nklmish

import javax.annotation.Nonnull;
import javax.annotation.meta.TypeQualifierDefault;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target(ElementType.PACKAGE)
@Nonnull
@TypeQualifierDefault({ElementType.METHOD, ElementType.PARAMETER})

nklmish

import javax.annotation.Nonnull;
import javax.annotation.meta.When;

public class Message {

    private final String greetingMessage = "Hello ";

    @Nonnull
    public String getEchoMessage() {
        return greetingMessage;

nklmish

import javax.annotation.Nonnull;
import javax.annotation.meta.When;

public class Message {
    private final String greetingMessage = "Hello ";
    
    @Nonnull(when = When.ALWAYS) //When.ALWAYS is default option and makes the annotated type as non-nullable    
    public String getEchoMessage() {
        return greetingMessage;
    }

nklmish

public class Message {
    private final String greetingMessage = "Hello ";
    public String getEchoMessage() {
        return greetingMessage;
    }
}