TestMap

public class TestMap {
public static void main(String[] args) { Map map=new HashMap();
//在此映射中关联指定值与指定键。
map.put("key1", "value1");
map.put("key2", "value2");
map.put("key3", "value2");
map.put("key4", "value4");
map.put("key5", "value5");
map.put("key6",null);
map.put(null,null);System.out.println(map.size());
System.out.println(map.get(null));for(int i=1;i<map.size();i++){
System.out.println(map.get(i));
}Iterator it=map.keySet().iterator();
while(it.hasNext()){
Object key=it.next();
Object value=map.get(key);
System.out.println("key="+key+"-->"+"value="+value);
}Iterator itt=map.entrySet().iterator();
while(itt.hasNext()){
System.out.println(itt.next());
}
Iterator it2=map.entrySet().iterator();
while(it2.hasNext()){
Map.Entry key=(Map.Entry) it2.next();
System.out.println("key="+key.getKey()+"-->"+"value="+key.getValue());
} }
}

package com.example.demo;import org.testng.annotations.Test;import java.util.HashMap;public class TestHashMap { @Test(description = "Hashmap的存值")
public void TestPut() {
///*Integer*/map.put("1", 1);
// 向map中添加值(返回这个key以前的值,如果没有返回null)
HashMap<String, Integer> map=new HashMap<>();
Integer flag_a = map.put("1", 1);
Integer flag_b = map.put("1", 2);
System.out.println(flag_a);//null
System.out.println(flag_b);//1
} @Test(description = "Hashmap的取值")
public void TestGet() {
HashMap<String, Integer> map=new HashMap<>();
map.put("key", 1);
/*Value的类型*///得到map中key相对应的value的值
System.out.println(map.get("1"));//null
System.out.println(map.get("key"));//1
} @Test(description = "Hashmap的判断为空")
public void TestCheckEmpty() {
HashMap<String, Integer> map=new HashMap<>();
/*boolean*///判断map是否为空
System.out.println(map.isEmpty());//true
map.put("key", 1);
System.out.println(map.isEmpty());//false
} @Test(description = "Hashmap判断是否含有key")
public void TestCheckKey() {
HashMap<String, Integer> map=new HashMap<>();
/*boolean*///判断map中是否存在这个key
System.out.println(map.containsKey("key"));//false
map.put("key", 1);
System.out.println(map.containsKey("key"));//true
} @Test(description = "Hashmap判断是否含有value")
public void TestCheckValue() {
HashMap<String, Integer> map=new HashMap<>();
/*boolean*///判断map中是否存在这个value
boolean flag_a = map.containsValue(1);
System.out.println(flag_a);//false
map.put("key", 1);
boolean flag_b = map.containsValue(1);
System.out.println(flag_b);//true
} @Test(description = "Hashmap删除这个key值下的value")
public void TestDelValue() {
HashMap<String, Integer> map=new HashMap<>();
/*Integer*///删除key值下的value
Integer result =map.remove("key1");
System.out.println(result);//null
map.put("key1", 2);
System.out.println(map.remove("key1"));//2(删除的值)
} @Test(description = "Hashmap显示所有的value值")
public void TestGetAllValue() {
HashMap<String, Integer> map=new HashMap<>();
/*Collection<Integer>*///显示所有的value值
System.out.println(map.values());//[]
map.put("key1", 1);
map.put("key2", 2);
System.out.println(map.values());//[1, 2]
} @Test(description = "Hashmap的元素个数")
public void TestGetNumber() {
HashMap<String, Integer> map=new HashMap<>();
/*int*///显示map里的值得数量
System.out.println(map.size());//0
map.put("key1", 1);
System.out.println(map.size());//1
map.put("key2", 2);
System.out.println(map.size());//2
} @Test(description = "Hashmap删除这个key值下的value")
public void TestDelKeysValue() {
HashMap<String, Integer> map=new HashMap<>();
/*SET<String>*///显示map所有的key
System.out.println(map.keySet());//[]
map.put("key1", 1);
System.out.println(map.keySet());//[key1]
map.put("key2", 2);
System.out.println(map.keySet());//[key1, key2]
} @Test(description = "Hashmap删除这个key值下的value")
public void TestShow() {
HashMap<String, Integer> map=new HashMap<>();
/*SET<map<String,Integer>>*///显示所有的key和value
System.out.println(map.entrySet());//[]
map.put("key1", 1);
System.out.println(map.entrySet());//[key1=1]
map.put("key2", 2);
System.out.println(map.entrySet());//[key1=1, key2=2]
} @Test(description = "Hashmap添加另一个同一类型的map下的所有内容")
public void TestContainShow() {
HashMap<String, Integer> map=new HashMap<>();
HashMap<String, Integer> map1=new HashMap<>();
/*void*///将同一类型的map添加到另一个map中
map1.put("key11", 1);
map.put("key", 2);
System.out.println(map);//{key=2}
map.putAll(map1);
System.out.println(map);//{key11=1, key11=2}
} @Test(description = "Hashmap删除这个key和value")
public void TestDelKeyandValue() {
HashMap<String, Integer> map=new HashMap<>();
/*boolean*///删除这个键值对
map.put("TestDelKeyandValue1", 1);
map.put("TestDelKeyandValue2", 2);
System.out.println(map);//{DEMO1=1, DEMO2=2}
System.out.println(map.remove("TestDelKeyandValue2", 1));//false
System.out.println(map.remove("TestDelKeyandValue2", 2));//true
System.out.println(map);//{TestDelKeyandValue1=1}
} @Test(description = "Hashmap替换这个key的value：(java8)")
public void TestReplaceValue() {
HashMap<String, Integer> map=new HashMap<>();
/*value*///判断map中是否存在这个key
map.put("key1", 1);
map.put("key2", 2);
map.put("key3", 3);
System.out.println(map);//{key1=1, key2=2}
System.out.println(map.replace("key2", 22));//2
System.out.println(map.replace("key3", 33));//3
System.out.println(map);//{key1=1, key2=22}
} @Test(description = "清空这个hashmap")
public void TestClear() {
HashMap<String, Integer> map=new HashMap<>();
/*void*///清空map
map.put("key1", 1);
map.put("key2", 2);
System.out.println(map);//{key1=1, key2=2}
map.clear();//2
System.out.println(map);//{}
} @Test(description = "Hashmap的克隆")
public void TestClone() {
HashMap<String, Integer> map=new HashMap<>();
/*object*///克隆这个map
map.put("DEMO1", 1);
map.put("DEMO2", 2);
System.out.println(map.clone());//{DEMO1=1, DEMO2=2}
Object clone = map.clone();
System.out.println(clone);//{DEMO1=1, DEMO2=2}
}}

