棧和隊列是Java中非常重要的數據結構，它們被廣泛應用于許多程序的實現中。下面我們通過幾個例子來學習一下棧和隊列在Java中的應用。

例1：用棧進行表達式求值

public static double evaluate(String expression) {
Stack<Double> operands = new Stack<>();
Stack<Character> operators = new Stack<>();
expression = expression.replaceAll(" ", "");
int i = 0;
while (i < expression.length()) {
char c = expression.charAt(i);
if (Character.isDigit(c)) {
StringBuilder sb = new StringBuilder();
while (i < expression.length() && (Character.isDigit(expression.charAt(i)) || expression.charAt(i) == '.')) {
sb.append(expression.charAt(i));
i++;
}
operands.push(Double.parseDouble(sb.toString()));
continue;
}
if (c == '(') {
operators.push(c);
} else if (c == ')') {
while (operators.peek() != '(') {
double result = applyOperator(operands.pop(), operands.pop(), operators.pop());
operands.push(result);
}
operators.pop();
} else {
while (!operators.isEmpty() && precedence(c) <= precedence(operators.peek())) {
double result = applyOperator(operands.pop(), operands.pop(), operators.pop());
operands.push(result);
}
operators.push(c);
}
i++;
}
while (!operators.isEmpty()) {
double result = applyOperator(operands.pop(), operands.pop(), operators.pop());
operands.push(result);
}
return operands.pop();
}
private static int precedence(char operator) {
switch (operator) {
case '+':
case '-':
return 1;
case '*':
case '/':
return 2;
case '^':
return 3;
}
throw new IllegalArgumentException("Unknown operator: " + operator);
}
private static double applyOperator(double b, double a, char operator) {
switch (operator) {
case '+':
return a + b;
case '-':
return a - b;
case '*':
return a * b;
case '/':
return a / b;
case '^':
return Math.pow(a, b);
}
throw new IllegalArgumentException("Unknown operator: " + operator);
}

例2：用隊列實現二叉樹的層次遍歷

public List<Integer> levelOrderTraversal(TreeNode root) {
List<Integer> result = new ArrayList<>();
if (root == null) {
return result;
}
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(root);
while (!queue.isEmpty()) {
int size = queue.size();
for (int i = 0; i < size; i++) {
TreeNode node = queue.poll();
result.add(node.val);
if (node.left != null) {
queue.offer(node.left);
}
if (node.right != null) {
queue.offer(node.right);
}
}
}
return result;
}

通過以上的例子，我們對Java中的棧和隊列有了一定的了解。我們可以看到，棧和隊列在程序設計中具有重要作用，并且它們的應用場景非常廣泛。