What Is an Expression in Java

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What Is an Expression
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An expression is composed of one or more operands The simplest form of an expression consists of a single literal constant or object The result, in general, is the operand's rvalue For example, here are three simple expressions:
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void mumble() { 314159; "melancholia"; upperBound; }
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The result of 314159 is 314159 Its type is double The result of "melancholia" is the address in memory of the first element of the string Its type is const char* The result of upperBound is its associated value Its type is determined by its definition More generally, an expression consists of one or more operands and an operation to be applied to them For example, the following are all expressions (we've left out the object definitions; the appropriate operation is applied automatically based on the type of the operand(s)):
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salary + raise ivec[ size/2 ] * delta first_name + " " + last_name
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The operations applied to the operands are represented by operators For example, in the first expression, the floating point addition operator is applied to the operands salary and raise In the second expression, the operand size is divided by 2 The result is used to index into the integer array ivec Its value is multiplied by the operand delta In the third expression, two string operands and a string literal are concatenated to form a new string using the instance of the addition operator defined by the standard library string class Operators that act on one operand are unary operators, such as the address-of (&) and dereference (*) operators, whereas operators that act on two operands, such as the addition and subtraction operators, are binary operators Some operators represent both a unary
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file:///F|/WinDDK/resources/CPPPrimer/c++primerhtm (140 / 1065) [2001-3-29 11:32:04]
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and a binary operation (more exactly, the same symbol is used to represent two different operators) For example,
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*ptr
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represents the unary dereference operator It returns the value stored in the object that ptr addresses However,
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var1 * var2
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represents the binary multiplication operator It computes the product of its two operands: var1 and var2 The evaluation of an expression performs one or more operations, yielding a result Except when noted otherwise, the result of an expression is an rvalue The data type of the result of an arithmetic expression is determined by the data type of the operand(s) When more than one data type is present, type conversions take place following a predefined set of type conversion rules (We look at type conversions in detail in Section 414) When two or more operators are combined, the expression is referred to as a compound expression For example, the purpose of the following expression is to determine whether the pointer ptr addresses an object (it addresses an object if its value is not zero) and whether the object addressed has a value other than zero:
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The explicit test against zero is optional The following expression is equivalent and is the preferred real-world C++ program idiom: ptr && *ptr
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ptr != 0 && *ptr != 0
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The full expression is composed of three subexpressions: the inequality test of ptr against zero, the dereference of ptr, and the inequality test against zero of the value resulting from the dereference If ptr is defined as follows
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int ival = 1024; int *ptr = &ival;
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the result of the dereference subexpression is 1,024, and the results of the two subexpression tests for inequality to zero are both true The result of the full expression is also true: ptr is not set to zero, and the object it addresses is not set to zero (The && operator is referred to as the AND operator: it evaluates to true if both its left- and right-hand subexpressions evaluate to true; otherwise, it evaluates to false) If we look at our compound expression closely, we will notice that its successful evaluation is dependent on the order of subexpression evaluation For example, if the second half of the expression is evaluated first that is, if ptr is dereferenced before it is confirmed not to equal zero the program is likely to fail or be corrupted at run-time whenever ptr in fact is set to 0 In the case of the AND operator, the order of subexpression evaluation is strictly defined: If the left subexpression evaluates as false, the right subexpression is not evaluated; the program error cannot occur The order of subexpression evaluation in practice is a frequent source of program error for the beginning C and C++ programmer It is difficult to uncover, because a visual inspection of our program does not reveal the error unless we understand the rules of subexpression evaluation In general, the order of subexpression evaluation is determined by the precedence and associativity of its operators We'll look at this in some detail in Section 413 after we look at the set of operators supported by C++ The following
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