1. String --> InputStream
InputStream String2InputStream(String str){
ByteArrayInputStream stream = new ByteArrayInputStream(str.getBytes());
return stream;
}

2. InputStream --> String
String inputStream2String(InputStream is){
BufferedReader in = new BufferedReader(new InputStreamReader(is));
StringBuffer buffer = new StringBuffer();
String line = "";
while ((line = in.readLine()) != null){
buffer.append(line);
}
return buffer.toString();
}

今天从网上看到了另一种方法，特拿来分享

String all_content=null;
try {
all_content =new String();

InputStream ins = 获取的输入流;

ByteArrayOutputStream outputstream = new ByteArrayOutputStream();
byte[] str_b = new byte[1024];
int i = -1;
while ((i=ins.read(str_b)) > 0) {
outputstream.write(str_b,0,i);
}
all_content = outputstream.toString();
} catch (Exception e) {

e.printStackTrace();
}

此两种方法上面一种更快，但是比较耗内存，后者速度慢，耗资源少

3、File --> InputStream
InputStream in = new InputStream(new FileInputStream(File));

4、InputStream --> File
public void inputstreamtofile(InputStream ins,File file){
OutputStream os = new FileOutputStream(file);
int bytesRead = 0;
byte[] buffer = new byte[8192];
while ((bytesRead = ins.read(buffer, 0, 8192)) != -1) {
os.write(buffer, 0, bytesRead);
}
os.close();
ins.close();
}

from: http://apps.hi.baidu.com/share/detail/19357535