Java: 从文本文件导入 ArrayList - 越界错误
Java: importing from text file to ArrayList - out of bounds errors
我是一名学习 java 的学生，我很难弄清楚为什么会出现越界错误。关于如何编码的任何输入
openFileList(); 方法会非常有用。
准确的错误是：
java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
at java.util.ArrayList.rangeCheck(Unknown Source)
at java.util.ArrayList.get(Unknown Source)
at MathClassMrA.openFileList(ps25openfilemra.java:100)
at PS25OpenFileMrA.main(ps25openfilemra.java:9)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:272)
代码如下：
import java.util.Scanner;
import java.util.ArrayList;
import java.io.*;
class PS25OpenFileMarian {
public static void main(String [] arg) throws IOException { // note throws IOException in main()
MathClassMarian objA = new MathClassMarian();
objA.openFileList(); // You need to code this method
objA.displayArrayList();
//objA.openFileArray(); // You need to code this method.
// objA.displayArrayObjects();
} // ---------end of main() -----------------------
} // ######### end of PS25OpenFile Program ################################
/* **************************************************************************
* @Author: Marian @Date: today
* This class is design to hold data that can be manipulated by it's child class.
* ************************************************************************* */
class Student {
//INSTANCE VARIABLES
private String name;
private int [] marks;
//CONSTRUCTORS
Student()
{// default
name = null;
marks = new int[10];
for (int i=0; i < marks.length; i++)
marks[i] = -1;
}
//INSTANCE METHODS
public String getName() { return name; }
public void setName(String N) { name = N; }
public int getMark(int index) { return marks[index]; }
public void setMark(int index, int M) { marks[index] = M; }
public void displayInfo() {
System.out.println("--------------------");
System.out.printf("Name of Student: %s \n", name);
System.out.println("====Marks===");
for (int i=0; i < marks.length; i++)
if (marks[i] != -1)
System.out.printf("Mark %d: = %d \n " , i+1, marks[i]);
System.out.printf("Overall Average: = %.2f \n", calcAverage());
} //---end of displayInfo() method
public double calcAverage() {
double average = 0;
int NoOfTests = 0;
for (int i=0; i < marks.length; i++)
{
if (marks[i] != -1)
{
average += marks[i]; // average = average + marks[i];
NoOfTests++;
}
}
return (average/NoOfTests);
} // --end of calcAverage() mMethod --
} // end of Student class
/* *************************************************************************
* @Author: You @Date: Today
* This class manipulates the data in either an Array of Objects or an ArrayList
* ************************************************************************** */
class MathClassMarian extends Student {
//INSTANCE VARIABLES ----
public static int records = 0; // used to count the number of records in the Array of Objects.
ArrayList<Student> objRefList;
Student [] objRefArray;
//CONSTRUCTORS -------------
MathClassMarian()
{ // default constructor
objRefList = new ArrayList<Student>(); // array list null
objRefArray = new Student[10]; // array of objects null.
}
//INSTANCE METHODS --------------
/* *******************************************************************
* Activity #1: @Author: You @Date: today The method below should connect
* to a file called studentdata.txt and load the information in the file into the ArrayList
* call the method openFileList(): Note: don't forget the throws IOException in the method
* header.
* @param: none @return: none
* *******************************************************************/
public void openFileList() throws IOException{
File file = new File("studentdata.txt");
Scanner scan = new Scanner (file);
// objRefList.add(new student()) ;
ArrayList <Student> objRefList = new ArrayList<Student>();
int i =0, j=0;
String temp ;
int temp2;
while (scan.hasNext()) {
temp = scan.nextLine();
objRefList.get(i).setName(temp);
i++;
}
do {
temp2 = scan.nextInt();
objRefList.get(j).setMark(j,temp2);
j++;
} while (scan.hasNextInt());*/
}
/* *******************************************************************
* Activity #2: @Author: You @Date: today The method below should connect
* to a file called studentdata.txt and load the information in the file into the Array of Objects
* Call the file openFileArray(). Note: need to increment records variable, it keeps track
* of the number of objects in my Array of Objects. And of course don't forget the
* thows IOException in the method header.
public void displayArrayList()
{
System.out.println("ArrayList Information -------");
for (int index = 0; index < objRefList.size(); index++)
objRefList.get(index).displayInfo();
System.out.println("================");
} //--------end of displayArrayList() method --------
public void displayArrayObjects()
{
System.out.println("Array of Objects Information ----------");
for (int index = 0; index < records; index++)
objRefArray[index].displayInfo();
System.out.println("===============");
} // -------end of displayArrayObjects() method -----
} // end of Student class
您必须改为使用 objRefList.add()。由于 arrayList 为空，行 objRefList.get(i).setName(temp); 不起作用。
您可能需要在 openFileList 方法中执行以下操作：
Student student = new Student();
student.setName(temp);
objRefList.add(student);
应该使用 Student class，使用数组来存储 RandomAccessFile 中保存的对象，并像这样打印：
public Persona[] readAll()throws IOException{
Persona[] n = new Persona[this.N];
for(int i=0;i<n.length;i++){
pos = 4+i*MAX;
raf.seek(pos);
Persona p1 = new Persona();
p1.setNombre(raf.readUTF());
p1.setApellido(raf.readUTF());
p1.setEdad(raf.readInt());
p1.setTelefono(raf.readInt());
n[i]=p1;
}
return n;
}
我在 class 中使用非结构化的 OOP 管理文件。更灵活。
我是一名学习 java 的学生，我很难弄清楚为什么会出现越界错误。关于如何编码的任何输入
openFileList(); 方法会非常有用。
准确的错误是：
java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
at java.util.ArrayList.rangeCheck(Unknown Source)
at java.util.ArrayList.get(Unknown Source)
at MathClassMrA.openFileList(ps25openfilemra.java:100)
at PS25OpenFileMrA.main(ps25openfilemra.java:9)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:272)
代码如下：
import java.util.Scanner;
import java.util.ArrayList;
import java.io.*;
class PS25OpenFileMarian {
public static void main(String [] arg) throws IOException { // note throws IOException in main()
MathClassMarian objA = new MathClassMarian();
objA.openFileList(); // You need to code this method
objA.displayArrayList();
//objA.openFileArray(); // You need to code this method.
