16.doc.html

java语言规范

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<h3>16.1.7    The Boolean Operator <code>|</code></h3>
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<li><i>V</i><i></i> is definitely assigned after <i>a</i> <code>|</code> <i>b</i> when true iff at least one of the following is true:
<ul>
<a name="26085"></a>
<li><i>V</i><i></i> is definitely assigned after <i>b</i>. (Note that if <i>V</i><i></i> is definitely assigned after <i>a</i>, it follows that <i>V</i><i></i> is definitely assigned after <i>b</i>.)
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<li><i>V</i><i></i> is definitely assigned after <i>a</i> when true and <i>V</i><i></i> is definitely assigned after <i>b</i> when true.
</ul>
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<li><i>V</i><i></i> is definitely assigned after <i>a</i> <code>|</code> <i>b</i> when false iff <i>V</i><i></i> is definitely assigned after <i>a</i> when false or <i>V</i><i></i> is definitely assigned after <i>b</i> when false.
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<li><i>V</i><i></i> is definitely assigned before <i>a</i> iff <i>V</i><i></i> is definitely assigned before <i>a</i> <code>|</code> <i>b</i>.
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<li><i>V</i><i></i> is definitely assigned before <i>b </i>iff <i>V</i><i></i> is definitely assigned after <i>a</i>.
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<h3>16.1.8    The Boolean Operator <code>^</code></h3>
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<li><i>V</i><i></i> is definitely assigned after <i>a</i> <code>^</code> <i>b</i> when true iff at least one of the following is true:
<ul>
<a name="26093"></a>
<li><i>V</i><i></i> is definitely assigned after <i>b</i>.
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<li><i>V</i><i></i> is definitely assigned after <i>a</i> when true and <i>V</i><i></i> is definitely assigned after <i>b</i> when true.
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<li><i>V</i><i></i> is definitely assigned after <i>a</i> when false and <i>V</i><i></i> is definitely assigned after <i>b</i> when false.
</ul>
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<li><i>V</i><i></i> is definitely assigned after <i>a</i> <code>^</code> <i>b</i> when false iff at least one of the following is true:
<ul>
<a name="26097"></a>
<li><i>V</i><i></i> is definitely assigned after <i>b</i>.
<a name="26098"></a>
<li><i>V</i><i></i> is definitely assigned after <i>a</i> when true and <i>V</i><i></i> is definitely assigned after <i>b</i> when false.
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<li><i>V</i><i></i> is definitely assigned after <i>a</i> when false and <i>V</i><i></i> is definitely assigned after <i>b</i> when true.
</ul>
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<li><i>V</i><i></i> is definitely assigned before <i>a</i> iff <i>V</i><i></i> is definitely assigned before <i>a</i> <code>^</code> <i>b</i>.
<a name="26101"></a>
<li><i>V</i><i></i> is definitely assigned before <i>b </i>iff <i>V</i><i></i> is definitely assigned after <i>a</i>.
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<h3>16.1.9    The Boolean Operator <code>==</code></h3>
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<li><i>V</i><i></i> is definitely assigned after <i>a</i> <code>==</code> <i>b</i> when true iff at least one of the following is true:
<ul>
<a name="26104"></a>
<li><i>V</i><i></i> is definitely assigned after <i>b</i>.
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<li><i>V</i><i></i> is definitely assigned after <i>a</i> when true and <i>V</i><i></i> is definitely assigned after <i>b</i> when false.
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<li><i>V</i><i></i> is definitely assigned after <i>a</i> when false and <i>V</i><i></i> is definitely assigned after <i>b</i> when true.
</ul>
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<li><i>V</i><i></i> is definitely assigned after <i>a</i> <code>==</code> <i>b</i> when false iff at least one of the following is true:
<ul>
<a name="26108"></a>
<li><i>V</i><i></i> is definitely assigned after <i>b</i>.
