YouTipJava Hashset
[ Java Collection Framework](#)
HashSet is implemented based on HashMap and is a collection that does not allow duplicate elements.
HashSet allows null values.
HashSet is unordered, meaning it does not record the insertion order.
HashSet is not thread-safe. If multiple threads try to modify a HashSet at the same time, the final result is uncertain. You must explicitly synchronize concurrent access to HashSet when accessing it from multiple threads.
HashSet implements the Set interface.
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The elements in HashSet are actually objects. Some common primitive types can use their wrapper classes.
The wrapper class table corresponding to primitive types is as follows:
| Primitive Type | Reference Type |
| --- | --- |
| boolean | Boolean |
| byte | Byte |
| short | Short |
| int | Integer |
| long | Long |
| float | Float |
| double | Double |
| char | Character |
The HashSet class is in the java.util package and needs to be imported before use. The syntax is as follows:
import java.util.HashSet; // Import HashSet class
In the following example, we create a HashSet object sites to store string elements:
HashSet sites = new HashSet();
### Add Elements
The HashSet class provides many useful methods. You can use the add() method to add elements:
## Example
// Import HashSet class
import java.util.HashSet;
public class TutorialTest {
public static void main(String[] args){
HashSet sites =new HashSet();
sites.add("Google");
sites.add("Tutorial");
sites.add("Taobao");
sites.add("Zhihu");
sites.add("Tutorial");// Duplicate elements will not be added
System.out.println(sites);
}
}
Execute the above code, and the output is as follows:
[Google, Tutorial, Zhihu, Taobao]
In the above example, Tutorial was added twice, but it will only appear once in the collection, because each element in the collection must be unique.
### Check if Element Exists
We can use the contains() method to determine whether an element exists in the collection:
## Example
// Import HashSet class
import java.util.HashSet;
public class TutorialTest {
public static void main(String[] args){
HashSet sites =new HashSet();
sites.add("Google");
sites.add("Tutorial");
sites.add("Taobao");
sites.add("Zhihu");
sites.add("Tutorial");// Duplicate elements will not be added
System.out.println(sites.contains("Taobao"));
}
}
Execute the above code, and the output is as follows:
true
### Delete Elements
We can use the remove() method to delete elements from the collection:
## Example
// Import HashSet class
import java.util.HashSet;
public class TutorialTest {
public static void main(String[] args){
HashSet sites =new HashSet();
sites.add("Google");
sites.add("Tutorial");
sites.add("Taobao");
sites.add("Zhihu");
sites.add("Tutorial");// Duplicate elements will not be added
sites.remove("Taobao");// Delete element, returns true if successful, false otherwise
System.out.println(sites);
}
}
Execute the above code, and the output is as follows:
[Google, Tutorial, Zhihu]
To delete all elements in the collection, you can use the clear method:
## Example
// Import HashSet class
import java.util.HashSet;
public