Table of contents
Create a new HashMap() object
This is the first step in using HashMap. The creation statement is as follows:
HashMap<Integer, Integer> map = new HashMap<>();
Note that the Key and Value of HashMap cannot be basic data types, but must be reference data types or wrapper data types. The reason for this is related to the underlying implementation principle of HashMap. Simply put, the basic data types cannot call the hashcode() and equals() methods.
Traverse HashMap()
for(Integer key: map.keySet()){
}
The keySet() method can traverse the HashMap object, and can perform corresponding operations in the for statement block.
Add key-value pairs to HashMap object
Use the put(key, value) method to add key-value pairs to the HashMap object. Note that the data type must be consistent with the data type defined.
Find a value in a HashMap object
The get(key) method can return the corresponding value. When I was working on a force button question before, I thought I had to traverse to find out whether the key already existed. Later, I found that it can be solved by directly using the get(key) method and judging whether the return value is empty. This is also the characteristic of HashMap One, can reduce running time consumption.
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