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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.dataflow.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;

/**
 * A method is called side-effect-free if it has no visible side-effects, such as setting a
 * field of an object that existed before the method was called.
 *
 * Only the visible side-effects are important. The method is allowed to cache the answer to a
 * computationally expensive query, for instance. It is also allowed to modify newly-created
 * objects, and a constructor is side-effect-free if it does not modify any objects that existed
 * before it was called.
 *
 * 
This annotation is important to pluggable type-checking because if some fact about an object
 * is known before a call to such a method, then the fact is still known afterwards, even if the
 * fact is about some non-final field. When any non-{@code @SideEffectFree} method is called, then a
 * pluggable type-checker must assume that any field of any accessible object might have been
 * modified, which annuls the effect of flow-sensitive type refinement and prevents the pluggable
 * type-checker from making conclusions that are obvious to a programmer.
 *
 * 
Also see {@link Pure}, which means both side-effect-free and {@link Deterministic}.
 *
 * 
Analysis: The Checker Framework performs a conservative analysis to verify a
 * {@code @SideEffectFree} annotation. The Checker Framework issues a warning if the method uses any
 * of the following Java constructs:
 *
 * 

 *   Assignment to any expression, except for local variables and method parameters.
 *   
A method invocation of a method that is not {@code @SideEffectFree}.
 *   
Construction of a new object where the constructor is not {@code @SideEffectFree}.
 * 
 *
 * These rules are conservative: any code that passes the checks is side-effect-free, but the
 * Checker Framework may issue false positive warnings, for code that uses one of the forbidden
 * constructs but is side-effect-free nonetheless. In particular, a method that caches its result
 * will be rejected.
 *
 * In fact, the rules are so conservative that checking is currently disabled by default, but can
 * be enabled via the {@code -AcheckPurityAnnotations} command-line option.
 *
 * 
 *
 * @checker_framework.manual #type-refinement-purity Side effects, determinism, purity, and
 *     flow-sensitive analysis
 * @author Stefan Heule
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD, ElementType.CONSTRUCTOR})
public @interface SideEffectFree {}