Create a JPanel. Add a combo box. The following code is generated:

    private javax.swing.JComboBox jComboBox1;
        jComboBox1 = new javax.swing.JComboBox();
        jComboBox1.setModel(new javax.swing.DefaultComboBoxModel(new String[] { "Item 1", "Item 2", "Item 3", "Item 4" }));

When trying to compile you get 3 [rawtypes] warnings, which is understandable. So add String type parameter, and the generated code is like this:

    private javax.swing.JComboBox<String> jComboBox1;
        jComboBox1 = new javax.swing.JComboBox<String>();
        jComboBox1.setModel(new javax.swing.DefaultComboBoxModel(new String[] { "Item 1", "Item 2", "Item 3", "Item 4" }));

That's better, but you still get one warning:

T:\Work\Sources\NewJPanel.java:32: warning: [rawtypes] found raw type: DefaultComboBoxModel
        jComboBox1.setModel(new javax.swing.DefaultComboBoxModel(new String[] { "Item 1", "Item 2", "Item 3", "Item 4" }));
  missing type arguments for generic class DefaultComboBoxModel<E>
  where E is a type-variable:
    E extends Object declared in class DefaultComboBoxModel

The code to init the combo box should use the diamond operator:

    jComboBox1.setModel(new javax.swing.DefaultComboBoxModel<>(new String[] { "Item 1", "Item 2", "Item 3", "Item 4" }));