@Test
    public void testFilter() throws Exception {

        // Test string
        final String test = "3.yes,1.A;2.no,1.B;3.yes,1.C;3.yes,1.D;4.nope,1.E;";

        GuacamoleReader reader = new FilteredGuacamoleReader(new ReaderGuacamoleReader(new StringReader(test)),
                                                             new TestFilter());

        GuacamoleInstruction instruction;

        // Validate first instruction
        instruction = reader.readInstruction();
        assertNotNull(instruction);
        assertEquals("yes", instruction.getOpcode());
        assertEquals(1, instruction.getArgs().size());
        assertEquals("A", instruction.getArgs().get(0));

        // Validate second instruction
        instruction = reader.readInstruction();
        assertNotNull(instruction);
        assertEquals("yes", instruction.getOpcode());
        assertEquals(1, instruction.getArgs().size());
        assertEquals("C", instruction.getArgs().get(0));

        // Validate third instruction
        instruction = reader.readInstruction();
        assertNotNull(instruction);
        assertEquals("yes", instruction.getOpcode());
        assertEquals(1, instruction.getArgs().size());
        assertEquals("D", instruction.getArgs().get(0));

        // Should be done now
        instruction = reader.readInstruction();
        assertNull(instruction);

    }
Filter which allows through "yes" instructions but drops all others.
guacamole-common/src/test/java/org/apache/guacamole/protocol/FilteredGuacamoleReaderTest.java