How to create a check to keep the user from adding an existing value

all. I'm writing a GUI program that allows the user to enter a student id, name, and major. They can either insert these values as a record into the student database, delete them, find them, or update them. What I want to do is create a check- maybe an if statement- to keep the user from inserting an id that's already in the Hashmap.

processButton.addActionListener(new ActionListener() {

        @Override
        public void actionPerformed(ActionEvent ae){
            int idKey = Integer.parseInt(idText.getText());
            String nameValue = nameText.getText();
            String majorValue = majorText.getText();

            String s = (String)comboList.getSelectedItem();

            switch(s) {
                case "Insert":
                    hmap.put(idKey, nameValue);
                    JOptionPane.showMessageDialog(null, "Student added.");
                    idText.setText("");
                    nameText.setText("");
                    majorText.setText("");
                    break;
                case "Delete":
                    hmap.remove(idKey);
                    JOptionPane.showMessageDialog(null, "Student removed.");
                    idText.setText("");
                    nameText.setText("");
                    majorText.setText("");
                    break;
                case "Find":
                    String var = hmap.get(idKey);
                    JOptionPane.showMessageDialog(null, "Student found."
                    + "\n" + var);
                    idText.setText("");
                    nameText.setText("");
                    majorText.setText("");
                    break;
                case "Update":
                    JFrame frame = new JFrame();
                    Object[] grades = {"A", "B", "C", "D", "F"};
                    String gradeAdded = (String)JOptionPane.showInputDialog(frame, "Choose grade:", 
                            "", JOptionPane.QUESTION_MESSAGE, null, grades, grades[0]);
                    Object[] credits = {"3", "6"};
                    String creditsAdded = (String)JOptionPane.showInputDialog(frame, "Choose credits:",
                            "", JOptionPane.QUESTION_MESSAGE, null, credits, credits[0]);
                    idText.setText("");
                    nameText.setText("");
                    majorText.setText("");
                    break;
           }               
        }
    });

This is my code for the processing button actionlistener. In the insert case I would like to create this check. Thanks!