集合嵌套
public static void main(String[] args) {//集合嵌套//定义一个集合 放集合one,two 以ArrayList<Person> 为泛型ArrayList<ArrayList<Person>> list=new ArrayList();ArrayList<Person> one=new ArrayList();ArrayList<Person> two=new ArrayList();one.add(new Person("张三",23));one.add(new Person("李四",24));one.add(new Person("王五",25));two.add(new Person("李雷",18));two.add(new Person("韩梅梅",19));two.add(new Person("王铁钢",20));//将两个集合放入list集合中list.add(one);list.add(two);//循环遍历for (ArrayList<Person> lists : list) {for (Person p :lists ) {System.out.println(p.getName()+p.getAge());}}}Set集合
接口 无顺序 不可重复
HashSet:底层为哈希算法
:LinkedSet：Set体系下唯一可以保证存取顺序的
TreeSet:底层为二叉树 目的：对set集合中的数据进行排序
方法：与List中的方法相同
HashSet:
HashSet去重是底层通过哈希码和equals完成的
LinkedHashSet:特点可以保证怎么存就怎么取
TreeSet:
排序去重 底层会把存入TreeSet集合的数据进行自然顺序排序
里面有compare:return (x<y)? -1:((x==y)?0:1)
使用二叉树存储对象 排序去重 需要实现（implements）Comparable<对象>
重写compareTo方法 如果返回值为0 那么只存第一个对象
返回负数倒着存 返回正数就按顺序存
public class Person {private String name;private int age;private float score;public Person() {}public Person(String name, int age, float score) {this.name = name;this.age = age;this.score = score;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}public float getScore() {return score;}public void setScore(float score) {this.score = score;}@Overridepublic boolean equals(Object o) {if (this == o) return true;if (o == null || getClass() != o.getClass()) return false;Person person = (Person) o;return age == person.age && Float.compare(score, person.score) == 0 && Objects.equals(name, person.name);}@Overridepublic int hashCode() {return Objects.hash(name, age, score);}@Overridepublic String toString() {return "Person{" +"name='" + name + '\'' +", age=" + age +", score=" + score +'}';}
}public class ComPerson implements Comparator<Person> {@Overridepublic int compare(Person o1, Person o2) {int n1=(int)(o1.getScore()-o2.getScore());//分数int n2=o1.getAge()-o2.getAge();//年龄int n3=o1.getName().compareTo(o2.getName());//名字int result=0;//不是同一个人 按成绩降序if((n2+n3)!=0){result=-n1;//如果成绩相同 不去重 按年龄排序if(result==0){//成绩相同//比较年龄result=n2;//年龄相同result=result==0?1:result;//一前一后}}return result;}
} TreeSet<Person> ts=new TreeSet<>(new ComPerson());ts.add(new Person("张三",23,90f));ts.add(new Person("李四",24,95f));ts.add(new Person("李四",24,95f));ts.add(new Person("王五",22,91f));ts.add(new Person("王五",22,91f));System.out.println(ts);比较器（降低程序之间的耦合度）
1.创建比较器 实现 Comparator<类>
2.重写compareTo方法
3.修改想要比较的返回值
例：
public class CheckAge implements Comparator<Student> {//通过年龄排序@Overridepublic int compare(Student o1, Student o2) {return o1.getAge()-o2.getAge();}
}public class CheckName implements Comparator<Student> {//通过名字排序@Overridepublic int compare(Student o1, Student o2) {return o1.getName().compareTo(o2.getName());}
} public static void main(String[] args) {//创建一个带比较器的集合TreeSet<Student> ts=new TreeSet(new CheckName());//TreeSet<Student> ts=new TreeSet(new CheckAge());ts.add(new Student("张三",23));ts.add(new Student("张三",23));ts.add(new Student("李四",24));ts.add(new Student("李四",24));ts.add(new Student("王五",25));ts.add(new Student("王五",25));System.out.println(ts);}不去重 但排序
public class ComString implements Comparator<Character> {//不去重 但排序@Overridepublic int compare(Character o1, Character o2) {int num1=o1.compareTo(o2);int num2=num1==0?1:num1;return num2;}
