1. Create simple java class:

package test;

import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;

public class ClassA {

    public void m3() throws FileNotFoundException, IOException {
        BufferedReader br = new BufferedReader(new FileReader("C:\\test.txt"));
        String s;
        while ((s = br.readLine()) != null) {
            System.out.println(s);
        }
    }
}

2. Right click the body of ClassA and from pop-up menu select Refactor > Extract Interface

3. Type name "MyInterface", check method m3() and press Refactor

>> There are missing imports in interface:

package test;

public interface MyInterface {

    void m3() throws FileNotFoundException, IOException;
    
}

Product Version: NetBeans IDE 7.4 (Build 201310111528)
Java: 1.7.0_45; Java HotSpot(TM) 64-Bit Server VM 24.45-b08
Runtime: Java(TM) SE Runtime Environment 1.7.0_45-b18
System: Windows 7 version 6.1 running on amd64; Cp1252; en_US (nb)