public class Stack<A> implements StackI<A> {
    private A[] a= (A[])new Object[5];
    // "private A[] a= new A[5];" geht aus historischen Gruenden nicht
    private int tos=0;

    public void push(A n) {
        a[tos]=n;
        tos++;
    }
    public A pop() {
        tos--;
        return a[tos];
    }
    public boolean isempty() {
        return tos==0;
    }
    public String toString() {
        String s="";
        for (int i=0; i<tos; i++)
            s += a[i]+" ";
        return s;
    }
}