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<li><i>V</i><i></i> is definitely assigned after <i>a</i> when true and <i>V</i><i></i> is definitely assigned after <i>b</i> when true.
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<li><i>V</i><i></i> is definitely assigned after <i>a</i> when false and <i>V</i><i></i> is definitely assigned after <i>b</i> when false.
</ul>
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<li><i>V</i><i></i> is definitely assigned before <i>a</i> iff <i>V</i><i></i> is definitely assigned before <i>a</i> <code>==</code> <i>b</i>.
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<li><i>V</i><i></i> is definitely assigned before <i>b </i>iff <i>V</i><i></i> is definitely assigned after <i>a</i>.
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<h3>16.1.10    The Boolean Operator <code>!=</code></h3>
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The rules for <i>a</i> <code>!=</code> <i>b</i> are identical to the rules for <i>a</i> <code>^</code> <i>b</i> <a href="16.doc.html#26091">(&#167;16.1.8)</a>.
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<h3>16.1.11    The Boolean Operator <code>?&#32;:</code></h3>
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Suppose that <i>b</i> and <i>c</i> are boolean-valued expressions.
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<li><i>V</i><i></i> is definitely assigned after <i>a</i> <code>?</code> <i>b</i> <code>:</code> <i>c </i>when true iff both of the following are true:
<ul>
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<li><i>V</i><i></i> is definitely assigned before <i>b</i> or <i>V</i><i></i> is definitely assigned after <i>b</i> when true.
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<li><i>V</i><i></i> is definitely assigned before <i>c</i> or <i>V</i><i></i> is definitely assigned after <i>c</i> when true.
</ul>
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<li><i>V</i><i></i> is definitely assigned after <i>a</i> <code>?</code> <i>b</i> <code>:</code> <i>c </i>when false iff both of the following are true:
<ul>
<a name="26124"></a>
<li><i>V</i><i></i> is definitely assigned before <i>b</i> or <i>V</i><i></i> is definitely assigned after <i>b</i> when false.
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<li><i>V</i><i></i> is definitely assigned before <i>c</i> or <i>V</i><i></i> is definitely assigned after <i>c</i> when false.
</ul>
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<li><i>V</i><i></i> is definitely assigned before <i>a</i> iff <i>V</i><i></i> is definitely assigned before <i>a</i> <code>?</code> <i>b</i> <code>:</code> <i>c</i>.
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<li><i>V</i><i></i> is definitely assigned before <i>b </i>iff <i>V</i><i></i> is definitely assigned after <i>a</i> when true.
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<li><i>V</i><i></i> is definitely assigned before <i>c </i>iff <i>V</i><i></i> is definitely assigned after <i>a</i> when false.
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<h3>16.1.12    The Conditional Operator <code>?&#32;:</code></h3>
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Suppose that <i>b</i> and <i>c</i> are expressions that are not boolean-valued.
<p><ul><a name="26131"></a>
<li><i>V</i><i></i> is definitely assigned after <i>a</i> <code>?</code> <i>b</i> <code>:</code> <i>c </i>iff both of the following are true:
<ul>
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<li><i>V</i><i></i> is definitely assigned after <i>b</i>.
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<li><i>V</i><i></i> is definitely assigned after <i>c</i>.
</ul>
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<li><i>V</i><i></i> is definitely assigned before <i>a</i> iff <i>V</i><i></i> is definitely assigned before <i>a</i> <code>?</code> <i>b</i> <code>:</code> <i>c</i>.
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<li><i>V</i><i></i> is definitely assigned before <i>b </i>iff <i>V</i><i></i> is definitely assigned after <i>a</i> when true.
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<li><i>V</i><i></i> is definitely assigned before <i>c </i>iff <i>V</i><i></i> is definitely assigned after <i>a</i> when false.
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<h3>16.1.13    Boolean Assignment Expressions</h3>
<a name="26138"></a>
Suppose that an assignment expression <i>a</i> <code>=</code> <i>b</i>, <i>a</i> <code>&amp;=</code> <i>b</i>, <i>a</i> <code>|=</code> <i>b</i>, or <i>a</i> <code>^=</code> <i>b</i> is boolean-
valued.
