Analizador Lexico Pilas

===============================CLASE PILA==========================================
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package Paquete;

public class Pila {

    class Nodo {
        char simbolo;
        Nodo sig;
    }

    private Nodo raiz;

    Pila () {
        raiz=null;
    }

    public void insertar(char x) {
        Nodo nuevo;
        nuevo = new Nodo();
        nuevo.simbolo = x;
        if (raiz==null)
        {
            nuevo.sig = null;
            raiz = nuevo;
        }
        else
        {
            nuevo.sig = raiz;
            raiz = nuevo;
        }
    }

    public char extraer ()
    {
        if (raiz!=null)
        {
            char informacion = raiz.simbolo;
            raiz = raiz.sig;
            return informacion;
        }
        else
        {
            return Character.MAX_VALUE;
        }
    }

    public boolean vacia() {
        if (raiz==null) {
            return true;
        } else {
            return false;
        }
    }
}

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===================================================CLASE FORMULA====================================

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package Paquete;

import javax.swing.*;
import java.awt.event.*;
public class Formula extends JFrame implements ActionListener {
    private JTextField tf1;
    private JButton boton1;
    public Formula() {
        setLayout(null);
        tf1=new JTextField("{2*(4-5)-{3*4}-[4-5]}");
        tf1.setBounds(10,10,230,30);
        add(tf1);
        boton1=new JButton("Verificar fórmula.");
        boton1.setBounds(10,70,180,30);
        add(boton1);
        boton1.addActionListener(this);
    }

    public void actionPerformed(ActionEvent e) {
        if (e.getSource()==boton1) {
           if (balanceada()) {
               setTitle("Está correctamente balanceada.");
           } else {
               setTitle("No está correctamente balanceada.");
           }
        }
    }
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    public boolean balanceada() {
        Pila pila1;
        pila1 = new Pila ();
        String cadena=tf1.getText();
        for (int f = 0 ; f < cadena.length() ; f++)
        {
            if (cadena.charAt(f) == '(' || cadena.charAt(f) == '[' || cadena.charAt(f) == '{') {
                pila1.insertar(cadena.charAt(f));
            } else {
                if (cadena.charAt(f)==')') {
                    if (pila1.extraer()!='(') {
                        return false;
                    }
                } else {
                    if (cadena.charAt(f)==']') {
                        if (pila1.extraer()!='[') {
                            return false;
                        }
                    } else {
                        if (cadena.charAt(f)=='}') {
                            if (pila1.extraer()!='{') {
                                return false;
                            }
                        }
                    }
                }
        }
        }
        if (pila1.vacia()) {
            return true;
        } else {
        return false;
        }
    }


    public static void main(String[] ar) {
        Formula formula1=new Formula();
        formula1.setBounds(0,0,350,140);
        formula1.setVisible(true);
    }

}
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