集合類

Collection<>

常用方法

Collection<String> c = new ArrayList<String>(); c.add("java"); c.add("hello"); c.add("my"); System.out.println(c); // [java, hello, my]System.out.println(c.size()); // 3c.remove("java");System.out.println(c); // [hello, my]System.out.println(c.contains("java")); // falsec.clear();System.out.println(c); // []

遍歷

增強for循環

作用；方便遍歷 原理；內置迭代器 推薦：只要遍歷的時候推薦使用

/* 格式； for（元素數據類型 變量名 : 數據或Collection集合）{輸出變量名即可完成遍歷 } */ for (String i : l) {System.out.println(i); }

三種遍歷方法

Iterator<Student> it = l.iterator(); while (it.hasNext()) {Student s = it.next();System.out.println(s.getName()); } for (int i = 0; i < l.size(); i++) {Student s = l.get(i);System.out.println(s.getName()); } for (Student s : l) {System.out.println(s.getName()); }

List<>

有序，可重復

常用方法

List<String> l = new ArrayList<>(); l.add("add"); l.add("java"); // void add(int index, E element) l.add(1, "hello"); System.out.println(l); // [add, hello, java]// E remove(int index), E remove(Object o) l.remove(0); l.remove("java"); System.out.println(l); // [hello]// E set(int index, E element) l.set(0, "java"); System.out.println(l); // [java]// E get(int index) l.get(0); // java

并發修改異常

ConcurrentModificationException

實際修改值和預期修改值不相等

解決方案：用for循環遍歷

for (int i = 0; i < l.size(); i++) {String s = l.get(i);System.out.println(s); }

ArrayList<>

數組

// 用多態創建對象 Collection<String> c = new ArrayList<String>(); c.add("java"); c.add("hello"); System.out.println(c); // [java, hello]

LinkedList<>

鏈表

LinkedList<String> c = new LinkedList<>();; c.add("hello"); c.add("world"); c.add("java"); System.out.println(c); // [hello, world, java]c.addFirst("first"); c.addLast("last"); System.out.println(c); // [first, hello, world, java, last]System.out.println(c.getFirst()); System.out.println(c.getLast()); c.removeFirst(); c.removeLast(); System.out.println(c); // [hello, world, java]

ListIterator<>

extends List<>

常用方法:

正向遍歷

ListIterator<String> lit = l.listIterator(); while (lit.hasNext()) {String s = lit.next();System.out.println(s); }

逆向遍歷

ListIterator<String> lit = l.listIterator(); while (lit.hasPrevious()) {String s = lit.previous();System.out.println(s); }

add（）不會造成并發修改異常，add方法里讓實際操作值和預期的相等

ListIterator<String> lit = l.listIterator(); while (lit.hasNext()) {String s = lit.next();if (s.equals("hello")) {lit.add("world");} } System.out.println(l); // [add, java, hello, world]

Set<>

不包含重復元素，沒有索引，不能用for循環遍歷

Set<String> set = new HashSet<>(); set.add("java"); set.add("hello"); set.add("world"); set.add("java"); for (String s : set) {System.out.println(s); } /*輸出 HashSet不不保證迭代順序 java world hello 沒有出現兩個"java" */

HashSet<>

為了使HashSet具有元素唯一性，要重寫equals()和hashCode()

@Override public boolean equals(Object o) {if (this == o) return true;if (o == null || getClass() != o.getClass()) return false;Student student = (Student) o;if (age != student.age) return false;return name != null ? name.equals(student.name) : student.name == null; }@Override public int hashCode() {int result = name != null ? name.hashCode() : 0;result = 31 * result + age;return result; }

LinkedHashSet<>

保證迭代的有序性，同時具有唯一性

LinkedHashSet<String> lhs = new LinkedHashSet<>(); lhs.add("hello"); lhs.add("java"); lhs.add("world"); lhs.add("java");for (String s : lhs) {System.out.println(s); }/*hellojavaworld*/

TreeSet<>

有序：元素按照一定的規則進行排序，有自然排序和比較器排序

沒有索引，不能用普通for循環遍歷

TreeSet<Integer> t = new TreeSet<>(); t.add(11); t.add(12); t.add(9); t.add(8); for (Integer i : t) {System.out.println(i); } //8 //9 //11 //12

Comparable<>

比較器

public class Student implements Comparable<Student>{@Override public int compareTo(Object o) {// return 0; // 只返回第一個// return 1; // 按序返回 第二個比第一個大// return -1; // 倒序返回// 按年齡從大到小進行排序int num = this.age - s.age;return num; } // this 升前降后 升：從小到大

用TreeSet<>帶參構造

TreeSet<Student> t = new TreeSet<>(new Comparator<Student>() {@Overridepublic int compare(Student s1, Student s2) {int num = s1.getAge() - s2.getAge(); // s1 相當于this，s2為上文sreturn num;} });

