public abstract class Stack {

    private int maximalgroesse;
    private Object first;

    public static void main(String[] args) {
        Stack st = new Stack(10);
        st.push("a");
        st.push("b");
        st.push("c");
        System.out.println(st.pop());
        System.out.println(st.pop());
        System.out.println(st.pop());
        System.out.println(st.isEmpty());

    }

    class Object {

        String value;
        Object next;

        public Object() {
        }

        public Object(String value, Object next) {
            this.value = value;
            this.next = next;
        }

        public String getValue() {
            return this.value;
        }

        public void setValue(String value) {
            this.value = value;
        }

        public Object getNext() {
            return this.next;
        }

        public void setNext(Object next) {
            this.next = next;
        }
    }

    public Stack(int maximalgroesse) {
        this.maximalgroesse = maximalgroesse;
        first = null;
    }

    public void push(Object o) {
        o.setNext(first);
        first = o;
    }

    public Object pop() {
        return first;
    }

    public boolean isEmpty() {
        if(first == null) {
            return true;
        } else {
            return false;
        }
    }
}