Map集合的获取功能
V get(Object key);//根据键获取值
Set<K> keySet();//获取所有键的集合
Collection<K> values();//获取所有值的集合
Set<Map.Entry<K,V>>entrySet();//获取所有键值对对象的集合
相关应用：
//获取值
System.out.println(map.get("1"));//如果存在，返回相应键值 反之，返回null
//获取所有键的集合
import java.util.*;
public class Main {
public static void main(String[] args) {
Map<String,String> map=new HashMap<String,String>();
map.put("1","lili");
map.put("2","zhangliang");
map.put("3","wangwu");
Set<String> keySet=map.keySet();
for(String ss:keySet){
System.out.println(ss);
}
}
}
//获取所有值的集合
import java.util.*;
public class Main {
public static void main(String[] args) {
Map<String,String> map=new HashMap<String,String>();
map.put("1","lili");
map.put("2","zhangliang");
map.put("3","wangwu");
Collection<String> cc=map.values();
for(String ss:cc){
System.out.println(ss);
}
}
}
Map集合的遍历方式
//方式一
import java.util.*;
public class Main {
public static void main(String[] args){
Map<String> mm=new HashMap<String>();
mm.put("1","zhangliang");
mm.put("2","lili");
mm.put("3","wangwu");
Set<String> ss=map.keySet();
for(String s:ss){
String value=mm.get(s);
System.out.println(s+","+value);
}
}
}
//方式二
import java.util.*;
public class Main {
public static void main(String[] args) {
Map<String,String> map=new HashMap<String,String>();
map.put("1","lili");
map.put("2","zhangliang");
map.put("3","wangwu");
Set<Map.Entry<String,String>> ss=map.entrySet();
for(Map.Entry<String,String> mm:ss){
String key=mm.getKey();
String value=mm.getValue();
System.out.println(key+","+value);
}
}
}
案例：HashMap集合存储学生对象并遍历
//Main.java
import java.util.*;
public class Main {
public static void main(String[] args) {
Map<String,Student> map=new HashMap<String,Student>();
Student s1=new Student("zhangliang",19);
Student s2=new Student("wangwu",20);
Student s3=new Student("lili",18);
map.put("12",s1);
map.put("23",s2);
map.put("34",s3);
//方式一
Set<String> sss=map.keySet();
for(String sa:sss){
Student value=map.get(sa);
System.out.println(sa+","+value.getName()+","+value.getAge());
}
//方式二
Set<Map.Entry<String,Student>> ss=map.entrySet();
for(Map.Entry<String,Student> me:ss){
String key=me.getKey();
Student value=me.getValue();
System.out.println(key+","+value.getName()+","+value.getAge());
}
}
}
//Student.java已经省略
HashMap集合存储学生对象并遍历
//Main.java
import java.util.*;
public class Main {
public static void main(String[] args) {
Map<Student, String> map=new HashMap<Student,String>();
Student s1=new Student("wangwu",19);
Student s2=new Student("lili",18);
Student s3=new Student("lisi",20);
map.put(s1,"shi");
map.put(s2,"jia");
map.put(s3,"zhuang");
Set<Student> set=map.keySet();
//方式一
for(Student s:set){
String value=map.get(s);
System.out.println(s.getName()+","+s.getAge()+","+value);
}
System.out.println("----------------------");
//方式二
Set<Map.Entry<Student,String>> ss=map.entrySet();
for(Map.Entry<Student,String> me:ss){
Student key=me.getKey();
String value=me.getValue();
System.out.println(key.getName()+","+key.getAge()+","+value);
}
}
}
//Student.java已省略
案例：ArrayList集合存储HashMap元素并遍历
import java.util.*;
public class Main {
public static void main(String[] args) {
ArrayList<HashMap<String,String>> arr=new ArrayList<HashMap<String,String>>();
HashMap<String,String> hh=new HashMap<String,String>();
hh.put("1","3");
hh.put("2","4");
hh.put("5","7");
arr.add(hh);
for(HashMap<String,String> me:arr){
Set<String> keyset=hh.keySet();
for(String ss:keyset){
String s1=hh.get(ss);
System.out.println(ss+","+s1);
}
}
}
}
案例：HashMap集合存储ArrayList元素并遍历
import java.util.*;
public class Main {
public static void main(String[] args) {
HashMap<String,ArrayList<String>> hm=new HashMap<String,ArrayList<String>>();
ArrayList<String> arr=new ArrayList<String>();
arr.add("shi");
hm.put("石",arr);
Set<String> keyset=hm.keySet();
for(String s:keyset){
ArrayList<String> aa=hm.get(s);
for(String mm:aa){
System.out.println(mm);
}
}
}
}
案例：统计字符串中每个字符出现的次数
import java.util.*;
public class Main {
public static void main(String[] args) {
Scanner sc=new Scanner(System.in);
//输入字符串
String s=sc.nextLine();
HashMap<Character,Integer> hm=new HashMap<Character,Integer>();
