The use of HashMap in java projects and small cases

HashMap is one of the most commonly used data structures in Java. It is implemented based on a hash table and has good performance in insertion and lookup.

How to use HashMap:

1. Import the HashMap class:import java.util.HashMap;
2. Create a HashMap object:HashMap<KeyType, ValueType> map = new HashMap<>();
3. Insert key-value pairs into HashMap:map.put(key, value);
4. Get value by key:ValueType value = map.get(key);
5. Determine whether the HashMap contains the specified key:boolean containsKey = map.containsKey(key);
6. Delete the specified key-value pair:map.remove(key);
7. Get the key set in HashMap:Set<KeyType> keySet = map.keySet();
8. Get the value collection in HashMap:Collection<ValueType> values = map.values();
9. Traverse the key-value pairs in the HashMap:

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   for (KeyType key : map.keySet()) {
       ValueType value = map.get(key);
       // 进行操作
   }
   

10. Get the size of the HashMap (the number of key-value pairs):int size = map.size();
11. Clear all key-value pairs in HashMap:map.clear();

HashMap case example:

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import java.util.HashMap;

public class HashMapExample {
    public static void main(String[] args) {
        HashMap<String, Integer> grades = new HashMap<>();

        // 添加学生的成绩
        grades.put("Alice", 85);
        grades.put("Bob", 90);
        grades.put("Charlie", 75);

        // 获取指定学生的成绩
        int aliceGrade = grades.get("Alice");
        System.out.println("Alice's grade: " + aliceGrade);

        // 判断是否包含指定学生
        boolean containsBob = grades.containsKey("Bob");
        System.out.println("Contains Bob: " + containsBob);

        // 删除指定学生的成绩
        grades.remove("Charlie");
        System.out.println("Size of grades: " + grades.size());

        // 遍历所有学生的成绩
        for (String student : grades.keySet()) {
            int grade = grades.get(student);
            System.out.println(student + "'s grade: " + grade);
        }

        // 清空所有学生的成绩
        grades.clear();
        System.out.println("Size of grades after clear: " + grades.size());
    }
}

This example demonstrates how to use HashMap to store students' grades, and perform operations such as obtaining, deleting, determining whether it is included, traversing, and clearing.