寫入CSV文件。 myPackage.MyClass不能轉換爲java.lang.CharSequence

Question

我需要將List<MyClass> myList寫入CSV文件。特別是我需要寫values。 MyClass的有以下實現：

public class MyClass { 
    private Object[] values; 
//... 
    @Override 
    public String toString() 
     { 
     String line = ""; 
     for (int i=0; i<this.getSize(); i++) { 
      //if (this.values[i] != null) { 
       line = line + this.values[i] + " "; 
      //} 
     } 
     return line; 
    } 
} 

的代碼如下：

private void saveSolutionToCSV(List<MyClass> solution) { 
    int columnSize = solution.get(0).getSize(); 
    try { 
     FileWriter writer = new FileWriter("test.csv"); 
     Iterator result = solution.iterator(); 
     while(result.hasNext()) { 
     for(int i = 0; i < columnSize; i++) { 
      CharSequence element = (CharSequence)result.next(); 
      writer.append(element); 
      if(i < columnSize - 1) 
      writer.append(','); 
     } 
     writer.append('\n'); 
     } 
    } 
    catch(Exception e) 
    { 
     e.printStackTrace(); 
    } 
} 

和錯誤消息如下：

java.lang.ClassCastException: myPackage.MyClass cannot be cast to java.lang.CharSequence 

如何解決這個問題呢？謝謝。

Answer 1

嘗試：

String element = result.next().toString(); 
writer.append(element); 
if(i < columnSize - 1) 
    writer.append(','); 

Answer 2

您需要鑄造的CharSequence之前調用toString（）。

Answer 3

嘗試調用toString（）方法明確地像

CharSequence element = (CharSequence)result.next().toString(); 

Answer 4

CsvWriter.java

import java.io.ByteArrayOutputStream; 
import java.io.Closeable; 
import java.io.FilterOutputStream; 
import java.io.IOException; 
import java.io.OutputStream; 
import java.io.PrintWriter; 
import java.util.Collection; 

/** 
* Composes CSV data for collection of objects. Relays on theirs {@link Object#toString toString()} implementation.<br> 
* Could use memory buffer for the data or write straight to the output stream. 
* <p> 
* Example 1: 
* 
* <pre> 
* Object[][] data = ... 
* 
* CsvWriter writer = new CsvWriter(); 
* writer.write("Column 1", "Column 2", ... "Column N"); 
* for (Object[] values : data) 
*  writer.write(values); 
* ... 
* System.out.println(writer.toString()); 
* </pre> 
* <p> 
* Example 2: 
* 
* <pre> 
* Object[][] data = ... 
* 
* CsvWriter writer = null; 
* try { 
*  writer = new CsvWriter(new FileOutputStream(new File("data.csv"))); 
*  writer.write("Column 1", "Column 2", ... "Column N"); 
*  for (Object[] values : data) 
*   writer.write(values); 
* } finally { 
*  writer.close(); 
* } 
* </pre> 
* 
* @author Mykhaylo Adamovych 
*/ 
public class CsvWriter implements Closeable { 
    public static final String NULL_MARK = ""; 
    public static final String QUOTE = "\""; 
    private String nullMark = NULL_MARK; 
    private final PrintWriter pw; 
    private ByteArrayOutputStream baos; 

    /** 
    * Creates temporary buffer in the memory. <br> 
    * Use {@link #toString()} thereafter. No need to close. 
    */ 
    public CsvWriter() { 
     baos = new ByteArrayOutputStream(); 
     pw = new PrintWriter(baos, true); 
    } 

    /** 
    * Doesn't consume memory for CSV data, writes straight to the output stream. Just like {@link FilterOutputStream}, but deal with objects and relays on 
    * theirs {@link Object#toString toString()} implementation.<br> 
    * Close writer thereafter. 
    * 
    * @param os 
    *   output stream to write data. 
    */ 
    public CsvWriter(OutputStream os) { 
     pw = new PrintWriter(os, true); 
    } 

    protected String composeRecord(Object... values) { 
     if (values == null || values.length == 0) 
      return ""; 
     final StringBuffer csvRecord = new StringBuffer(); 
     csvRecord.append(QUOTE); 
     csvRecord.append(composeValue(values[0])); 
     csvRecord.append(QUOTE); 
     for (int i = 1; i < values.length; i++) { 
      csvRecord.append("," + QUOTE); 
      csvRecord.append(composeValue(values[i])); 
      csvRecord.append(QUOTE); 
     } 
     return csvRecord.toString(); 
    } 

    protected String composeValue(Object value) { 
     if (value == null) 
      return nullMark; 
     return value.toString().replaceAll(QUOTE, QUOTE + QUOTE); 
    } 

    public String getNullMark() { 
     return nullMark; 
    } 

    public void setNullMark(String nullMarker) { 
     nullMark = nullMarker; 
    } 

    @Override 
    public String toString() { 
     if (baos == null) 
      throw new UnsupportedOperationException(); 
     return baos.toString(); 
    } 

    /** 
    * Writes collection of objects as CSV record. 
    * 
    * @param values 
    */ 
    public void write(Collection<?> values) { 
     write(values.toArray()); 
    } 

    /** 
    * Writes collection of objects as CSV record. 
    * 
    * @param values 
    */ 
    public void write(Object... values) { 
     pw.println(composeRecord(values)); 
    } 

    @Override 
    public void close() throws IOException { 
     if (baos != null) 
      throw new UnsupportedOperationException(); 
     pw.close(); 
    } 
} 

例如：

public class MyClass { 
    private Object[][] data; 
//... 
    @Override 
    public String toCsv() { 
     CsvWriter writer = new CsvWriter(); 
     csvWriter.write("Column1", "Column2") 
     for (final Object[] values : data) 
      csvWriter.write(values); 
     return writer.toString(); 
    } 
}