Java HashSet.clone() – Examples

In this tutorial, we will learn about the Java HashSet.clone() method, and learn how to use this method to make a shallow copy of this HashSet, with the help of examples.

clone()

HashSet.clone() returns a shallow copy of this HashSet instance: the elements themselves are not cloned, but a new HashSet instance is.

Syntax

The syntax of clone() function is

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HashSet.clone()

Returns

The method returns Object. You may need to type cast it to HashSet<T>.

Example 1 – clone()

In this example, we will create a HashSet with String values, and clone this HashSet using clone() method.

Java Program

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import java.util.Arrays;
import java.util.HashSet;

public class Example {  
	public static void main(String[] args) {  
		HashSet<String> hashSet = new HashSet<String>(Arrays.asList("a", "b", "c"));
		System.out.println("Original HashSet : " + hashSet);
		
		HashSet<String> hashSetCloned = (HashSet<String>) hashSet.clone();
		System.out.println("Cloned HashSet  : " + hashSetCloned);
    }  
}

Output

Original HashSet : [a, b, c]
Cloned HashSet  : [a, b, c]

Any modifications done to the cloned HashSet will not reflect in the original HashSet, since these original and cloned HashSet s are two different instances.

In the following program, we will add elements to the cloned HashSet, and this should not affect the original hashSet.

Java Program

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import java.util.Arrays;
import java.util.HashSet;

public class Example {  
	public static void main(String[] args) {  
		HashSet<String> hashSet = new HashSet<String>(Arrays.asList("a", "b", "c"));
		System.out.println("Original HashSet : " + hashSet);
		
		HashSet<String> hashSetCloned = (HashSet<String>) hashSet.clone();
		hashSetCloned.add("d");
		hashSetCloned.add("e");
		System.out.println("Cloned HashSet   : " + hashSetCloned);
    }  
}

Output

Original HashSet : [a, b, c]
Cloned HashSet   : [a, b, c, d, e]

Example 2 – clone() – HashSet with User-defined Objects

In this example, we will define and initialize a HashSet with objects of type Car. We will clone this HashSet, using clone() method. And following this, we will provide another program to understand the “shallow copy”.

Java Program

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import java.util.HashSet;

public class Example {  
	public static void main(String[] args) {
		Car car1 = new Car(1, "Tesla");
		Car car2 = new Car(2, "BMW");
		Car car3 = new Car(3, "Toyota");
		
		HashSet<Car> hashSet = new HashSet<Car>();
		hashSet.add(car1);
		hashSet.add(car2);
		hashSet.add(car3);
		System.out.println("Original HashSet : " + hashSet);
		
		HashSet<Car> hashSetCloned = (HashSet<Car>) hashSet.clone();
		System.out.println("Cloned   HashSet : " + hashSetCloned);
    }  
}

class Car {
	int id;
	String name;
	public Car(int id, String name) {
		this.id = id;
		this.name = name;
	}
	
	@Override
	public String toString() {
		return id + "-" + name;
	}
}

Output

Original HashSet : [1-Tesla, 3-Toyota, 2-BMW]
Cloned   HashSet : [1-Tesla, 3-Toyota, 2-BMW]

If any of the Car objects is modified, the changes effect both original HashSet and cloned HashSet.

In the following program, we will modify the name of Car in car3, and let us observe how this effects original and cloned HashSet.

Java Program

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import java.util.HashSet;

public class Example {  
	public static void main(String[] args) {
		Car car1 = new Car(1, "Tesla");
		Car car2 = new Car(2, "BMW");
		Car car3 = new Car(3, "Toyota");
		
		HashSet<Car> hashSet = new HashSet<Car>();
		hashSet.add(car1);
		hashSet.add(car2);
		hashSet.add(car3);
		
		HashSet<Car> hashSetCloned = (HashSet<Car>) hashSet.clone();
		
		car3.name = "Audi";
		
		System.out.println("Original HashSet : " + hashSet);
		System.out.println("Cloned   HashSet : " + hashSetCloned);
    }  
}

class Car {
	int id;
	String name;
	public Car(int id, String name) {
		this.id = id;
		this.name = name;
	}
	
	@Override
	public String toString() {
		return id + "-" + name;
	}
}

Output

Original HashSet : [1-Tesla, 3-Audi, 2-BMW]
Cloned   HashSet : [1-Tesla, 3-Audi, 2-BMW]

Hence, the shallow copy by clone() method.

Conclusion

In this Java Tutorial, we have learnt the syntax of Java HashSet.clone() method, and also learnt how to use this method with the help of examples.