GZIP 解压缩

[wangsufu77]

package GZIPUtils;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.util.zip.GZIPInputStream;
import java.util.zip.GZIPOutputStream;

public class ZipUtil2 {

// 压缩
public static String compress(String str) throws IOException {
if (str == null || str.length() == 0) {
return str;
}
ByteArrayOutputStream out = new ByteArrayOutputStream();
GZIPOutputStream gzip = new GZIPOutputStream(out);
gzip.write(str.getBytes());
gzip.close();
return out.toString("ISO-8859-1");
}

// 解压缩
public static String uncompress(String str) throws IOException {
if (str == null || str.length() == 0) {
return str;
}
ByteArrayOutputStream out = new ByteArrayOutputStream();
ByteArrayInputStream in = new ByteArrayInputStream(str
.getBytes("ISO-8859-1"));
GZIPInputStream gunzip = new GZIPInputStream(in);
byte[] buffer = new byte[256];
int n;
while ((n = gunzip.read(buffer))>= 0) {
out.write(buffer, 0, n);
}
// toString()使用平台默认编码，也可以显式的指定如toString("GBK")
return out.toString();
}

// 测试方法
public static void main(String[] args) throws IOException {

//测试字符串
String str="hello中国画";

System.out.println("原长度："+str.length());

System.out.println("压缩后："+ZipUtil2.compress(str).length());

System.out.println("解压缩："+ZipUtil2.uncompress(ZipUtil2.compress(str)));
}
}

上面的解压缩没有问题，但是如果把编码方式ISO-8859-1改为utf-8则报错"Not in GZIP format"
什么情况啊？