{% code %} package _27javacollect;
import java.time.DayOfWeek; import java.util.*;
class Student { public String name; public int score;
public Student(String name, int score) {
this.name = name;
this.score = score;
}
} class UserComparator implements Comparator<User></User> { public int compare(User u1, User u2) { if (u1.number.charAt(0) == u2.number.charAt(0)) { // 如果两人的号都是A开头或者都是V开头,比较号的大小: return u1.number.compareTo(u2.number); } if (u1.number.charAt(0) == 'V') { // u1的号码是V开头,优先级高: return -1; } else { return 1; } } }
class User { public final String name; public final String number;
public User(String name, String number) {
this.name = name;
this.number = number;
}
public String toString() {
return name + "/" + number;
}
}
public class List_exe {
public static void main(String[] args) {
/** 使用List*/
List<String> list = new ArrayList<>();
list.add("apple");
list.add("pear");
list.add("apple");
list.add(null);
System.out.println(list);
// 迭代器遍历list
for(Iterator<String> it=list.iterator();it.hasNext();){
String s = it.next();
System.out.println("迭代器："+s);
}
//再次简化
for(String s:list){
System.out.println("简化："+s);
}
//list转换array，不常用
Object[] array = list.toArray();
for(Object o:array){
System.out.println("list转array，不常用："+o);
}
//list转换array，常用
String[] array1 = list.toArray(new String[list.size()]);
for (String n : array1) {
System.out.println("list转array，常用："+n);
}
//array变list
Integer[] array2 = { 1, 2, 3 };
List<Integer> list2 = Arrays.asList(array2);
System.out.println(list2);
/**使用map */
Student s = new Student("Xiao Ming", 99);
Map<String, Student> map = new HashMap<>();
map.put("Xiao Ming", s); // 将"Xiao Ming"和Student实例映射并关联
Student target = map.get("Xiao Ming"); // 通过key查找并返回映射的Student实例
System.out.println(target == s); // true，同一个实例
System.out.println(target.score); // 99
Student another = map.get("Bob"); // 通过另一个key查找
System.out.println(another); // 未找到返回null
//key不能重复，value可以重复
Map<String, Integer> map1 = new HashMap<>();
map1.put("apple", 123);
map1.put("pear", 123);
System.out.println(map1.get("apple")); // 123
map1.put("apple", 789); // 再次放入apple作为key，但value变为789
System.out.println(map1.get("apple")); // 789
//遍历key
for (String key : map1.keySet()) {
Integer value = map1.get(key);
System.out.println(key + " = " + value);
}
//同时遍历key,value
for (Map.Entry<String, Integer> entry : map1.entrySet()) {
String key = entry.getKey();
Integer value = entry.getValue();
System.out.println(key + " = " + value);
}
/**使用EnumMap */
Map<DayOfWeek, String> map2 = new EnumMap<>(DayOfWeek.class);
map2.put(DayOfWeek.MONDAY, "星期一");
map2.put(DayOfWeek.TUESDAY, "星期二");
map2.put(DayOfWeek.WEDNESDAY, "星期三");
map2.put(DayOfWeek.THURSDAY, "星期四");
map2.put(DayOfWeek.FRIDAY, "星期五");
map2.put(DayOfWeek.SATURDAY, "星期六");
map2.put(DayOfWeek.SUNDAY, "星期日");
System.out.println(map2);
System.out.println(map2.get(DayOfWeek.MONDAY));
/**使用TreeMap 对Key进行排序*/
Map<String, Integer> map3 = new TreeMap<>();
map3.put("orange", 1);
map3.put("apple", 2);
map3.put("pear", 3);
for (String key : map3.keySet()) {
System.out.println(key);
}
/**Set */
//Set存储不重复的元素组合
Set<String> set = new HashSet<>();
System.out.println(set.add("abc")); // true
System.out.println(set.add("xyz")); // true
System.out.println(set.add("xyz")); // false，添加失败，因为元素已存在
System.out.println(set.contains("xyz")); // true，元素存在