}案例：
public static void main(String[] args) {//让其有序(按字典顺序) 不去重String s="liaoninglianningliaoning";sort(s);}public static void sort(String s){TreeSet<Character> ts=new TreeSet(new ComString());for(int i=0;i<s.length();i++){char c=s.charAt(i);ts.add(c);}System.out.println(ts);}字符串转换回基本数据类型（parse）
public static void main(String[] args) {int a=Integer.parseInt("123");//123float b=Float.parseFloat("123f");//123.0short c=Short.parseShort("123");//123byte d=Byte.parseByte("1");//1double e=Double.parseDouble("123");//123.0// double f=Double.parseDouble("a");//报错 转换异常System.out.println(a);System.out.println(b);System.out.println(c);System.out.println(d);System.out.println(e);// System.out.println(f);}Map集合
HashMap:重点 springMVC携带服务器数据到前端
Map<key，value>键，值
Map集合添加元素特点：map.put();
如果键 是第一次存储 就直接存储元素 返回null
如果键 不是第一存储 就用值将以前的值替换掉 返回以前的值
Map<Integer,String> m=new HashMap();
根据键获取值：
m.get(键)；
返回所有键的集合：
方法1（有迭代器）
m.keySet();//.var
Set<Integer> s = m.keySet();
//遍历map 通过遍历装有键的Set集合 获得对应的所有值
Iterator<Integer> i=s.iterator();
while(i.hasNext()){
Integer key = i.next();
String value = m.get(key);
System.out.println("key"+key+",value"+value);
}
方法2（增强for循环）
for (Integer key : m.keySet())
{ System.out.println("key"+key+",value"+m.get(key)); }
方法3（映射Map.Entry）
Map<Integer,String> m=new HashMap();
m.put(1,"沈阳"); m.put(2,"大连"); m.put(3,"营口");
Set<Map.Entry<Integer,String>> me=m.entrySet();//视图
for (Map.Entry<Integer, String> mm : me) {
mm.getKey();
mm.getValue();
System.out.println( mm.getKey()+ mm.getValue());
}
获取集合中所有值的集合
Collection<String> values = m.values().var;
获取键值对的个数
m.size();
LinkedHashMap(底层是链表)
特点：怎么存就怎么取
public static void main(String[] args) {LinkedHashMap<Integer,String> lhm=new LinkedHashMap<>();lhm.put(1,"营口");lhm.put(1,"营口");lhm.put(3,"大连");lhm.put(3,"大连");lhm.put(2,"沈阳");lhm.put(2,"沈阳");Set<Map.Entry<Integer, String>> ent = lhm.entrySet();Iterator<Map.Entry<Integer, String>> i=ent.iterator();while(i.hasNext()){Map.Entry<Integer, String>me= i.next();Integer key = me.getKey();String value = me.getValue();System.out.println(key+value);}}TreeMap
public class Student {private String name;private int age;public Student() {}public Student(String name, int age) {this.name = name;this.age = age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}@Overridepublic String toString() {return "Student{" +"name='" + name + '\'' +", age=" + age +'}';}@Overridepublic boolean equals(Object o) {if (this == o) return true;if (o == null || getClass() != o.getClass()) return false;Student student = (Student) o;return age == student.age && Objects.equals(name, student.name);}@Overridepublic int hashCode() {return Objects.hash(name, age);}
}public class ComStu implements Comparator<Student> {@Overridepublic int compare(Student o1, Student o2) {return o1.getAge()-o2.getAge();}
} public static void main(String[] args) {TreeMap<Student,String> tm=new TreeMap(new ComStu());tm.put(new Student("张三",23),"沈阳");tm.put(new Student("张三",23),"沈阳");tm.put(new Student("李四",22),"大连");tm.put(new Student("李四",22),"大连");tm.put(new Student("王五",25),"营口");tm.put(new Student("王五",25),"营口");System.out.println(tm);}