所以我应该制作3个类，并给予第4个类用于用户界面。一个类（DBBinding）应该有一个String键和String值，并采取类似名称：Alien或star：harry dean和make name或star是“key”，另一个是“value”下一个类（DBrecord） ）是将一组这些“绑定”作为一个记录。我选择在ArrayList中保留一组这些绑定。第三个类（DBTable）是另一个ArrayList但是。我正处于从文件中读取一行txt的位置，其中每行txt将成为一个我们知道格式正确的DBrecord（键：值，键：值，键：值等等）上）。
我遇到麻烦的地方在DBrecord课程中。我有一个方法（private void addBindingToRecord（String key_，String value_））从DBrecord类中调用（public static DBrecord createDBrecord（String record））这里是每个方法代码。 我在使用addBindingToRecord方法时遇到问题...它在第一次使用时出现空指针异常。我认为它与sytax有关，我如何调用“this.myDBrecord.add（myDBBinding）;”......已经尝试了多种方法，结果相同....
public static DBrecord createDBrecord(String record)//takes a string and breaks it into DBBindings and makes a record with it.
{
DBrecord myRecord=new DBrecord();
String temp[];
temp=record.split(",",0);
if(temp!=null)
{
for(int i=0; i<Array.getLength(temp); i++)
{
System.out.println("HERE");//for testing
String temp2[];
temp2=temp[i].split(":",0);
myRecord.addBindingToRecord(temp2[0], temp2[1]);
}
}
return myRecord;
}
private void addBindingToRecord(String key_, String value_)
{
DBBinding myDBBinding=new DBBinding(key_, value_);
if(myDBBinding!=null)//////////////ADDED
this.myDBrecord.add(myDBBinding);///Here is where my null pointer exception is.
}
我将在这里发布我所有课程的完整代码，以便在需要时查看。感谢任何帮助，提示，想法。
package DataBase;
import java.io.*;
public class CommandLineInterface {
public static void main(String[] args) {
DBTable db = new DBTable(); // DBTable to use for everything
try {
// Create reader for typed input on console
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
String line;
while (true) {
int length = 0;
int selectedLength = 0;
// YOUR CODE HERE
System.out.println("\n" + length + " records (" + selectedLength + " selected)");
System.out.println("r read, p print, sa select and, so select or, da ds du delete, c clear sel");
System.out.print("db:");
line = reader.readLine().toLowerCase();
if (line.equals("r")) {
System.out.println("read");
String fname;
System.out.print("Filename:");
//fname = reader.readLine();////ADD BACK IN AFTER READ DEBUGED
// YOUR CODE HERE
fname="movie.txt";
db.readFromFile(fname);
}
else if (line.equals("p")) {
System.out.println("print");
// YOUR CODE HERE
DBTable.print();
}
else if (line.equals("da")) {
System.out.println("delete all");
// YOUR CODE HERE
}
else if (line.equals("ds")) {
System.out.println("delete selected");
// YOUR CODE HERE
}
else if (line.equals("du")) {
System.out.println("delete unselected");
// YOUR CODE HERE
}
else if (line.equals("c")) {
System.out.println("clear selection");
/// YOUR CODE HERE
}
else if (line.equals("so") || line.equals("sa")) {
if (line.equals("so")) System.out.println("select or");
else System.out.println("select and");
System.out.print("Criteria record:");
String text = reader.readLine(); // get text line from user
// YOUR CODE HERE
}
else if (line.equals("q") || line.equals("quit")) {
System.out.println("quit");
break;
}
else {
System.out.println("sorry, don't know that command");
}
}
}
catch (IOException e) {
System.err.println(e);
}
}
}
package DataBase;
import java.util.*;
import java.io.*;
public class DBTable {
static ArrayList<DBrecord> myDBTable;
public DBTable()
{
ArrayList<DBrecord> myDBTable= new ArrayList<DBrecord>();
}
public static void addRecordToTable(DBrecord myRecord)//added static when added addRecordToTable in readFromFile
{
if(myRecord!=null)
{myDBTable.add(myRecord);}
}
public static void readFromFile(String FileName)
{
try
{
FileReader myFileReader=new FileReader(FileName);
String line="Start";
BufferedReader myBufferdReader=new BufferedReader(myFileReader);
while(line!="\0")
{
line=myBufferdReader.readLine();
if(line!="\0")
{
System.out.println(line);//TEST CODE
addRecordToTable(DBrecord.createDBrecord(line));// made addRecordToTable static.
}
}
}catch(IOException e)
{System.out.println("File Not Found");}
}
public static void print()
{
if (myDBTable==null)
{
System.out.println("EMPTY TABLE");
return;
}
else
{
for (int i=0; i<myDBTable.size(); i++)
{
System.out.println(myDBTable.get(i).toString());
}
}
}
}
package DataBase;
import java.util.*;
import java.lang.reflect.Array;
//import DataBase.*;//did not help ... ?
public class DBrecord {
boolean select;
String key;
//need some type of collection to keep bindings.
ArrayList<DBBinding> myDBrecord;
public DBrecord()
{
//DBrecord myRecord=new DBrecord();
select=false;
ArrayList<DBBinding> myDbrecord=new ArrayList<DBBinding>();
}
private void addBindingToRecord(String key_, String value_)
{
DBBinding myDBBinding=new DBBinding(key_, value_);
//System.out.println(myDBBinding.toString());//for testing
if(myDBBinding!=null)//////////////ADDED
this.myDBrecord.add(myDBBinding);
System.out.println(key_);//for testing
System.out.println(value_);//for testing
}
public String toString()
{
//out put key first then all values in collection/group/record. use correct formatting.
StringBuilder myStringbuilder=new StringBuilder();
for (int i=0;i<this.myDBrecord.size();i++)
{
myStringbuilder.append(myDBrecord.get(i).toString());
myStringbuilder.append(", ");
}
myStringbuilder.delete(myStringbuilder.length()-2, myStringbuilder.length()-1);//delete last ", " thats extra
return myStringbuilder.toString();
}
public static DBrecord createDBrecord(String record)//takes a string and breaks it into DBBindings and makes a record with it.
{
//System.out.println("HERE");//for testing
DBrecord myRecord=new DBrecord();
String temp[];
temp=record.split(",",0);
if(temp!=null)
{
//System.out.println("HERE");//for testing
//for(int i=0; i<Array.getLength(temp); i++) ///for testing
//{System.out.println(temp[i]);}
for(int i=0; i<Array.getLength(temp); i++)
{
System.out.println("HERE");//for testing
String temp2[];
temp2=temp[i].split(":",0);
System.out.println(temp2[0]);//for testing
System.out.println(temp2[1]);//for testing
myRecord.addBindingToRecord(temp2[0], temp2[1]);
System.out.println(temp2[0]+ " "+ temp2[1]);////test code
}
}
return myRecord;
}
}
package DataBase;
public class DBBinding {
private String key;
private String value;
public DBBinding(String key_, String value_)
{
key =key_;
value=value_;
}
public String getKey()
{return key;}
public String getValue()
{return value;}
public String toString()
{return key+": "+value;}
}
答案 0 :(得分：1)
在您的构造函数中：ArrayList<DBBinding> myDbrecord=new ArrayList<DBBinding>();
您只需创建一个名为myDbrecord的局部变量并对其进行初始化，而不是初始化字段myDBrecord。
你可能想要：
myDBrecord = new ArrayList<DBBinding>();