Java 9 – @SafeVarargs Annotation

In this post, we will see about @SafeVarargs Annotation in Java 9.
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The @SafeVarargs is an annotation that is used to perform safe operations. When a method takes variable arguments, then it may cause to unsafe operation, so the @SafeVarargs annotation tells to the compiler to perform safe operations. For example, if we don’t use the annotation the compiler reports warning: Type safety: Potential heap pollution via varargs parameter.

We can use this annotation to final and static and private (from Java 9) methods only of a class. See the example below.

@SafeVarargs annotation

Let’s see few examples with final and static methods.

@SafeVarargs with Final Method

In this example, we have a final methods that takes var-args parameters so we used @SafeVarargs annotation. See the example.

import java.util.ArrayList;
import java.util.List;

class DemoPrint{
    @SafeVarargs
    final void display(List... lists) {
        for (List list : lists) {  
            System.out.println(list);  
        }  
    }
}
class Main {
    public static void main(String[] args){
        DemoPrint dp = new DemoPrint();  
        List list = new ArrayList();  
        list.add("Python");  
        list.add("Java");  
        dp.display(list);
    }
}

Output

@SafeVarargs with Static Method

Java allows using @SafeVarargs annotation with static methods as well. See the example with below.

import java.util.ArrayList;
import java.util.List;

class DemoPrint{
    @SafeVarargs
    static void display(List... lists) {
        for (List list : lists) {  
            System.out.println(list);  
        }  
    }
}
class Main {
    public static void main(String[] args){
        List list = new ArrayList();  
        list.add("Python");  
        list.add("Java");  
        DemoPrint.display(list);
    }
}

Output

Java 9 @SafeVarargs Improvement

In Java 9, It is allowed to use @SafeVarargs annotation with private methods as well. Here, we have a private method displayString() that takes string type var-args.

import java.util.ArrayList;
import java.util.List;

class DemoPrint{
    @SafeVarargs
    static void display(List... lists) {
        for (List list : lists) {  
            System.out.println(list);  
        }  
    }
}
class Main {
    @SafeVarargs
    private void displayString(String...strings ) {
        for (String str : strings) {
            System.out.println(str);
        }
    }

    public static void main(String[] args){
        List list = new ArrayList();  
        list.add("Python");  
        list.add("Java");  
        DemoPrint.display(list);
        Main main = new Main();
        main.displayString("Java","is","a","Awesome","Language");
    }
}

Output

Please note that if you use @SafeVarargs in private method before Java 9, you will get a compilation error.

Without @SafeVarargs annotation

In case, we don’t use the annotation the compile will report a warning message to console with output.

class Main {

    private void displayString(String...strings ) {
        for (String str : strings) {
            System.out.println(str);
        }
    }

    public static void main(String[] args){
        Main main = new Main();
        main.displayString("Java","is","a","Awesome","Language");
    }
}

Output

That’s all about Java 9 @SafeVarargs Annotation.