for(int i=0;i<s.length();i++){
char key= s.charAt(i);//键
Integer ii=hm.get(key);
if(ii==null){
hm.put(key,1);
}else{
ii++;
hm.put(key,ii);
}
}
//分割
StringBuilder sb=new StringBuilder();
Set<Character> keyset=hm.keySet();
for(Character key:keyset){
Integer value=hm.get(key);
//拼接
sb.append(key).append("(").append(value).append(")");
}
//转换为字符串
String result=sb.toString();
//输出
System.out.println(result);
}
}
Collections
针对集合操作的工具类
public static <T extends Comparable<?super T>> void sort(List<T> list);//升序排列
public static void reverse(List<?> list);//反转指定列表中元素的顺序
public static void shuffle(List<?> list);//使用默认的随机源随机排列指定的列表
相关应用：
//升序排列
Collections.sort(ll);
//反转既定序列
Collections.reverse(ll);
//随机排序
Coolections.shuffle(ll);
案例：ArrayList存储学生对象并排序
import java.util.*;
public class Main {
public static void main(String[] args) {
ArrayList<Student> arr=new ArrayList<Student>();
Student s1=new Student("lili",19);
Student s2=new Student("wangwu",19);
Student s3=new Student("zhangliang",18);
arr.add(s1);
arr.add(s2);
arr.add(s3);
Collections.sort(arr, new Comparator<Student>() {
@Override
public int compare(Student s1, Student s2) {
int num=s1.getAge()-s2.getAge();
int num2=num==0?s1.getName().compareTo(s2.getName()):num;
return num2;
}
});
for(Student s:arr){
String name=s.getName();
int age=s.getAge();
System.out.println(name+","+age);
}
}
}
案例：模拟斗地主
//基础版
import java.util.*;
public class Main {
public static void main(String[] args) {
ArrayList<String> arr=new ArrayList<String>();
//定义花色数组
String[] colors={"♥","♠","♦","♣"};
//定义数字数组
String[] numbers={"A","2","3","4","5","6","7","8","9","10","J","Q","K"};
for(String color:colors){
for(String number:numbers){
arr.add(color+number);
}
}
arr.add("小王");
arr.add("大王");
//洗牌
Collections.shuffle(arr);
//发牌
ArrayList<String> aarr=new ArrayList<String>();
ArrayList<String> barr=new ArrayList<String>();
ArrayList<String> carr=new ArrayList<String>();
ArrayList<String> darr=new ArrayList<String>();
for(int i=0;i<arr.size();i++){
String poker=arr.get(i);
if(i>=arr.size()-3){
darr.add(poker);
}else if(i%3==0){
aarr.add(poker);
}else if(i%3==1){
barr.add(poker);
}else{
carr.add(poker);
}
}
looker("a",aarr);
looker("b",barr);
looker("c",carr);
looker("d",darr);
}
public static void looker(String name,ArrayList<String> arr){
System.out.println(name+"的牌是：");
for(String poker:arr){
System.out.println(poker+" ");
}
}
}
//升级版
import java.util.*;
public class Main {
public static void main(String[] args) {
HashMap<Integer,String> hm=new HashMap<Integer,String>();
ArrayList<Integer> arr=new ArrayList<Integer>();
//定义花色数组
String[] colors={"♥","♠","♦","♣"};
//定义数字数组
String[] numbers={"A","2","3","4","5","6","7","8","9","10","J","Q","K"};
int index=0;
for(String number:numbers){
for(String color:colors){
hm.put(index,color+number);
arr.add(index);
index++;
}
}
hm.put(index,"小王");
arr.add(index);
index++;
hm.put(index,"大王");
arr.add(index);
//洗牌
Collections.sort(arr);
//发牌
TreeSet<Integer> aarr=new TreeSet<Integer>();
TreeSet<Integer> barr=new TreeSet<Integer>();
TreeSet<Integer> carr=new TreeSet<Integer>();
TreeSet<Integer> darr=new TreeSet<Integer>();
for(int i=0;i<arr.size();i++){
if(i>=arr.size()-3){
darr.add(arr.get(i));
}else if(i%3==0){
aarr.add(arr.get(i));
}else if(i%3==1){
barr.add(arr.get(i));
}else{
carr.add(arr.get(i));
}
}
looker("a",aarr,hm);
looker("b",barr,hm);
looker("c",carr,hm);
looker("d",darr,hm);
}
public static void looker(String name,TreeSet<Integer> ts,HashMap<Integer,String> hm){
System.out.println(name+"的牌是： ");
for(Integer key:ts){
String poker=hm.get(key);
System.out.println(poker+" ");
}
}
}
File类
是文件和目录路径名的抽象表示
File(String pathname);//将字符串转变为抽象路径名
File(String parent,String child);//从父路径名字符串和子路径名字符串创建新的File实例
File(File parent,String child);//从父抽象路径名和子路径名字符串创建新的File实例
相关应用：
//仅仅能够显示，并不要求该文件存在
File f1=new File("E:\\itcast\\java.txt");
System.out.println(f1);
//显示内容与上述一致
File f1=new File("E:\\itcast","java.txt");
System.out.println(f1);
//显示内容与上述一致
File f1=new File("E:\\itcast");
File f2=new File(f1,"java.txt");
System.out.println(f2);