<p><ul><a name="26139"></a>
<li><i>V</i><i></i> is definitely assigned before <i>a</i> iff <i>V</i><i></i> is definitely assigned before the assignment expression.
<a name="26140"></a>
<li><i>V</i><i></i> is definitely assigned before <i>b</i> iff <i>V</i><i></i> is definitely assigned after <i>a</i>.
<a name="26141"></a>
<li><i>V</i><i></i> is definitely assigned after <i>a</i> <code>=</code> <i>b</i> when true iff either <i>a</i> is <i>V</i><i></i> or <i>V</i><i></i> is definitely assigned after the right-hand operand expression when true.
<a name="26142"></a>
<li><i>V</i><i></i> is definitely assigned after <i>a</i> <code>=</code> <i>b</i> when false iff either <i>a</i> is <i>V</i><i></i> or <i>V</i><i></i> is definitely assigned after the right-hand operand expression when false.
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<li><i>V</i><i></i> is definitely assigned after <i>a</i> <code>&amp;=</code> <i>b</i> when true iff either <i>a</i> is <i>V</i><i></i> or <i>V</i><i></i> would be definitely assigned after <i>a</i> <code>&amp;</code> <i>b</i> (in the same context) when true.
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<li><i>V</i><i></i> is definitely assigned after <i>a</i> <code>&amp;=</code> <i>b</i> when false iff either <i>a</i> is <i>V</i><i></i> or <i>V</i><i></i> would be definitely assigned after <i>a</i> <code>&amp;</code> <i>b</i> (in the same context) when false.
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<li><i>V</i><i></i> is definitely assigned after <i>a</i> <code>|=</code> <i>b</i> when true iff either <i>a</i> is <i>V</i><i></i> or <i>V</i><i></i> would be definitely assigned after <i>a</i> <code>|</code> <i>b</i> (in the same context) when true.
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<li><i>V</i><i></i> is definitely assigned after <i>a</i> <code>|=</code> <i>b</i> when false iff either <i>a</i> is <i>V</i><i></i> or <i>V</i><i></i> would be definitely assigned after <i>a</i> <code>|</code> <i>b</i> (in the same context) when false.
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<li><i>V</i><i></i> is definitely assigned after <i>a</i> <code>^=</code> <i>b</i> when true iff either <i>a</i> is <i>V</i><i></i> or <i>V</i><i></i> would be definitely assigned after <i>a</i> <code>^</code> <i>b</i> (in the same context) when true.
<a name="26148"></a>
<li><i>V</i><i></i> is definitely assigned after <i>a</i> <code>^=</code> <i>b</i> when false iff either <i>a</i> is <i>V</i><i></i> or <i>V</i><i></i> would be definitely assigned after <i>a</i> <code>^</code> <i>b</i> (in the same context) when false.
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Note that if <i>a</i> is <i>V</i><i></i> and <i>V</i><i></i> is not definitely assigned before a compound assignment such as <i>a</i> <code>&amp;=</code> <i>b</i>, then a compile-time error will necessarily occur. The rules stated above include the disjunct "<i>a</i> is <i>V</i><i></i>" so that <i>V</i><i></i> will be considered to have been definitely assigned at later points in the code. Including the disjunct "<i>a</i> is <i>V</i><i></i>" does not affect the binary decision as to whether a program is acceptable or will result in a compile-time error, but it affects <i>how many</i> different points in the code may be regarded as erroneous, and so in practice it can improve the quality of error reporting.<p>
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<h3>16.1.14    Other Assignment Expressions</h3>
<a name="26151"></a>
Suppose that an assignment expression <i>a</i> <code>=</code> <i>b</i>, <i>a</i> <code>+=</code> <i>b</i>, <i>a</i> <code>-=</code> <i>b</i>, <i>a</i> <code>*=</code> <i>b</i>, <i>a</i> <code>/=</code> <i>b</i>, <i>a</i> <code>%=</code> <i>b</i>, 
<i>a</i> <code>&lt;&lt;=</code> <i>b</i>, <i>a</i> <code>&gt;&gt;=</code> <i>b</i>, <i>a</i> <code>&gt;&gt;&gt;=</code> <i>b</i>, <i>a</i> <code>&amp;=</code> <i>b</i>, <i>a</i> <code>|=</code> <i>b</i>, or <i>a</i> <code>^=</code> <i>b</i> is not boolean-valued.
