### Question:
DoubleConverter extends AbstractConverter { @Override protected Object convert(Object value) { if (value instanceof Number) { return ((Number)value).doubleValue(); } return super.convert(value); } DoubleConverter();   }### Answer:
@Test public void test() { Assert.assertEquals(100.1, converter.convert("100.1")); Assert.assertEquals(-100.2, converter.convert("-100.2")); Assert.assertEquals((double) 1000000000, converter.convert("1000000000")); Assert.assertEquals(null, converter.convert("-")); Assert.assertEquals(null, converter.convert("abc.100")); Assert.assertEquals(null, converter.convert("100a")); Assert.assertEquals(10.0, converter.convert(10)); }