2021-2-1
1.Map collection
2. Common methods of Map collection
3. Get function of Map collection
package com.wc.MapDemo;
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
/**
* @author wc
* @Date: 2021/02/01/16:57
*/
public class MapDemo1 {
public static void main(String[] args) {
Map<String, String> map=new HashMap<>();
map.put("张三","李四");
map.put("王五","赵柳");
map.put("skr","阿伟");
//根据对应的键获取值，不存在就返回null
System.out.println(map.get("张三"));
//获取所有键集合
Set<String> keySet = map.keySet();
for (String s : keySet) {
System.out.println(s);
}
//获取所有值集合
Collection<String> values = map.values();
for (String value : values) {
System.out.println(value);
}
System.out.println(map);
}
}
4. The traversal of the Map collection (case enumeration)
package com.wc.MapDemo;
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
/**
* @author wc
* @Date: 2021/02/01/16:57
*/
public class MapDemo1 {
public static void main(String[] args) {
Map<String, String> map=new HashMap<>();
map.put("张三","李四");
map.put("王五","赵柳");
map.put("skr","阿伟");
Set<String> keySet = map.keySet();
for (String key : keySet) {
String value = map.get(key);
System.out.println(key+","+value);
}
}
}
Case study
package com.wc.MapDemo;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
/**
* @author wc
* @Date: 2021/02/01/16:57
*/
public class MapDemo2 {
public static void main(String[] args) {
HashMap<String, Student> hashMap=new HashMap<>();
Student student=new Student("思琪",250);
Student student1=new Student("日辰",18);
Student student2=new Student("橙子",20);
hashMap.put("zz",student);
hashMap.put("帅气",student1);
hashMap.put("勇敢",student2);
Set<String> set = hashMap.keySet();
for (String keySet : set) {
Student value = hashMap.get(keySet);
System.out.println(keySet+","+value.getName()+","+value.getAge());
}
System.out.println("-----------------------------");
Set<Map.Entry<String, Student>> entrySet = hashMap.entrySet();
for (Map.Entry<String, Student> entry : entrySet) {
String key = entry.getKey();
Student value = entry.getValue();
System.out.println(key+","+value.getName()+","+value.getAge());
}
}
}
Case study
package com.wc.MapDemo;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
/**
* @author wc
* @Date: 2021/02/01/16:57
*/
public class MapDemo3 {
public static void main(String[] args) {
HashMap<String, String> hm1 = new HashMap<>();
hm1.put("张三", "李四");
hm1.put("王五", "赵柳");
hm1.put("skr", "阿伟");
ArrayList<HashMap<String, String>> arrayList = new ArrayList<>();
arrayList.add(hm1);
for (HashMap<String, String> hashMap : arrayList) {
Set<String> set = hashMap.keySet();
for (String keySet : set) {
String value = hashMap.get(keySet);
System.out.println(keySet+","+value);
}
}
}
}
Case study
package com.wc.MapDemo;
import java.util.HashMap;
import java.util.Scanner;
import java.util.Set;
/**
* @author wc
* @Date: 2021/02/02/9:19
*/
public class StringBuilderDemo {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
System.out.println("请输入字符串：");
String line = scanner.nextLine();
HashMap<Character, Integer> hm = new HashMap<>();
//遍历数组，得到每一个字符，并根据每个字符找到对应的value
for (int i = 0; i < line.length(); i++) {
char key = line.charAt(i);
Integer value = hm.get(key);
//如果返回null，说明该字符串不存在，1作为存储值
if (value == null) {
hm.put(key, 1);
//如果不为null，说明该字符串存在，value作为存储值，每有一个加1
} else {
value++;
hm.put(key, value);
}
}
//遍历HashMap并拼接
StringBuilder sb = new StringBuilder();
for (Character key : hm.keySet()) {
Integer value = hm.get(key);
sb.append(key).append("(").append(value).append(")");
}
//StringBuilder类是一个可变的字符序列。
String result = sb.toString();
System.out.println(result);
}
}
5.Collections
