null 检查语句是否在 java 中不起作用

Question

i want to check a instanciate object is null in java when i press enter, i have a class with a constructor and i pass a parameter

private String codigo;
    private String nombre;
    private String descripcion;
    private String foto;
    private String stock;

public Producto(String codigo) {
   this.codigo = codigo;
        this.nombre = nombre;
        this.descripcion = descripcion;
}
/*then getters and setters


*/


//this is in a java form 
private void txtCodigoProductoActionPerformed(java.awt.event.ActionEvent evt) { 
Producto producto = new Producto(txtCodigoProducto.getText());
System.out.println(producto);


if (producto==null) {
            JOptionPane.showMessageDialog(this, "No existe este producto");

        }else{
            lbl1.setText(producto.getNombre());
            lbl2.setText(producto.getDescripcion());
         }

}

System.out.println(producto) shows null when i press enter since i wrote an incorrect number so all the attributes of producto are null but then why it doesn't check if is null and doesn't show me the the message dialog? I change the condition to one of the attributes since all of them are null:

if (producto.getStock()==null) {
            JOptionPane.showMessageDialog(this, "No existe este producto");

        }else{
            lbl1.setText(producto.getNombre());
            lbl2.setText(producto.getDescripcion());
         }
}

and it worked and showed me the messageDialog, so why producto == null doesn't work?

Answer 1

In Java, a variable of object type, such as producto in your example, can either refer to an object, or be null. If you set it to an object, like you did when you wrote producto = new Producto(txtCodigoProducto.getText());then it's not null any more.

Inside an object there can be other variables, called fields. Each of these (if it has object type) can also either refer to an object, or be null. The object that you created with new Producto(txtCodigoProducto.getText()); has five variables inside. The one called codigo refers to an object. The other four are null.

Checking if (producto.getStock()==null) will only work if producto refers to an object. It (presumably) asks that object for the value of its stock field, then checks whether that value is null, or refers to an object.

That's a completely different check from if(producto == null) , which only checks the producto variable itself.

Answer 2

Producto producto = new Producto(...); absolutely guarantees that producto is not null . System.out.println(producto) therefore calls producto.toString() .

It is the toString() implementation of Producto that returns null or the text "null" .

if (producto == null) is a meaningless statement, since producto cannot be null .