Java集合与映射：深入解析与实践

### Java集合与映射：深入解析与实践 #### 1. 集合与队列操作 在Java编程中，集合和队列是常用的数据结构。下面通过代码示例来展示栈和队列的基本操作。 ```java import java.util.Arrays; import java.util.ArrayDeque; public class CollectionExample { public static void main(String[] args) { // 使用ArrayDeque作为栈 String[] elementArray = new String[] {"sway", "and", "twist", "stacks", "tall"}; System.out.println("Array of elements: " + Arrays.toString(elementArray)); ArrayDeque<String> stack = new ArrayDeque<String>(); for (String string : elementArray) stack.push(string); System.out.println("Stack before: TOP->" + stack + "<-BOTTOM"); System.out.print("Popping stack: "); while (!stack.isEmpty()) { System.out.print(stack.pop() + " "); } System.out.println("\n"); // 使用ArrayDeque作为FIFO队列 elementArray = new String[] {"Waiting", "in", "queues", "is", "boring"}; System.out.println("Array of elements: " + Arrays.toString(elementArray)); ArrayDeque<String> fifoQueue = new ArrayDeque<String>(); for (String string : elementArray) fifoQueue.offerLast(string); System.out.println("Queue before: HEAD->" + fifoQueue + "<-TAIL"); System.out.print("Polling queue: "); while (!fifoQueue.isEmpty()) { String string = fifoQueue.pollFirst(); System.out.print(string.toUpperCase() + " "); } System.out.println(); } } ``` 上述代码展示了如何使用`ArrayDeque`作为栈和FIFO队列。栈遵循后进先出（LIFO）原则，而队列遵循先进先出（FIFO）原则。 流程图如下： ```mermaid graph TD; A[开始] --> B[创建元素数组]; B --> C[使用ArrayDeque作为栈]; C --> D[将元素压入栈]; D --> E[弹出栈元素]; E --> F[使用ArrayDeque作为队列]; F --> G[将元素插入队列尾部]; G --> H[从队列头部移除元素]; H --> I[结束]; ``` #### 2. 集合相关的复习问题 以下是一些关于集合的复习问题及解析： ##### 2.1 关于集合的正确陈述 问题：Which statements about collections are true? （选择两个正确答案） 选项： - (a) Some operations on a collection may throw an UnsupportedOperationException. - (b) Methods calling optional operations in a collection must either catch an UnsupportedOperationException or declare it in their throws clause. - (c) A List can have duplicate elements. - (d) An ArrayList can only accommodate a fixed number of elements. - (e) The Collection interface contains a method named get. 答案：(a)和(c)。有些集合操作可能会抛出`UnsupportedOperationException`，`List`可以包含重复元素。 ##### 2.2 程序编译和运行结果 问题：What will be the result of attempting to compile and run the following program? ```java import java.util.ArrayList; import java.util.Collection; import java.util.HashSet; import java.util.LinkedList; import java.util.TreeSet; public class Sets { public static void main(String[] args) { HashSet<Integer> set1 = new HashSet<Integer>(); addRange(set1, 1); ArrayList<Integer> list1 = new ArrayList<Integer>(); addRange(list1, 2); TreeSet<Integer> set2 = new TreeSet<Integer>(); addRange(set2, 3); LinkedList<Integer> list2 = new LinkedList<Integer>(); addRange(list2, 5); set1.removeAll(list1); list1.addAll(set2); list2.addAll(list1); set1.removeAll(list2); System.out.println(set1); } static void addRange(Collection<Integer> col, int step) { for (int i = step*2; i<=25; i+=step) col.add(i); } } ``` 选项： - (a) The program will fail to compile, since operations are performed on incompatible collection implementations. - (b) The program will fail to compile, since the TreeSet referenced by set2 has not been given a Comparator to use when sorting its elements. - (c) The program will compile without error, but will throw an UnsupportedOperationException, when run. - (d) The program will compile without error and will print all primes below 25, when run. 答案：(d)。程序将编译无误并在运行时打印出25以下的所有质数。 ##### 2.3 `Collection<E>`接口中定义的方法 问题：Which of these methods are defined in the Collection<E> interface? （选择三个正确答案） 选项： - (a) add(E o) - (b) retainAll(Collection<?> c) - (c) get(int index) - (d) iterator() - (e) indexOf(Object o) 答案：(a)、(b)和(d)。`add(E o)`用于添加元素，`retainAll(Collection<?> c)`用于保留指定集合中的元素，`iterator()`用于获取迭代器。 ##### 2.4 程序输出结果 问题：What will the following program print? ```java import java.util.ArrayList; import java.util.List; import java.util.ListIterator; public class Iterate { public static void main(String[] args) { List<String> l = new ArrayList<String>(); l.add("A"); l.add("B"); l.add("C"); l.add("D"); l.add("E"); ListIterator<String> i = l.listIterator(); i.next(); i.next(); i.next(); i.next(); i.remove(); i.previous(); i.previous(); i.remove(); System.out.println(l); } } ``` 选项： - (a) It will