System.out.println(set.contains("XYZ")); // false，元素不存在
System.out.println(set.remove("hello")); // false，删除失败，因为元素不存在
System.out.println(set.size()); // 2，一共两个元素
//HashSet无序
Set<String> set1 = new HashSet<>();
set1.add("apple");
set1.add("banana");
set1.add("pear");
set1.add("orange");
for (String s1 : set1) {
System.out.println("HashSet："+s1);
}
//TreeSet有序
Set<String> set2 = new TreeSet<>();
set2.add("apple");
set2.add("banana");
set2.add("pear");
set2.add("orange");
for (String s2 : set2) {
System.out.println("TreeSet："+s2);
}
/**Queue*/
Queue<String> q = new LinkedList<>();
// 添加3个元素到队列:
q.offer("apple");
q.offer("pear");
q.offer("banana");
q.add("apple"); //添加失败抛出异常
q.offer("apple"); //添加失败返回false
q.remove(); //获取队首并删除失败抛出异常
q.poll(); //获取队首并删除失败返回null
q.element(); //获取队首不删除失败抛出异常
q.peek(); //获取队首不删除失败返回null
// 从队列取出元素:
System.out.println(q.poll()); // apple
System.out.println(q.poll()); // pear
System.out.println(q.poll()); // banana
System.out.println(q.poll()); // null,因为队列是空的
// 队首永远都是apple，因为peek()不会删除它:
System.out.println(q.peek()); // apple
System.out.println(q.peek()); // apple
System.out.println(q.peek()); // apple
/**PriorityQueue 优先队列，顺序决定优先级*/
Queue<String> q2 = new PriorityQueue<>();
// 添加3个元素到队列:
q2.offer("apple");
q2.offer("pear");
q2.offer("banana");
System.out.println(q2.poll()); // apple
System.out.println(q2.poll()); // banana
System.out.println(q2.poll()); // pear
System.out.println(q2.poll()); // null,因为队列为空
Queue<User> q3 = new PriorityQueue<>(new UserComparator());
// 添加3个元素到队列:
q3.offer(new User("Bob", "A1"));
q3.offer(new User("Alice", "A2"));
q3.offer(new User("Boss", "V1"));
System.out.println(q3.poll()); // Boss/V1
System.out.println(q3.poll()); // Bob/A1
System.out.println(q3.poll()); // Alice/A2
System.out.println(q3.poll()); // null,因为队列为空
/**Deque 两头进，两头出*/
Deque<String> deque = new LinkedList<>();
deque.addFirst("a");
deque.addLast("a");
deque.offerFirst("C");
deque.offerLast("A");
deque.getFirst();
deque.getLast();
deque.peekFirst();
deque.peekLast();
deque.removeFirst();
deque.removeLast();
deque.pollFirst();
deque.pollLast();
System.out.println(deque.pollFirst()); // C, 剩下B -> A
System.out.println(deque.pollLast()); // B
System.out.println(deque.pollFirst()); // A
System.out.println(deque.pollFirst()); // null
/**Stack*/
/**Java中没有单独的接口，只能用Deque代替Stack，只调用push(),pop(),peek()*/
deque.push("a");
deque.push("c");
System.out.println(deque.pop());
System.out.println(deque.peek());
/**Collections*/
////创建空集合
List<String> list3 = Collections.emptyList();
System.out.println(list3);
////排序
List<String> list4 = new ArrayList<>();
list4.add("apple");
list4.add("pear");
list4.add("orange");
// 排序前:
System.out.println(list4);
Collections.sort(list4);
// 排序后:
System.out.println(list4);
////洗牌
List<Integer> list5 = new ArrayList<>();
for (int i=0; i<10; i++) {
list5.add(i);
}
// 洗牌前:
System.out.println(list5);
Collections.shuffle(list5);
// 洗牌后:
System.out.println(list5);
////不可变集合
List<String> mutable = new ArrayList<>();
mutable.add("apple");
mutable.add("pear");
// 变为不可变集合:
List<String> immutable = Collections.unmodifiableList(mutable);
mutable.add("orange");
try{
immutable.add("orange"); // UnsupportedOperationException!
}
catch (UnsupportedOperationException u){
System.out.println(u);
}
System.out.println(immutable);
}
} {% endcode %}