public static void main(String[] args) {
HashMap<String, Integer> map=new HashMap<>();
/*boolean*///判断map中是否存在这个key
map.put("DEMO1", 1);
map.put("DEMO2", 2);
System.out.println(map);//{DEMO1=1, DEMO2=2}
System.out.println(map.putIfAbsent("DEMO1", 12222));//1
System.out.println(map.putIfAbsent("DEMO3", 12222));//null
System.out.println(map);//{DEMO1=1, DEMO2=2}
}

public static void main(String[] args) {
HashMap<String, Integer> map=new HashMap<>();
/*boolean*///当这个value为null时为1，否则为3
map.put("DEMO1", 1);
map.put("DEMO2", 2);
System.out.println(map);//{DEMO1=1, DEMO2=2}
map.compute("DEMO2", (k,v)->v==null?1:3);
System.out.println(map);//{DEMO1=1, DEMO2=3}
}

package com.example.demo;import java.util.*;public class TestSort { public static Map<String, List<Integer>> getElementPositions(List<String> list) {
Map<String, List<Integer>> positionsMap = new HashMap<>(); for (int i = 0; i < list.size(); i++) {
String str = list.get(i);
List<Integer> positions = positionsMap.get(str); if (positions == null) { // 如果 positionsMap 还不存在 str 这个键及其对应的 List<Integer>
positions = new ArrayList<>(1);
positionsMap.put(str, positions); // 将 str 及其对应的 positions 放入 positionsMap
} positions.add(i); // 将索引加入 str 相关联的 List<Integer> 中
} return positionsMap;
} public static void main(String[] args) throws Exception {
List<String> list = Arrays.asList("a", "b", "b", "c", "c", "c", "d", "d", "d", "f", "f", "g"); System.out.println("使用 Java8 之前的 API：");
Map<String, List<Integer>> elementPositions = getElementPositions(list);
System.out.println(elementPositions);
}
}

package com.example.demo;import java.util.*;public class TeatSort2 { public static Map<String, List<Integer>> getElementPositions(List<String> list) {
Map<String, List<Integer>> positionsMap = new HashMap<>(); for (int i = 0; i < list.size(); i++) {
String str = list.get(i);
positionsMap.putIfAbsent(str, new ArrayList<>(1)); // 如果 positionsMap 不存在键 str 或者 str 关联的 List<Integer> 为 null，那么就会进行 put；否则不执行 put
positionsMap.get(str).add(i);
} return positionsMap;
} public static void main(String[] args) throws Exception {
List<String> list = Arrays.asList("a", "b", "b", "c", "c", "c", "d", "d", "d", "f", "f", "g"); System.out.println("使用 putIfAbsent：");
Map<String, List<Integer>> elementPositions = getElementPositions(list);
System.out.println(elementPositions);
}
}