我正在嘗試創(chuàng)建一個管理訂票系統(tǒng)的 Java 程序。我有一個電影課： public class Film { private String title; private Double price; private String ageRestriction; private double rating; private String genre; private String location; private String screenDay;基于兩個參數(shù)（位置和周）創(chuàng)建電影項的 ArrayList 并排序的 FilmList 類 public class FilmList { public FilmList(ArrayList<Film> filmArrayList) { this.filmArrayList = filmArrayList; } public FilmList (){ this.filmArrayList = new ArrayList<>(); } public ArrayList <Film> filmArrayList; public void addFilm(Film films){ this.filmArrayList.add(films); } private String showLocation; private String screenWeek; public void setScreenWeek(String screenDay) { this.screenWeek = screenDay; } public String getScreenWeek() { return screenWeek; } public void setShowLocation(String location) { this.showLocation = showLocation; } public String getShowLocation() { return showLocation; } public Optional<Film> searchFilm(){ Optional<Film> movieFounded = filmArrayList.stream().filter(i -> i.getLocation().contains(getShowLocation()) && i.getScreenDay().contains(getScreenWeek())).findAny(); return movieFounded; }setShowLocation 參數(shù)是通過單擊按鈕來設(shè)置的（每個劇院都有一個，而 setScreenWeek 是由 Combobox 設(shè)置的圖形單元與控制臺的接口。請注意，如果我按下按鈕而不選擇組合框上的任何內(nèi)容，則會出現(xiàn)錯誤。
1 回答
函數(shù)式編程
TA貢獻1807條經(jīng)驗 獲得超9個贊
所以
FilmList filmList = new FilmList();
filmList.addFilm
System.out.println(searchFilm().toString());
您的代碼有點奇怪，但我想您的意思是將Film實例傳遞給addFilm，然后使用filmList.searchFilm().
反正
filter(
i -> i.getLocation().contains(getShowLocation()) &&
i.getScreenDay().contains(getScreenWeek())
)
在這里，您正在過濾filmArrayList，此時它包含一個元素。和
i.getLocation().contains(getShowLocation())
基本上意味著
i.getLocation().contains(null)
因為該showLocation字段未初始化。
這同樣適用于第二個條件，使用screenWeek.
我實際上很驚訝它沒有拋出 a NullPointerException，因為
public boolean contains(CharSequence s) {
return indexOf(s.toString()) > -1; // NullPointerException at s.toString()
}
但是無論如何，假設(shè)您初始化了這些字段，然后唯一的元素被filter操作丟棄，這就是您看到Optional.empty.
final FilmList filmList = new FilmList();
filmList.setShowLocation("Your location");
filmList.setScreenWeek("Screen week");
filmList.addFilm(filmInstance);
System.out.println(filmList.searchFilm().toString());
您顯然需要一個完全構(gòu)造的Film實例
final Film filmInstance = new Film();
filmInstance.title = "The NullPointerException adventure";
filmInstance.price = 12D;
filmInstance.ageRestriction = "+18";
filmInstance.rating = 18D;
filmInstance.genre = "Horror";
filmInstance.location = "US";
filmInstance.screenDay = "Monday";
filmList.addFilm(filmInstance);
問題出在FilmList#setShowLocation方法上。
您正在分配showLocation給自己，并且該location參數(shù)未使用。
public void setShowLocation(String location) {
this.showLocation = showLocation;
}
這應(yīng)該是
public void setShowLocation(String location) {
this.showLocation = location;
}
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