Double-sided queue problem

问题

I am trying to run this method to insert a generic value (EltType) into an double sided queue(deque), but I keep getting an outOfBoundsException that I just can't figure out. Would anyone please be able to help me with this ? This is just an extract from the code, but I think it can be pieced together from this!

  private final int CAPACITY = 10;
  private int capacity;
  private int end;
  private EltType deque[];  

  public ArrayBasedDeque() {
    this.capacity = CAPACITY;
    deque = (EltType[]) (new Object[capacity]);  
  }

    public void insertFirst(EltType first) {
        if(!isEmpty()) {
        EltType[] tempArray;
        tempArray = (EltType[]) new Object[CAPACITY+1];
        for (int i=0;i<=deque.length;i++) {
          tempArray[i+1] = deque[i]; 
        }
        deque = tempArray; 
        }
       deque[0] = first;
      }

  public boolean isEmpty() {
    boolean returned;
    if (deque.length < 1) {
     returned = true; 
    }else {
     returned = false; 
    }
    return returned;
  }

Error :

java.lang.ArrayIndexOutOfBoundsException: 10
    at ArrayBasedDeque.insertFirst(ArrayBasedDeque.java:48)
    at TestABD.main(TestABD.java:5)
    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:271)

回答1:

for (int i=0;i<=deque.length;i++) {

should be changed into

for (int i=0;i<deque.length;i++) {

You used a "smaller than or equal", but the last item of an array has (length-1) for index.

回答2:

In addition to the other answers about <=, you're setting the temp array size to CAPACITY+1, which will always be 11. You probably meant:

tempArray = (EltType[]) new Object[capacity+1];

回答3:

As other posters have mentioned, you have an "off-by-one" error, also called a fencepost error.

Also, you can simplify your isEmpty() method as follows:

public boolean isEmpty() {
    return end == 0;
}

I'm assuming that when end is zero, then that means there are no elements in the deque. You shouldn't check deque.length, because that simply tells you how many elements the array can store, not how many are currently in the array.

回答4:

Where are you changing your capacity? It probably shouldn't be a constant. The size is also not being incremented when you add.

回答5:

    for (int i=0;i<=deque.length;i++) {

You should use <, not <=.

回答6:

B/c you are using <=, you go up to deque.length which is 10, but deque only has 9 indices.

 for (int i=0;i<=deque.length;i++) {
          tempArray[i+1] = deque[i]; 
 }

Use < instead in the for loop

回答7:

As a side note:

public boolean isEmpty() {
  boolean returned;
  if (deque.length < 1) {
   returned = true; 
  }else {
   returned = false; 
  }
  return returned;
}

Doesn't:

public boolean isEmpty() {
  deque.length < 1
}

look simplier?

来源：https://stackoverflow.com/questions/4927026/double-sided-queue-problem

标签

java

deque