### Question:
SwapperSet { synchronized boolean free( int id ) { checkId( id ); SwapperMapping[] swapperMappings = this.swapperMappings; SwapperMapping current = swapperMappings[id]; if ( current == null || current == TOMBSTONE ) { throw new IllegalStateException( "PageSwapper allocation id " + id + " is currently not allocated. Likely a double free bug." ); } swapperMappings[id] = TOMBSTONE; this.swapperMappings = swapperMappings; freeCounter++; if ( freeCounter == 20 ) { freeCounter = 0; return true; } return false; }    }### Answer:
@Test void freeOfIdZeroMustThrow() { assertThrows( IllegalArgumentException.class, () -> set.free( 0 ) ); }