why does indxof doesnt work me in 2 methodes

I have 2 methodes to get protections from jTextField18.gettext a string
First method:

public boolean isEditValid(String number){
    for(int i=0;i<=number.length();i++)
        if(number.indexOf(i)<='a' && number.indexOf(i)>='z')



   return true;

   return false; 
}

Second method:

private void jButton18ActionPerformed(java.awt.event.ActionEvent evt) {                                          
    if(jTextField18.getText().isEmpty())
        JOptionPane.showMessageDialog(this, "The text field is emty");
    else if (isEditValid(jTextField18.getText())==true)
        JOptionPane.showMessageDialog(this, "Edit isnt valid ONLY NUMBERS!");
}       

When i write on text field a number and a string its says me Edit isnt valid ONLY NUMBERS why?