cdm/src/main/java/ucar/nc2/Group.java
public Group setImmutable() {
    super.setImmutable();
    variables = Collections.unmodifiableList(variables);
    dimensions = Collections.unmodifiableList(dimensions);
    groups = Collections.unmodifiableList(groups);
    attributes.setImmutable();
    return this;
  }
public Group setImmutable() {
    super.setImmutable();
    variables = Collections.unmodifiableList(variables);
    dimensions = Collections.unmodifiableList(dimensions);
    groups = Collections.unmodifiableList(groups);
    attributes.setImmutable();
    return this;
  }
Make this immutable.

@return this