问题
This is what I'm trying to do: I'm reading a file in from the command line. The file contains a list of data,below this paragraph is what it looks like. The problem I'm having is with the if statements.
import java.util.*;
import java.io.*;
public class VehicleTest {
public static void main(String[] args) throws FileNotFoundException {
String vehicle = "vehicle";
String car = "car";
String americanCar = "american car";
String foreignCar = "foreign car";
String truck = "truck";
String bicycle = "bicycle";
File file = new File(args[0]);
Scanner input = new Scanner(file);
String[] autos = new String[100];
ArrayList allVehicles = new ArrayList();
for (int i = 0; i < autos.length; i++) {
autos[i] = input.nextLine();
}
int j = 0;
int i = 0;
while (i++ < autos.length) {
if (vehicle.equalsIgnoreCase(autos[j++])) {
Vehicle v = new Vehicle();
v.setOwnerName(autos[j]);
allVehicles.add(v);
}else if(car.equalsIgnoreCase(autos[j++])){
Car c = new Car();
c.setOwnerName(autos[j]);
allVehicles.add(c);
}
}
for(Object a: allVehicles){
System.out.println(a);
}
}
}
In pseudo code this would be:
while i is less than the length of the string array
if you see the word vehicle create a new vehicle object and add it to the arrayList.
if you see the word car create a new car object and add it to the arrayList.
.....
The problems is that I get an arrayOutOfBounds exception with the code I'm using.
I understand that j++ is what is wrong, but how else am I supposed to iterate through the string array so that I can read each line and create the appropriate objects? I'm at a loss as to what to do. I need some help.
foreign car
aMarioy
Mario's house
(777) 777-7777
gmario@mario.com
false
black
Italy
4415.91
truck
aDougy
Doug's house
(123) 456-7890
hdoug@doug.com
30
61234.56
8/10/2003
vehicle
aRobby
Rob's house
(987) 654-3210
irob@rob.com
bicycle
bTommy
Tom's house
(246) 810-1214
jtom@tom.com
7
truck
bGeorge
George's house
(666) 666-6666
kgeorge@george.com
25
51234.56
12/4/2004
vehicle
bTim
Tim's house
(111) 111-1111
tim@tim.com
bicycle
bJim
Jim's house
(555) 555-5555
Ajim@jim.com
5
american car
bJohn
John's house
(888) 888-8888
Bjohn@john.com
true
green
false
true
car
cKen
Ken's house
(999) 999-9999
Cken@ken.com
false
orange
foreign car
cMario
Mario's house
(777) 777-7777
Dmario@mario.com
false
black
Italy
4415.91
american car
gSam
Sam's house
(333) 333-3333
Hsam@sam.com
false
blue
true
false
回答1:
A couple of problems:
- You're incrementing j in both "if" tests. I haven't checked to be certain (it's quite convoluted code, to be honest) but if you make sure that you only increment j when you've found a match, that will help.
- Your test using
ibasically means it will try to read as many vehicles as there are lines in the file, rather than stopping when you've reached the end of the file. Basically you don't needihere.
Here's one changed version:
while (j < autos.length) {
if (vehicle.equalsIgnoreCase(autos[j])) {
j++;
Vehicle v = new Vehicle();
v.setOwnerName(autos[j++]);
allVehicles.add(v);
} else if(car.equalsIgnoreCase(autos[j])){
j++;
Car c = new Car();
c.setOwnerName(autos[j++]);
allVehicles.add(c);
}
}
It would be slightly cleaner to extract the type once though - then you can do the comparisons separately:
while (j < autos.length) {
String type = autos[j++];
if (vehicle.equalsIgnoreCase(type)) {
Vehicle v = new Vehicle();
v.setOwnerName(autos[j++]);
allVehicles.add(v);
} else if(car.equalsIgnoreCase(type)){
Car c = new Car();
c.setOwnerName(autos[j++]);
allVehicles.add(c);
}
}
It's still not quite how I'd do it, but it's closer...
My next step would be to use the scanner more appropriately:
while (scanner.hasNext()) {
String type = scanner.nextLine();
if (type.equalsIgnoreCase("vehicle")) {
allVehicles.add(new Vehicle(scanner));
} else if (type.equalsIgnoreCase("car")) {
allVehicles.add(new Car(scanner));
}
// ...
}
Then make the constructor for Vehicle, Car etc do the parsing directly from the scanner.
The next step would be to separate out the construction from the iteration. Introduce a new method:
// Use a base type in real code
private static Object parseNextVehicle(Scanner scanner) {
String type = scanner.nextLine();
if (type.equalsIgnoreCase("vehicle")) {
return new Vehicle(scanner);
} else if (type.equalsIgnoreCase("car")) {
return new Car(scanner);
}
// ... throw an exception indicating an unknown vehicle type
}
// ... and then in the main method, use it like this:
while (scanner.hasNextLine()) {
allVehicles.add(parseNextVehicle(scanner));
}
回答2:
Every line that does not equal “vehicle” will (incorrectly) increment j so approximately after line 50 you will get the exception.
There are multiple solutions to this:
- Increment
jonly once per loop. - Read the lines into another
ArrayList. - Don’t read the lines into a data structure but process them as they are being read. This way you are more independent of the size of your data.
回答3:
Don't use j++ in the subscript; increment it once after the entire loop, rather than once or twice depending on which condition holds.
Probably better to do this:
- replace all your in-line increments (x++) with increment statements (x = x + 1)
- figure out where they need to go to make the code do what you want
- turn them back to inline pre/post increments once you've gotten it working, if it seems appropriate
回答4:
Put your increments and decrements in their own statement. This will make the code easier to understand in most cases.
In your case, j++ is called twice if the first if fails. This is probably not what you want.
I would convert your while loop into a for loop, like so:
for (int i = 0, j = 0; i < autos.length; ++i, ++j) {
if (vehicle.equalsIgnoreCase(autos[j])) {
// ...
If i == j always, just use the same variable for both.
来源：https://stackoverflow.com/questions/690028/array-index-out-of-bounds