AbstractStack.java

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package aud.adt;
 
import java.util.NoSuchElementException;
 
public abstract class AbstractStack<T> {
 
  protected AbstractStack() {}
 
  public abstract boolean is_empty();
 
  public abstract T top();
 
  public abstract T pop();
 
  public abstract void push(T x);
 
  @Override
  public String toString() {
    if (!is_empty()) {
      T x=pop();
      String bottom=toString();
      push(x);
      return (bottom+(bottom.length()>1 ? "|":""))+x;
    }
    else
      return "|";
  }
}