<p><ul><a name="26152"></a>
<li><i>V</i><i></i> is definitely assigned after the assignment expression iff either <i>a</i> is <i>V</i><i></i> or <i>V</i><i></i> is definitely assigned after <i>b</i>.
<a name="26153"></a>
<li><i>V</i><i></i> is definitely assigned before <i>a</i> iff <i>V</i><i></i> is definitely assigned before the assignment expression.
<a name="26154"></a>
<li><i>V</i><i></i> is definitely assigned before <i>b</i> iff <i>V</i><i></i> is definitely assigned after <i>a</i>.
</ul><a name="26155"></a>
<h3>16.1.15    Operators <code>++</code> and <code>--</code></h3>
<ul><a name="26156"></a>
<li><i>V</i><i></i> is definitely assigned after a preincrement, predecrement, postincrement, or postdecrement expression iff either the operand expression is <i>V</i><i></i> or <i>V</i><i></i> is definitely assigned after the operand expression.
<a name="26157"></a>
<li><i>V</i><i></i> is definitely assigned before the operand expression iff <i>V</i><i></i> is definitely assigned before the preincrement, predecrement, postincrement, or postdecrement expression. 
</ul><a name="26158"></a>
<h3>16.1.16    Other Expressions</h3>
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If an expression is not boolean-valued and is not a conditional-operator expression 
or assignment expression, the following rules apply:
<p><ul><a name="26160"></a>
<li>If the expression has no subexpressions, <i>V</i><i></i> is definitely assigned after the expression iff <i>V</i><i></i> is definitely assigned before the expression. This case applies to literals, simple names, <code>this</code>, <code>super</code>, and <code>null</code>.
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<li>If the expression has subexpressions, <i>V</i><i></i> is definitely assigned after the expression iff <i>V</i><i></i> is definitely assigned after its rightmost immediate subexpression.
</ul><a name="26162"></a>
For any immediate subexpression <i>y</i> of an expression <i>x</i>, <i>V</i><i></i> is definitely assigned 
before <i>y</i> iff <i>V</i><i></i> is definitely assigned before <i>x</i> or one of the following situations is 
true:
<p><ul><a name="26163"></a>
<li><i>y</i> is the right-hand operand of a binary operator and <i>V</i><i></i> is definitely assigned after the left-hand operand.
<a name="26164"></a>
<li><i>x</i> is an array reference, <i>y</i> is the subexpression within the brackets, and <i>V</i><i></i> is definitely assigned after the subexpression before the brackets.
<a name="26165"></a>
<li><i>x</i> is a method invocation expression for an object; <i>y</i> is the first argument expression in the method invocation expression; there is a subexpression whose value is an object to the left of the dot, method name, and left parenthesis of the method invocation expression; and <i>V</i><i></i> is definitely assigned after this subexpression.
<a name="26166"></a>
<li><i>x</i> is a method invocation expression or class instance creation expression; <i>y</i> is an argument expression, but not the first; and <i>V</i><i></i> is definitely assigned after the argument expression to the left of <i>y</i>.
<a name="26167"></a>
<li><i>x</i> is an class instance creation expression; <i>y</i> is a dimension expression, but not the first; and <i>V</i><i></i> is definitely assigned after the dimension expression to the left of <i>y</i>.
</ul><a name="44017"></a>