### Question:
SwapperSet { SwapperMapping getAllocation( int id ) { checkId( id ); SwapperMapping swapperMapping = swapperMappings[id]; if ( swapperMapping == null || swapperMapping == TOMBSTONE ) { return null; } return swapperMapping; }    }### Answer:
@Test void gettingAllocationZeroMustThrow() { assertThrows( IllegalArgumentException.class, () -> set.getAllocation( (short) 0 ) ); }