code: static public byte[] expandByteArray(byte[] src, byte[] dst, int srcPos, int srcLength, int dstPos, int dstLength) {
          byte[] rc = null;
          int totalLen = 0;
          if (null != src) {
              totalLen += srcLength;
          }
          if (null != dst) {
              totalLen += dstLength;
          }
          if (0 < totalLen) {
              rc = new byte[totalLen];
              if (TraceComponent.isAnyTracingEnabled() && tc.isDebugEnabled()) {
                  Tr.debug(tc, "Created byte[] of size " + totalLen);
              }
              try {
                  if (null != src) {
                      System.arraycopy(src, srcPos, rc, 0, srcLength);
                  }
                  if (null != dst) {
                      System.arraycopy(dst, dstPos, rc, srcLength, dstLength);
                  }
              } catch (Exception e) {
                  // no FFDC required
                  // any error from arraycopy, we'll just return null
                  if (TraceComponent.isAnyTracingEnabled() && tc.isDebugEnabled()) {
                      Tr.debug(tc, "Exception " + e + " while copying.");
                  }
                  rc = null;
              }
          }
          if (TraceComponent.isAnyTracingEnabled() && tc.isDebugEnabled()) {
              Tr.debug(tc, "expandByteArray returning: [" + getEnglishString(rc) + "]");
          }
          return rc;
      }
docs: Helper function to concat two byte[]s into a new, larger one.
  
  @param src
  @param dst
  @param srcPos
  @param srcLength
  @param dstPos
  @param dstLength
  @return byte[] (null if any errors)