How Java loads binaries and text files into String

public String read_file(String strFile_name)
    {
        System.out.println("read_file():strFile_name=["+strFile_name+"]");

        try{
            String strRead_data="";
            byte buf[] = new byte [1024];
            intiLength = 0;

            FileInputStream input=newFileInputStream(strFile_name);

            while((iLenh=input.read(buf))!=-1){
//              ↓ With this process, index over falls.
//              strRead_data+ = new String(buf, "UTF-8").substring(0, iLength);

                String strBuf = new String(buf, "UTF-8");
                System.out.println("iLengh=["+iLengh+"]");
                System.out.println("strBuf=["+strBuf+"]");
                strRead_data+=strBuf;// Even if only 50 bytes are read this time, 1024-50 bytes of data previously read are added after 50 bytes.

//              This makes an exception for index over
//              strRead_data+=strBuf.substring(0,iLenh);

// Have you tried emptying the buf??        Arrays.fill(buf, (byte)0);
            }

            input.close();

            return strRead_data;
        }
        catch(Exceptione){
            e.printStackTrace();
        }
        return null;
    }