Android（Java）-通過遞歸方法填充ArrayList

Question

我正在開發可打開epub文件的閱讀器。我在互聯網上進行了大量搜索，最后在本教程鏈接中找到了解決方案。

---

但是現在我在使用該方法時遇到了麻煩

logTableOfContents（列出tocReferences，int深度）

我把它從空改成字符串，這是我的代碼

 private String logTableOfContents(List<TOCReference> tocReferences, int depth) {

    if (tocReferences == null) {




        return null;
    }

    for (TOCReference tocReference : tocReferences) {
        StringBuilder tocString = new StringBuilder();
        for (int i = 0; i < depth; i++) {
            tocString.append("\t");
        }

        tocString.append(tocReference.getTitle());

        //Toast.makeText(this, ""+ logTableOfContents(tocReference.getChildren(), depth + 1), Toast.LENGTH_SHORT).show();

        return tocString.toString();

    }

    return null;
}

---

可以清楚地看到，准備該教程的開發人員已使用遞歸方法來獲取目錄 。

---

我將方法void更改為String以獲取並將它們存儲在ArrayList中，但是無法從遞歸方法中單獨獲取Book的目錄。嘗試記錄ArrayList的元素時出現nullPointerException。

---

現在，我想尋求某人的幫助來解決此問題，歡迎提出任何建議，謝謝。

---

我使用的教程中的代碼

 import java.io.IOException; import java.io.InputStream; import java.util.List; import nl.siegmann.epublib.domain.Book; import nl.siegmann.epublib.domain.TOCReference; import nl.siegmann.epublib.epub.EpubReader; import android.app.Activity; import android.content.res.AssetManager; import android.graphics.Bitmap; import android.graphics.BitmapFactory; import android.os.Bundle; import android.util.Log; /** * Log the info of 'assets/books/testbook.epub'. * * @author paul.siegmann * */ public class LogTestBookInfo extends Activity { /** Called when the activity is first created. */ @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); AssetManager assetManager = getAssets(); try { // find InputStream for book InputStream epubInputStream = assetManager .open("books/testbook.epub"); // Load Book from inputStream Book book = (new EpubReader()).readEpub(epubInputStream); // Log the book's authors Log.i("epublib", "author(s): " + book.getMetadata().getAuthors()); // Log the book's title Log.i("epublib", "title: " + book.getTitle()); // Log the book's coverimage property Bitmap coverImage = BitmapFactory.decodeStream(book.getCoverImage() .getInputStream()); Log.i("epublib", "Coverimage is " + coverImage.getWidth() + " by " + coverImage.getHeight() + " pixels"); // Log the tale of contents logTableOfContents(book.getTableOfContents().getTocReferences(), 0); } catch (IOException e) { Log.e("epublib", e.getMessage()); } } /** * Recursively Log the Table of Contents * * @param tocReferences * @param depth */ private void logTableOfContents(List<TOCReference> tocReferences, int depth) { if (tocReferences == null) { return; } for (TOCReference tocReference : tocReferences) { StringBuilder tocString = new StringBuilder(); for (int i = 0; i < depth; i++) { tocString.append("\\t"); } tocString.append(tocReference.getTitle()); Log.i("epublib", tocString.toString()); logTableOfContents(tocReference.getChildren(), depth + 1); } } } 

---

我使用的庫已在本教程中編寫。

Answer 1

我剛剛為我的問題找到了解決方案。

---

我向logTableOfContent（...，...，ArrayList ...）方法添加了一個新參數，並對其進行了修改：

---

 private void logTableOfContents(List<TOCReference> tocReferences, int depth, ArrayList<String> ListOfContents) { if (tocReferences == null) { return; } for (TOCReference tocReference : tocReferences) { StringBuilder tocString = new StringBuilder(); for (int i = 0; i < depth; i++) { tocString.append("\\t"); } tocString.append(tocReference.getTitle()); Log.v("TAG", tocString.toString()); ListOfContents.add(tocString.toString()); logTableOfContents(tocReference.getChildren(), depth + 1, ListOfContents); //return tocString.toString(); } } 

---

void方法可以用作

 ArrayList<String> TableOfContent = new ArrayList<>();

        logTableOfContents(book.getTableOfContents().getTocReferences(), 0, TableOfContent);

        for (String string:TableOfContent){

            Toast.makeText(this, ""+ string, Toast.LENGTH_SHORT).show();

        }

---

非常感謝那些抽出時間來審查問題的人。

---