Why am I getting null on this code?
<p>This is a part of my source (when I enter a button) of my JDilog form. I have made an object from the <code>SystemManagement</code> class which is management (I have written also some part of this class). When the user add her/his information, with the <code>addStudent()</code> method in <code>SystemManagement</code> class I add her/his information to the student's list but when I write </p>

<pre><code>System.out.println(management.getField());
</code></pre>

<p>It will return null! Why?</p>

<pre><code> private void submit() {
    String name = nameField.getText();
    String family = familyField.getText();
    String ye =  (String) yearComboBox.getSelectedItem();
    String ci = (String) cityComboBox.getSelectedItem();
    String fi = (String) fieldComboBox.getSelectedItem();
    if (ye == null) {
        JOptionPane.showMessageDialog(this, "You haven't chosen the year of entrance", "Error", JOptionPane.ERROR_MESSAGE);
        return;
    }
    if (ci == null) {
        JOptionPane.showMessageDialog(this, "You have not set the city", "Error", JOptionPane.ERROR_MESSAGE);
        return;
    }
    if (fi == null) {
        JOptionPane.showMessageDialog(this, " You have not set the field", "Error", JOptionPane.ERROR_MESSAGE);
        return;
    }
    if (name.equals("")) {
        JOptionPane.showMessageDialog(this, "You have not set your name ", "Error", JOptionPane.ERROR_MESSAGE);
        return;
    }
    if (family.equals("")) {
        JOptionPane.showMessageDialog(this, " You have not set the family", "Error ", JOptionPane.ERROR_MESSAGE);
        return;
    }
    management.addStudent(name, family, ye,ci, fi);
    System.out.println(management.getField());
    JOptionPane.showMessageDialog(this, management.toString(), " registered successefully", JOptionPane.INFORMATION_MESSAGE);
    clear();

}

public class SystemManagement implements Serializable {

private String siteName;
private List&lt;Fields&gt; fields;
private List&lt;Professors&gt; professors;
private List&lt;Students&gt; students;
private List&lt;Lessons&gt; lessons;
public Professors pro;
public Lessons les;


public SystemManagement(String siteName) {
    this.siteName = siteName;
    professors = new ArrayList&lt;Professors&gt;();
    students = new ArrayList&lt;Students&gt;();
    lessons = new ArrayList&lt;Lessons&gt;();
    fields = new ArrayList&lt;Fields&gt;();
}

public List&lt;Fields&gt; getFields() {
    return fields;
}

public void setField(String field) {
    this.field = field;
}

public String getField() {
    return field;
}


public void addStudent(String name, String family, String yearOfEntrance, String city, String field) {
    Students student = new Students(name, family, yearOfEntrance, city, field);
    students.add(student);

}
</code></pre>
