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<td align=right><i>Java Language Specification</i><br>
<font size="-1">Third Edition</font></td></tr></table>
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<p><strong>
CHAPTER
15 </strong></p>
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<h1>Expressions</h1>
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Much of the work in a program is done by evaluating <em>expressions</em>, either for their side effects, such as assignments to variables, or for their values, which can be used as arguments or operands in larger expressions, or to affect the execution sequence in statements, or both.<p>
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This chapter specifies the meanings of expressions and the rules for their evaluation.<p>
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<h2>15.1 Evaluation, Denotation, and Result</h2>
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When an expression in a program is <em>evaluated</em> (<em>executed</em>), the <em>result </em>denotes one of three things:<p>
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<li>A variable <a href="typesValues.html#18470">(§4.12)</a> (in C, this would be called an <em>lvalue</em>)
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<li>A value (<a href="typesValues.html#85587">§4.2</a>, <a href="typesValues.html#9317">§4.3</a>)
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<li>Nothing (the expression is said to be void)
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Evaluation of an expression can also produce side effects, because expressions may contain embedded assignments, increment operators, decrement operators, and method invocations.<p>
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An expression denotes nothing if and only if it is a method invocation <a href="expressions.html#20448">(§15.12)</a> that invokes a method that does not return a value, that is, a method declared <code>void</code> <a href="classes.html#40420">(§8.4)</a>. Such an expression can be used only as an expression statement <a href="statements.html#5984">(§14.8)</a>, because every other context in which an expression can appear requires the expression to denote something. An expression statement that is a method invocation may also invoke a method that produces a result; in this case the value returned by the method is quietly discarded.<p>
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Value set conversion <a href="conversions.html#184225">(§5.1.13)</a> is applied to the result of every expression that produces a value.<p>
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Each expression occurs in either:<p>
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<li>The declaration of some (class or interface) type that is being declared: in a field initializer, in a static initializer, in an instance initializer, in a constructor declaration, in an annotation, or in the code for a method.
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<li>An annotation of a package or of a top-level type declaration .
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<h2>15.2 Variables as Values</h2>
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If an expression denotes a variable, and a value is required for use in further evaluation, then the value of that variable is used. In this context, if the expression denotes a variable or a value, we may speak simply of the <em>value</em> of the expression.<p>
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If the value of a variable of type <code>float</code> or <code>double</code> is used in this manner, then value set conversion <a href="conversions.html#184225">(§5.1.13)</a> is applied to the value of the variable.<p>
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<h2>15.3 Type of an Expression</h2>
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If an expression denotes a variable or a value, then the expression has a type known at compile time. The rules for determining the type of an expression are explained separately below for each kind of expression.<p>
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<p>The value of an expression is assignment compatible <a href="conversions.html#184206">(§5.2)</a> with the type of the expression, unless heap pollution <a href="typesValues.html#111088">(§4.12.2.1)</a> occurs. Likewise the value stored in a variable is always compatible with the type of the variable, unless heap pollution occurs. In other words, the value of an expression whose type is <i>T</i> is always suitable for assignment to a variable of type <i>T</i>.</p>
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<p>Note that an expression whose type is a class type <i>F</i> that is declared <code>final</code> is guaranteed to have a value that is either a null reference or an object whose class is <i>F</i> itself, because <code>final</code> types have no subclasses.</p>
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<h2>15.4 FP-strict Expressions</h2>
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If the type of an expression is <code>float</code> or <code>double</code>, then there is a question as to what value set <a href="typesValues.html#9208">(§4.2.3)</a> the value of the expression is drawn from. This is governed by the rules of value set conversion <a href="conversions.html#184225">(§5.1.13)</a>; these rules in turn depend on whether or not the expression is <em>FP-strict</em>.<p>
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Every compile-time constant expression <a href="expressions.html#5313">(§15.28)</a> is FP-strict. If an expression is not a compile-time constant expression, then consider all the class declarations, interface declarations, and method declarations that contain the expression. If <em>any</em> such declaration bears the <code>strictfp</code> modifier, then the expression is FP-strict.<p>
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If a class, interface, or method, <i>X</i>, is declared <code>strictfp</code>, then <i>X</i> and any class, interface, method, constructor, instance initializer, static initializer or variable initializer within <i>X</i> is said to be <em>FP-strict</em>. Note that an annotation <a href="interfaces.html#253695">(§9.7)</a> element value <a href="interfaces.html#239527">(§9.6)</a> is always FP-strict, because it is always a compile-time constant <a href="expressions.html#5313">(§15.28)</a>.<p>
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It follows that an expression is not FP-strict if and only if it is not a compile-time constant expression <em>and</em> it does not appear within any declaration that has the <code>strictfp</code> modifier.<p>
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Within an FP-strict expression, all intermediate values must be elements of the float value set or the double value set, implying that the results of all FP-strict expressions must be those predicted by IEEE 754 arithmetic on operands represented using single and double formats. Within an expression that is not FP-strict, some leeway is granted for an implementation to use an extended exponent range to represent intermediate results; the net effect, roughly speaking, is that a calculation might produce "the correct answer" in situations where exclusive use of the float value set or double value set might result in overflow or underflow.<p>
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<h2>15.5 Expressions and Run-Time Checks</h2>
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If the type of an expression is a primitive type, then the value of the expression is of that same primitive type. But if the type of an expression is a reference type, then the class of the referenced object, or even whether the value is a reference to an object rather than <code>null</code>, is not necessarily known at compile time. There are a few places in the Java programming language where the actual class of a referenced object affects program execution in a manner that cannot be deduced from the type of the expression. They are as follows:<p>
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<li>Method invocation <a href="expressions.html#20448">(§15.12)</a>. The particular method used for an invocation <code>o.m(</code>...<code>)</code> is chosen based on the methods that are part of the class or
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