FilteredImageSource.addConsumer(null), removeConsumer(null) and 
isConsumer(null) throws NullPointerException. NullPointerException either needs to be specified or shoule not be thrown. isConsumer(null) is better to return
false.

import java.awt.image.*;

public class NullTest {
    public static void main(String[] args) {

        ImageProducer imgp = new MemoryImageSource(
                                 5, 5, new int[20], 0, 5);
        ImageFilter imgf = new ReplicateScaleFilter(3, 3);
        FilteredImageSource s = new FilteredImageSource(imgp, imgf);
        try {
            s.addConsumer((ImageConsumer) null);
        } catch (NullPointerException e) {
            System.out.println("addConsumer(null) throws NullPointerException");
            e.printStackTrace();
        }
        try {
            if (s.isConsumer((ImageConsumer) null)) {
                System.out.println("null should not be a consumer");
            }
        } catch (NullPointerException e) {
            System.out.println("isConsumer(null) throws NullPointerException");
            e.printStackTrace();
        }
        try {
            s.removeConsumer((ImageConsumer) null);
        } catch (NullPointerException e) {
            System.out.println("removeConsumer(null) throws NullPointerException");
            e.printStackTrace();
        }
        
    }
}

output:

addConsumer(null) throws NullPointerException
java.lang.NullPointerException
        at java.util.Hashtable.containsKey(Hashtable.java:308)
        at java.awt.image.FilteredImageSource.addConsumer(FilteredImageSource.ja
va:68)
        at NullTest.main(NullTest.java:11)
isConsumer(null) throws NullPointerException
java.lang.NullPointerException
        at java.util.Hashtable.containsKey(Hashtable.java:308)
        at java.awt.image.FilteredImageSource.isConsumer(FilteredImageSource.jav
a:83)
        at NullTest.main(NullTest.java:17)
removeConsumer(null) throws NullPointerException
java.lang.NullPointerException
        at java.util.Hashtable.get(Hashtable.java:329)
        at java.awt.image.FilteredImageSource.removeConsumer(FilteredImageSource
.java:93)
        at NullTest.main(NullTest.java:25)