偷看堆栈顶部导致越界异常

Question

在我开始之前，如果这很难读，我会道歉，它来自一本书并且我不允许改变它。我必须实现一个名为peek（）的方法来检查堆栈的顶部，然后显示它。

npm line-by-line

我有reset（）和size（）工作，但为什么peek（）无效？

interface ISimpleStack {

  // Push a character onto the stack.
  void push(char ch);

  // Pop a character from the stack.
  char pop();

  // Return true if the stack is empty.
  boolean isEmpty();

  // Return true if the stack is full.
  boolean isFull();

  void reset();

  char peek();

  int size();
}

// A fixed-length stack for characters.
class FixedLengthStack implements ISimpleStack {
  private char[] data; // this array holds the stack
  private int tos; // index of top of stack

  // Construct an empty stack given its size.
  FixedLengthStack(int size) {
    data = new char[size]; // create the array to hold the stack
    tos = 0;
  }

  // Construct a stack from a stack.
  FixedLengthStack(FixedLengthStack otherStack) {
    // size of new stack equals that of otherStack
    data = new char[otherStack.data.length];

    // set tos to the same position
    tos = otherStack.tos;

    // copy the contents
    for(int i = 0; i < tos; i++)
      data[i] = otherStack.data[i];
  }

  // Construct a stack with initial values.
  FixedLengthStack(char[] chrs) {
    // create the array to hold the initial values
    data = new char[chrs.length];
    tos = 0;

    // initialize the stack by pushing the contents
    // of chrs onto it
    for(char ch : chrs)
      push(ch);
  }

  // Push a character onto the stack.
  public void push(char ch) {
    if(isFull()) {
      System.out.println(" -- Stack is full.");
      return;
    }

    data[tos] = ch;
    tos++;
  }

  // Pop a character from the stack.
  public char pop() {
    if(isEmpty()) {
      System.out.println(" -- Stack is empty.");
      return (char) 0; // a placeholder value
    }

    tos--;
    return data[tos];
  }

  // Return true if the stack is empty.
  public boolean isEmpty() {
    return tos==0;
  }

  // Return true if the stack is full.
  public boolean isFull() {
    return tos==data.length;
  }

  public void reset() {
      tos = 0;
  }

  public char peek() {
    if(isEmpty()) {
      System.out.println(" -- Stack is empty.");
      return (char) 0; // a placeholder value
    }

    return data[tos];
  }

  public int size() {
      int size = 0;
      for(int i = 0; i <= tos; i++){
         size = i;
      }
      System.out.println("Size of stack is: " + size);
      return size;
  }
}

// A growable stack for characters.
class DynamicStack implements ISimpleStack {
  private char[] data; // this array holds the stack
  private int tos; // index of top of stack

  // Construct an empty stack given its size.
  DynamicStack(int size) {
    data = new char[size]; // create the array to hold the stack
    tos = 0;
  }

  // Construct a stack from a stack.
  DynamicStack(DynamicStack otherStack) {
    // size of new stack equals that of otherStack
    data = new char[otherStack.data.length];

    // set tos to the same position
    tos = otherStack.tos;

    // copy the contents
    for(int i = 0; i < tos; i++)
      data[i] = otherStack.data[i];
  }

  // Construct a stack with initial values.
  DynamicStack(char[] chrs) {
    // create the array to hold the initial values
    data = new char[chrs.length];
    tos = 0;

    // initialize the stack by pushing the contents
    // of chrs onto it
    for(char ch : chrs)
      push(ch);
  }

  // Push a character onto the stack.
  public void push(char ch) {

    // if there is no more room in the array,
    // expand the size of the stack
    if(tos == data.length) {
      // double the size of the existing array
      char[] t = new char[data.length * 2];

      // copy the contents of the stack into the larger array
      for(int i = 0; i < tos; i++)
        t[i] = data[i];

      // set data to refer to the new array
      data = t;
    }

    data[tos] = ch;
    tos++;
  }

  // Pop a character from the stack.
  public char pop() {
    if(isEmpty()) {
      System.out.println(" -- Stack is empty.");
      return (char) 0; // a placeholder value
    }

    tos--;
    return data[tos];
  }

  // Return true if the stack is empty.
  public boolean isEmpty() {
    return tos==0;
  }

  // Return true if the stack is full. For DynamicStack,
  // this method always returns false.
  public boolean isFull() {
    return false;
  }

  public void reset() {
      tos = 0;
  }

  public char peek() {
    if(isEmpty()) {
      System.out.println(" -- Stack is empty.");
      return (char) 0; // a placeholder value
    }

    return data[tos];
  }

  public int size() {
      int size = 0;
      for(int i = 0; i <= tos; i++){
         size = i;
      }
      System.out.println("Size of stack is: " + size);
      return size;
  }
}

// Demonstrate ISimpleStack.
class ISimpleStackDemo {
  public static void main(String[] args) {
    int i;
    char ch;

    // create an ISimpleStack interface variable
    ISimpleStack iStack;

    // Now, construct a FixedLengthStack and a DynamicStack
    FixedLengthStack fixedStack = new FixedLengthStack(10);
    DynamicStack dynStack = new DynamicStack(5);

    // first, use fixedStack through iStack
    iStack = fixedStack;

    // push characters onto fixedStack
    for(i = 0; !iStack.isFull(); i++)
      iStack.push((char) ('A'+i));

    iStack.size();

    System.out.print("Top of fixedStack: ");
    while(!iStack.isEmpty()) {
      ch = iStack.peek();
      System.out.print(ch);
    }

    // pop characters off fixedStack
    System.out.print("Contents of fixedStack: ");
    while(!iStack.isEmpty()) {
      ch = iStack.pop();
      System.out.print(ch);
    }

    System.out.print("\nContents of fixedStack after reset: ");
    iStack.reset();
      ch = iStack.pop();
      System.out.print(ch);
    System.out.println();

    // next, use dynStack through iStack
    iStack = dynStack;

    // push A through Z onto dynStack
    // this will result in three increases in its size
    for(i = 0; i < 26; i++)
      iStack.push((char) ('A'+i));

    iStack.size();
    // pop characters off dynStack
    System.out.print("Contents of dynStack: ");
    while(!iStack.isEmpty()) {
      ch = iStack.pop();
      System.out.print(ch);
    }
    System.out.print("\nContents of dynStack after reset: ");
    iStack.reset();
      ch = iStack.pop();
      System.out.print(ch);
  }
}

Answer 1

使用示例中的代码，对peek（）进行以下更改可解决粘贴bin中第二个示例中提到的输出问题。这与FixedLengthStack中出现的问题相同。事实上，稍微好一点的实现可能是有一个抽象的Stack类，它实现了size（），reset（），pop（），（顺便说一句，应该也可以将数组设置为一个空数组），并查看（）。

@Override
public char peek()
{
    if (isEmpty()) {
        System.out.println(" -- Stack is empty.");
        return (char) 0; // a placeholder value
    }

    return data[tos - 1];
}