It seems that whenever I use the Introduce Variable refactoring, the new
variable gets put in an inappropriate location. It is apparently always put at
the start of the current method, which almost never works. E.g.

void m() {
    // some stuff..
    if (test) {
        // some stuff...
        String x = ...;
        use(x + ".suffix");
    }
}

Now select 'x + ".suffix"' and use Introduce Var and call it y. You will get
something like

void m() {
    String y = x + ".suffix";
    // some stuff..
    if (test) {
        // some stuff...
        String x = ...;
        use(y);
    }
}

which of course does not compile.

The refactoring needs to examine the expression for all local variables and
introduce the new variable sometime after the definition of the last used local
variable.