how to set value with reflection

I am writing J-unit test case for one my method.

A.java

 void resetToolbar(final ListSelectionModel lastSelectionModel) {
        // attempt to restore the previous page
        if (toolbar != null && lastSelectionModel != null) {
            gridPanel.setSelectionModel(lastSelectionModel);
        }
       // Issue occur here..... In PagingToolbar.
        toolbar = new PagingToolbar();
    }

PagingToolbar.java

class PagingToolbar {
private I18nUtils i18n;   //It is a class How should I mock these. i18n

/**
 * Creates a new paging toolbar.
 */
public PagingToolbar() {
    super();
    setDisplayingItemsText(i18n.getText(this, "facebook")); 
// here i18n get null and throws null pointer-exception.

}

ATest.java

@Test(groups = { "unit" })
public class ATest {
    @Test
    public void resetToolbar(){

        I18nUtils i18n = createNiceMock(I18nUtils.class);

        // Invoke
        A tt = new A();

        // How to set i18n with reflection in pagingToolbar.
        tt.resetToolbar(listselectmodelMock);
    }
}

NOTE : In A class in resetToolbar() , I have to invoke pagingToolbar with new keyword.