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();
}

InputStream,String,File相互转化