現在、私は を使用しArrayStackてインフィックスをポストフィックスに変更し、ポストフィックスの評価を評価するプロジェクトに取り組んでいます。プログラムがそれを読み取ったときに表示される最初の演算子については、送り返します
java.lang.NullPointerException
at InfixToPostfix.convertString(InfixToPostfix.java:27)
at Postfix.main(Postfix.java:20)
プログラムをデバッグしましたが、.NET でプッシュ メソッドを処理する必要があることはわかっていますArrayStack。を取り除く方法がわかりませんNullPointerException。
これが私のArrayStackクラスです。
import java.util.*;
public class ArrayStack<T> implements StackADT<T>{
private int top = 0;
private static final int DEFAULT_CAPACITY = 70;
private T[] stack;
@SuppressWarnings("unchecked")
public ArrayStack()
{
top = 0;
stack = (T[])(new Object[DEFAULT_CAPACITY]);
}
@SuppressWarnings("unchecked")
public ArrayStack (int initialCapacity)
{
top = 0;
stack = (T[])(new Object[initialCapacity]);
}
public boolean isEmpty()
{
if(top==0)
return true;
else
return false;
}
public T pop() throws StackIsEmptyException
{
T retVal;
if(isEmpty())
throw new StackIsEmptyException("Stack is Empty");
else{
top--;
retVal = stack[top];
}
return retVal;
}
**public void push (T element)
{
if (size() == stack.length)
expandCapacity();
top++;
element = stack[top];
}**
public T peek() throws StackIsEmptyException
{
if (isEmpty())
throw new StackIsEmptyException("Stack is Empty");
return stack[top-1];
}
@SuppressWarnings("unchecked")
private void expandCapacity()
{
T[] larger = (T[])(new Object[stack.length*2]);
for (int index=0; index < stack.length; index++)
larger[index] = stack[index];
stack = larger;
}
@Override
public String toString() {
String result = "";
for (int scan=0; scan < top; scan++)
result = result + stack[scan].toString() + "\n";
return result;
}
@Override
public int size() {
return top;
}
}
InfixToPostfix クラス
public class InfixToPostfix {
private ArrayStack<Character> stack;
private String infix;
private String postfix= "";
public InfixToPostfix(String in, ArrayStack<Character> stack) {
infix = in;
this.stack = stack;
}
@SuppressWarnings("unused")
public String convertString (){
for (int i = 0; i< infix.length(); i++){
try {
char curChar = infix.charAt(i);
if(!isOperator(curChar)){
postfix = postfix + curChar;
if (i == (infix.length()-1)){
while(!stack.isEmpty()){
postfix += stack.pop();
}
}
}**else if(stack.isEmpty()&& isOperator(curChar)){
stack.push(curChar);**
}
else if(charPrec(curChar) == 4){
while (!stack.isEmpty() && stack.size() != '('){
postfix += stack.pop();
}
stack.pop();
}else if(!(stack.isEmpty())&&(precident(curChar,stack.peek()))){
stack.push(curChar);
if(charPrec(stack.peek())==3)
stack.push(curChar);
while (!(stack.isEmpty())&&(!precident(curChar,stack.peek()))){
postfix += stack.pop();
}
stack.push(curChar);
}
}
catch (StackIsEmptyException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
return postfix;
}
/**
* Evaluates the specified postfix expression. If an operand is
* encountered, it is pushed onto the stack. If an operator is
* encountered, two operands are popped, the operation is
* evaluated, and the result is pushed onto the stack.
* @param expr String representation of a postfix expression
* @return int value of the given expression
*/
public int evaluate (String expr)
{
int op1, op2, result = 0;
String token;
StringTokenizer tokenizer = new StringTokenizer (expr);
/* while (tokenizer.hasMoreTokens())
{
token = tokenizer.nextToken();
if (isOperator(token))
{
op2 = (stack.pop()).charValue();
op1 = (stack.pop()).charValue();
result = evalSingleOp (token.charAt(0), op1, op2);
stack.push (new Integer(result));
}
else
stack.push (new Integer(Integer.parseInt(token)));
} */
return result;
}
/**
* Determines if the specified token is an operator.
* @param token String representing a single token
* @return boolean true if token is operator
*/
private boolean isOperator (String token)
{
return ( token.equals("+") || token.equals("-") ||
token.equals("*") || token.equals("/") );
}
/**
* Performs integer evaluation on a single expression consisting of
* the specified operator and operands.
* @param operation operation to be performed
* @param op1 the first operand
* @param op2 the second operand
* @return int value of the expression
*/
private int evalSingleOp (char operation, int op1, int op2)
{
int result = 0;
switch (operation)
{
case '+':
result = op1 + op2;
break;
case '-':
result = op1 - op2;
break;
case '*':
result = op1 * op2;
break;
case '/':
result = op1 / op2;
}
return result;
}
private boolean isOperator(char ch) {
if(ch=='/'||ch=='*'||ch=='+'||ch=='-')
return true;
else
return false;
}
private boolean precident(char one, char two) {
if(charPrec(one) >= charPrec(two));
return true;
}
private int charPrec(char ch) {
switch(ch) {
case '-':
return 1;
case '+':
return 1;
case '*':
return 2;
case '/':
return 2;
case '(':
return 3;
case ')':
return 4;
}
return 0; }
}