {
e1.printStackTrace();
}
e.printStackTrace();
}

return str;
}

private static String convert(String str) {
String tmp;
StringBuffer sb = new StringBuffer(1000);
char c;
int i, j;
sb.setLength(0);
for (i = 0; i < str.length(); i++) {
c = str.charAt(i