Is it possible to use arrow keys alone to expand tree node in package explorer in Eclipse on Linux?

Put this into your ~/.gtkrc-2.0 and you should be good to go. The Left and Right lines make the requested change, the rest are just my personal additions to make the tree-view act more vim-like. Hope that helps!

binding "gtk-binding-tree-view" {
    bind "j"        { "move-cursor" (display-lines, 1) }
    bind "k"        { "move-cursor" (display-lines, -1) }
    bind "h"        { "expand-collapse-cursor-row" (1,0,0) }
    bind "l"        { "expand-collapse-cursor-row" (1,1,0) }
    bind "o"        { "move-cursor" (pages, 1) }
    bind "u"        { "move-cursor" (pages, -1) }
    bind "g"        { "move-cursor" (buffer-ends, -1) }
    bind "y"        { "move-cursor" (buffer-ends, 1) }
    bind "p"        { "select-cursor-parent" () }
    bind "Left"     { "expand-collapse-cursor-row" (0,0,0) }
    bind "Right"    { "expand-collapse-cursor-row" (0,1,0) }
    bind "semicolon" { "expand-collapse-cursor-row" (0,1,1) }
    bind "slash"    { "start-interactive-search" () }
}
class "GtkTreeView" binding "gtk-binding-tree-view"

then restart your Eclipse to apply new bindings

If anyone is wondering how to do this with GTK3 - simply open ~/.config/gtk-3.0/gtk.css and add the following:

@binding-set MyTreeViewBinding
{
    bind "Left"     { "expand-collapse-cursor-row" (0,0,0) };
    bind "Right"    { "expand-collapse-cursor-row" (0,1,0) };
}

GtkTreeView
{
  gtk-key-bindings: MyTreeViewBinding;
}

My version for GTK3 that behaves in more natural way. Add the following to ~/.config/gtk-3.0/gtk.css:

@binding-set MyTreeViewBinding
{
    bind "Left"     { "select-cursor-parent" ()
                      "expand-collapse-cursor-row" (0,0,0) };
    bind "Right"    { "expand-collapse-cursor-row" (0,1,0) };
}

GtkTreeView
{
    gtk-key-bindings: MyTreeViewBinding;
}