Java : @SafeVarargs (annotation) with Examples

SafeVarargs (Java SE 20 & JDK 20) with Examples.
You will find code examples on most SafeVarargs methods.

Summary

A programmer assertion that the body of the annotated method or constructor does not perform potentially unsafe operations on its varargs parameter.

Class diagram

An example with unchecked warnings :

public class Foo<T> {
    public void m(T... args) {
        System.out.println(Arrays.toString(args));
    }
}

//> javac -Xlint:unchecked Foo.java
//Foo.java:6: warning: [unchecked] Possible heap pollution from parameterized vararg type T
//    public void m(T... args) {
//                       ^
//  where T is a type-variable:
//    T extends Object declared in class Foo
//1 warning

An example with @SafeVarargs :

public class Foo<T> {
    @SafeVarargs
    public final void m(T... args) {
        System.out.println(Arrays.toString(args));
    }
}

> javac -Xlint:unchecked Foo.java
 <No errors and warnings>

An example with unsafe operations on varargs parameter :

public class Bar {
    public static List<String>[] m(List<String>... args) {
        if (0 < args.length) {
            Object[] objects = args;
            objects[args.length - 1] = List.of(123);
        }

        return args;
    }
}

List<String>[] listArray = Bar.m(
        List.of("aaa"),
        List.of("bbb"),
        List.of("ccc")
);

try {
    System.out.println("-- listArray --");
    for (final var list : listArray) {
        if (!list.isEmpty()) {
            String str = list.get(0);
            System.out.println(str);
        }
    }
} catch (ClassCastException e) {
    System.out.println("ClassCastException!");
}

// Result
// ↓
//-- listArray --
//aaa
//bbb
//ClassCastException!

API Examples

Java : @SafeVarargs (annotation) with Examples

Summary

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