一般要有主要條件和次要條件

三元運算符：如num==0?s1.getName().compareTo(s2.getName) : num

上述先判斷num是否等于0，true就返回s1.getName().compareTo(s2.getName)，false就返回num，而compareTo按字母排序比較字符串

產生十個不重復隨機數并遍歷

Set<Integer> s = new HashSet<>(); Random r = new Random();while (s.size() < 10) {int i = r.nextInt(20) + 1;s.add(i); } for (Integer i :s) {System.out.println(i); }

Map<>

基本功能

鍵有唯一性，重復的鍵就相當于修改的操作

Map<String, String> map = new HashMap<>(); map.put("hello", "java"); map.put("hi", "mysql"); map.put("hello", "navicat"); System.out.println(map); // {hi=mysql, hello=navica}System.out.println(map.size()); // 2map.remove("hello");System.out.println(map); // {hi=mysql}System.out.println(map.containsKey("hi")); // trueSystem.out.println(map.containsValue("mysql")); // truemap.clear();System.out.println(map.isEmpty()); // true

獲取功能

Map<String, String> map = new HashMap<>(); map.put("hello", "java"); map.put("hi", "mysql"); map.put("greet", "navicat");System.out.println(map.get("greet")); // navicat Set<String> ke = map.keySet(); for (String s : ke) {System.out.println(s); } /* 獲取鍵的集合 hi greet hello */ Collection<String> values = map.values(); for (String s1 : values) {System.out.println(s1); } /* 獲取值的集合 mysql navicat java */

遍歷

// 1.獲取鍵的集合 Set<String> keySet = map.keySet(); // 2.遍歷每一個元素 for (String key : keySet) {// 3.根據鍵找到值String values = map.get(key);System.out.println(key + "," + values); } // 1.獲取鍵值對的集合 Set<Map.Entry<String, String>> entrySet = map.entrySet(); // 2.遍歷每一個鍵值對 for (Map.Entry<String, String> my : entrySet) {String key = my.getKey();String value = my.getValue();System.out.println(key + "," + value); }

案例

1.HashMap<String, Student>案例

public static void main(String[] args) {HashMap<String, Student> hs = new HashMap<>();Student s1 = new Student("你", 10);Student s2 = new Student("它", 11);Student s3 = new Student("他", 15);Student s4 = new Student("我", 42);hs.put("daqi1", s1);hs.put("daqi2", s2);hs.put("daqi3", s3);hs.put("daqi4", s4);Set<String> keySet = hs.keySet();for (String key : keySet) {Student student = hs.get(key);System.out.println(key + "," + student.getName() + "," + student.getAge());}/*daqi3,他,15daqi4,我,42daqi1,你,10daqi2,它,11*/Set<Map.Entry<String, Student>> entrySet = hs.entrySet();for (Map.Entry<String, Student> me : entrySet) {String key = me.getKey();Student value = me.getValue();System.out.println(key + "," + value.getName() + "," + value.getAge());} }

ps：上述如果將HasMap<…>改為HasMap<Student, String>,為了確保鍵的唯一性，要重寫equals和HasCode方法，用快捷鍵Alt + Insert快速生成即可

2.嵌套

ArrayList<HashMap<String, String>>

ArrayList<HashMap<String, String>> array = new ArrayList<>();HashMap<String, String> hm1 = new HashMap<>(); hm1.put("java", "mysql"); hm1.put("Navicat", "Tomcat"); array.add(hm1);HashMap<String, String> hm2 = new HashMap<>(); hm1.put("notion", "music"); hm1.put("math", "Chinese");array.add(hm2);for (HashMap<String, String> hm : array) {Set<String> keySet = hm.keySet();for (String key : keySet) {String value = hm.get(key);System.out.println(key + "," + value);} }

3.統計字符串中每個字符出現的個數

public static void main(String[] args) {Scanner sc = new Scanner(System.in);System.out.println("請輸入字符串：");String line = sc.nextLine();HashMap<Character, Integer> hm = new HashMap<>();// TreeMap<Character, Integer> tm = new TreeMap<>(); // TreeMap 可以對鍵進行自然排序// 創建StringBuild來存儲字符串StringBuilder sb = new StringBuilder();// 遍歷字符串的每一個字符for (int i = 0; i < line.length(); i++) {char key = line.charAt(i);Integer value = hm.get(key);if (value == null) {hm.put(key, 1);} else {value++;hm.put(key, value);}}Set<Character> set = hm.keySet();for (Character key : set) {Integer value = hm.get(key);sb.append(key).append("(").append(value).append(")");}String s = sb.toString();System.out.println(s);}

Collections類

集合操作的工具類，靜態方法，直接調用

常用方法

List<Integer> l = new ArrayList<>(); // 添加元素 l.add(10); l.add(39); l.add(49); l.add(100); l.add(49);Collections.sort(l); System.out.println(l); // [10, 39, 49, 49, 100]Collections.reverse(l); System.out.println(l); // [100, 49, 49, 39, 10]Collections.shuffle(l); // 隨機排序 System.out.println(l); // [49, 49, 100, 10, 39]

總結

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