In spec is written:

public java.util.List<RowSorter.SortKey> getSortKeys()

    Returns the current sort keys. This must not return null.

    Specified by:
        getSortKeys in class RowSorter

    Returns:
        the current sort order


If I run the following code:
TableModel dataModel = new DefaultTableModel(5,3);
TableRowSorter rs = new TableRowSorter(dataModel);
LinkedList<RowSorter.SortKey> keys =
new LinkedList<RowSorter.SortKey>();
keys.add(new RowSorter.SortKey(0, SortOrder.DESCENDING));
rs.setSortKeys(keys);
rs.getSortKeys().add( new RowSorter.SortKey(0, SortOrder.DESCENDING));

I get:
java.lang.UnsupportedOperationException
at java.util.Collections$UnmodifiableCollection.add(Collections.java:1021)
at SorterChanged.convertRowIndexToModel0002(SorterChanged.java:265)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at javasoft.sqe.javatest.lib.MultiTest.invokeTestCase(MultiTest.java:399)
at javasoft.sqe.javatest.lib.MultiTest.run(MultiTest.java:195)
at javasoft.sqe.javatest.lib.MultiTest.run(MultiTest.java:127)
at SorterChanged.main(SorterChanged.java:74)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:86)

This exception is not specified.