Question: well,
I am currently developing an application which uses JFrame and Applet.

Why we can't extend both..

```
public class myClass extends Applet, javax.swing.JFrame {...}
//invalid...

```

The valid code is.

```
public class myClass extends Applet {
    javax.swing.JFrame frame = new javax.swing.JFrame();
    public void init(){
        frame.setSize(300, 400);
        frame.setVisible(true);
    }
}

```

Why so?
Why we can't extends more than one class

Answer: 
The designers of Java learned from the mistakes made in other languages such as C++ where the [`diamond problem`](http://www.programmerinterview.com/index.php/c-cplusplus/diamond-problem/) was an issue caused by multiple inheritance so decided to make Java a single inheritance language to simplify development.
Just like what Jakub said, multiple inheritance is not supported in java so you will have to use interfaces instead. It's just the nature of the language. I do hope that's what you're asking.