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The Checker Framework enhances Java's type system to make it more powerful and useful. This lets software developers detect and prevent errors in their Java programs. The Checker Framework includes compiler plug-ins ("checkers") that find bugs or verify their absence. It also permits you to write your own compiler plug-ins.

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package org.checkerframework.checker.nullness.qual;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.checkerframework.framework.qual.PreconditionAnnotation;

// TODO: In a fix for https://tinyurl.com/cfissue/1917, add the text:
// Every prefix expression must also be non-null; for example, {@code
// @RequiresNonNull(expression="a.b.c")} implies that both {@code a.b} and {@code a.b.c} must be
// non-null.
/**
 * Indicates a method precondition: the method expects the specified expressions to be non-null when
 * the annotated method is invoked.
 *
 * For example:
 * 
 *
 * 
 * class MyClass {
 *   @Nullable Object field1;
 *   @Nullable Object field2;
 *
 *   @RequiresNonNull({"field1", "other.field1"})
 *   void method1(@NonNull MyClass other) {
 *     field1.toString();           // OK, this.field1 is known to be non-null
 *     field2.toString();           // error, might throw NullPointerException
 *     other.field1.toString();     // OK, other.field1 is known to be non-null
 *     other.field2.toString();     // error, might throw NullPointerException
 *   }
 *
 *   void method2() {
 *     MyClass other = new MyClass();
 *
 *     field1 = new Object();
 *     other.field1 = new Object();
 *     method1();                   // OK, satisfies method precondition
 *
 *     field1 = null;
 *     other.field1 = new Object();
 *     method1();                   // error, does not satisfy this.field1 method precondition
 *
 *     field1 = new Object();
 *     other.field1 = null;
 *     method1();                   // error, does not satisfy other.field1 method precondition
 *   }
 * 
 *
 * Do not use this annotation for formal parameters (instead, give them a {@code @NonNull} type,
 * which is the default and need not be written). The {@code @RequiresNonNull} annotation is
 * intended for other expressions, such as field accesses or method calls.
 *
 * @checker_framework.manual #nullness-checker Nullness Checker
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD, ElementType.CONSTRUCTOR})
@PreconditionAnnotation(qualifier = NonNull.class)
public @interface RequiresNonNull {
    /**
     * The Java expressions that need to be {@link
     * org.checkerframework.checker.nullness.qual.NonNull}.
     *
     * @checker_framework.manual #java-expressions-as-arguments Syntax of Java expressions
     */
    String[] value();
}