// objA.displayArrayObjects();
} // ---------end of main() -----------------------
} // ######### end of PS25OpenFile Program ################################
/* **************************************************************************
* @Author: Marian @Date: today
* This class is design to hold data that can be manipulated by it's child class.
* ************************************************************************* */
class Student {
//INSTANCE VARIABLES
private String name;
private int [] marks;
//CONSTRUCTORS
Student()
{// default
name = null;
marks = new int[10];
for (int i=0; i < marks.length; i++)
marks[i] = -1;
}
//INSTANCE METHODS
public String getName() { return name; }
public void setName(String N) { name = N; }
public int getMark(int index) { return marks[index]; }
public void setMark(int index, int M) { marks[index] = M; }
public void displayInfo() {
System.out.println("--------------------");
System.out.printf("Name of Student: %s \n", name);
System.out.println("====Marks===");
for (int i=0; i < marks.length; i++)
if (marks[i] != -1)
System.out.printf("Mark %d: = %d \n " , i+1, marks[i]);
System.out.printf("Overall Average: = %.2f \n", calcAverage());
} //---end of displayInfo() method
public double calcAverage() {
double average = 0;
int NoOfTests = 0;
for (int i=0; i < marks.length; i++)
{
if (marks[i] != -1)
{
average += marks[i]; // average = average + marks[i];
NoOfTests++;
}
}
return (average/NoOfTests);
} // --end of calcAverage() mMethod --
} // end of Student class
/* *************************************************************************
* @Author: You @Date: Today
* This class manipulates the data in either an Array of Objects or an ArrayList
* ************************************************************************** */
class MathClassMarian extends Student {
//INSTANCE VARIABLES ----
public static int records = 0; // used to count the number of records in the Array of Objects.
ArrayList<Student> objRefList;
Student [] objRefArray;
//CONSTRUCTORS -------------
MathClassMarian()
{ // default constructor
objRefList = new ArrayList<Student>(); // array list null
objRefArray = new Student[10]; // array of objects null.
}
//INSTANCE METHODS --------------
/* *******************************************************************
* Activity #1: @Author: You @Date: today The method below should connect
* to a file called studentdata.txt and load the information in the file into the ArrayList
* call the method openFileList(): Note: don't forget the throws IOException in the method
* header.
* @param: none @return: none
* *******************************************************************/
public void openFileList() throws IOException{
File file = new File("studentdata.txt");
Scanner scan = new Scanner (file);
// objRefList.add(new student()) ;
ArrayList <Student> objRefList = new ArrayList<Student>();
int i =0, j=0;
String temp ;
int temp2;
while (scan.hasNext()) {
temp = scan.nextLine();
objRefList.get(i).setName(temp);
i++;
}
do {
temp2 = scan.nextInt();
objRefList.get(j).setMark(j,temp2);
j++;
} while (scan.hasNextInt());*/
}
/* *******************************************************************
* Activity #2: @Author: You @Date: today The method below should connect
* to a file called studentdata.txt and load the information in the file into the Array of Objects
* Call the file openFileArray(). Note: need to increment records variable, it keeps track
* of the number of objects in my Array of Objects. And of course don't forget the
* thows IOException in the method header.
public void displayArrayList()
{
System.out.println("ArrayList Information -------");
for (int index = 0; index < objRefList.size(); index++)
objRefList.get(index).displayInfo();
System.out.println("================");
} //--------end of displayArrayList() method --------
public void displayArrayObjects()
{
System.out.println("Array of Objects Information ----------");
for (int index = 0; index < records; index++)
objRefArray[index].displayInfo();
System.out.println("===============");
} // -------end of displayArrayObjects() method -----
} // end of Student class
您必须改为使用 objRefList.add()。由于 arrayList 为空，行 objRefList.get(i).setName(temp); 不起作用。
您可能需要在 openFileList 方法中执行以下操作：
Student student = new Student();
student.setName(temp);
objRefList.add(student);
应该使用 Student class，使用数组来存储 RandomAccessFile 中保存的对象，并像这样打印：
public Persona[] readAll()throws IOException{
Persona[] n = new Persona[this.N];
for(int i=0;i<n.length;i++){
pos = 4+i*MAX;
raf.seek(pos);
Persona p1 = new Persona();
p1.setNombre(raf.readUTF());
p1.setApellido(raf.readUTF());
p1.setEdad(raf.readInt());
p1.setTelefono(raf.readInt());
n[i]=p1;
}
return n;
}
我在 class 中使用非结构化的 OOP 管理文件。更灵活。