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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.framework.qual;

import java.lang.annotation.Annotation;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Repeatable;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * A conditional postcondition annotation to indicate that a method ensures that certain expressions
 * have a certain qualifier once the method has terminated, and if the result is as indicated by
 * {@code result}. The expressions for which the qualifier holds after the method's execution are
 * indicated by {@code expression} and are specified using a string. The qualifier is specified by
 * the {@code qualifier} annotation element.
 *
 * Here is an example use:
 *
 * 

 *   {@literal @}EnsuresQualifierIf(result = true, expression = "#1", qualifier = Odd.class)
 *    boolean isOdd(final int p1, int p2) {
 *        return p1 % 2 == 1;
 *    }
 * 
 *
 * This annotation is only applicable to methods with a boolean return type.
 *
 * 
Some type systems have specialized versions of this annotation, such as {@code
 * org.checkerframework.checker.nullness.qual.EnsuresNonNullIf} and {@code
 * org.checkerframework.checker.lock.qual.EnsuresLockHeldIf}.
 *
 * @see EnsuresQualifier
 * @checker_framework.manual #java-expressions-as-arguments Syntax of Java expressions
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD})
@InheritedAnnotation
@Repeatable(EnsuresQualifierIf.List.class)
public @interface EnsuresQualifierIf {
  /**
   * Returns the Java expressions for which the qualifier holds if the method terminates with return
   * value {@link #result()}.
   *
   * @return the Java expressions for which the qualifier holds if the method terminates with return
   *     value {@link #result()}
   * @checker_framework.manual #java-expressions-as-arguments Syntax of Java expressions
   */
  String[] expression();

  /**
   * Returns the qualifier that is guaranteed to hold if the method terminates with return value
   * {@link #result()}.
   *
   * @return the qualifier that is guaranteed to hold if the method terminates with return value
   *     {@link #result()}
   */
  Class qualifier();

  /**
   * Returns the return value of the method that needs to hold for the postcondition to hold.
   *
   * @return the return value of the method that needs to hold for the postcondition to hold
   */
  boolean result();

  /**
   * A wrapper annotation that makes the {@link EnsuresQualifierIf} annotation repeatable.
   *
   * Programmers generally do not need to write this. It is created by Java when a programmer
   * writes more than one {@link EnsuresQualifierIf} annotation at the same location.
   */
  @Documented
  @Retention(RetentionPolicy.RUNTIME)
  @Target({ElementType.METHOD})
  @InheritedAnnotation
  @interface List {
    /**
     * Return the repeatable annotations.
     *
     * @return the repeatable annotations
     */
    EnsuresQualifierIf[] value();
  }
}