code: public Jar setOutputStream(OutputStream os) {
          if (os == null)
              throw new NullPointerException("The OutputStream is null");
          if (jos != null)
              throw new IllegalStateException("Entries have already been added, the JAR has been written or setOutputStream has already been called.");
          this.os = os;
          return this;
      }
docs: Sets an {@link OutputStream} to which the JAR will be written.
  If used, this method must be called before any entries have been added or the JAR written. Calling this method prevents this
  object from using an internal buffer to store the JAR, and therefore, none of the other {@code write} methods can be called.
  
  @param os the target OutputStream of this JAR.
  @return {@code this}