@CanIgnoreReturnValue
    public Builder setSystemInstruction(Content systemInstruction) {
      checkNotNull(
          systemInstruction,
          "system instruction can't be null. "
              + "Use Optional.empty() if no system instruction should be provided.");
      this.systemInstruction = Optional.of(systemInstruction);
      return this;
    }
Sets a system instruction that will be used by default to interact with the generative model.
java-vertexai/google-cloud-vertexai/src/main/java/com/google/cloud/vertexai/generativeai/GenerativeModel.java