Java 9 @SafeVarargs Annotation

It is an annotation which applies on a method or constructor that takes varargs parameters. It is used to ensure that the method does not perform unsafe operations on its varargs parameters.

It was included in Java7 and can only be applied on

Final methods
Static methods
Constructors

From Java 9, it can also be used with private instance methods.

Note: The @SafeVarargs annotation can be applied only to methods that cannot be overridden. Applying to the other methods will throw a compile time error.

Let's see some examples, in first example, we are not using @SafeVarargs annotation and compiling code . See, what happens?

Java 9 @SafeVarargs Annotation Example

import java.util.ArrayList;
import java.util.List;
public class SafeVar{
	private void display(List<String>... products) { // Not using @SaveVarargs
		for (List<String> product : products) {
			System.out.println(product);
		}
	}
	public static void main(String[] args) {
		SafeVar p = new SafeVar();
		List<String> list = new ArrayList<String>();
		list.add("Laptop");
		list.add("Tablet");
		p.display(list);
	}	
}

It produces warning messages at compile time, but compiles without errors.

Output:

At compile time:
Note: SafeVar.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
At runtime:
[Laptop, Tablet]

This is a compiler generated warning regarding unsafe varargs type.

To avoid it, we should use @SaveVarargs notation to the method, as we did in the following example.

Java 9 @SafeVarargs Annotation Example

import java.util.ArrayList;
import java.util.List;
public class SafeVar{
	// Applying @SaveVarargs annotation
	@SafeVarargs
	private void display(List<String>... products) { // Not using @SaveVarargs
		for (List<String> product : products) {
			System.out.println(product);
		}
	}
	public static void main(String[] args) {
		SafeVar p = new SafeVar();
		List<String> list = new ArrayList<String>();
		list.add("Laptop");
		list.add("Tablet");
		p.display(list);
	}	
}

Now, compiler does not produce warning message, code compiles and runs successfully.

Output:

[Laptop, Tablet]

Note: To apply @SaveVarargs annotation on private instance methods, compile code using Java 9 or higher versions only.

What happens? If we compile the following code by using older versions of Java.

Java @SafeVarargs Annotation Example

import java.util.ArrayList;
import java.util.List;
public class SafeVar{	
	@SafeVarargs
	private void display(List<String>... products) {
		for (List<String> product : products) {
			System.out.println(product);
		}
	}
	public static void main(String[] args) {
		SafeVar v = new SafeVar();
		List<String> list = new ArrayList<String>();
		list.add("Laptop");
		list.add("Tablet");
		v.display(list);
	}	
}

Output:

SafeVar.java:6: error: Invalid SafeVarargs annotation. Instance method display(List<String>...) is not final.
	private void display(List<String>... products) {
	             ^
Note: SafeVar.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
1 error

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