Expressions, Statements, and Blocks - Java Tutorial 5.0 英文版

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Expressions, Statements, and Blocks

Variables and operators, which were discussed in the previous two sections, are the basic building blocks of programs. You combine literals, variables, and operators to form expressions — segments of code that perform computations and return values. Certain expressions can be made into statements — complete units of execution. By grouping statements together with braces, { and }, you create blocks of code.

Expressions

Expressions perform the work of a program. Among other things, expressions are used to compute and to assign values to variables and to help control the execution flow of a program. The job of an expression is twofold: (1) to perform the computation indicated by the elements of the expression, and (2) to return a value that is the result of the computation.

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Definition:  An expression is a series of variables, operators, and method invocations, which are constructed according to the syntax of the language, that evaluates to a single value.

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As discussed in the previous section, operators return a value, so the use of an operator is an expression. This partial listing of the MaxVariablesDemo program shows some of its expressions in boldface.

...
//other primitive types
char aChar = 'S';
boolean aBoolean = true;

//display them all
System.out.println("The largest byte value is "
                   + largestByte);
...

if (Character.isUpperCase(aChar)) {
    ...
}

Each expression performs an operation and returns a value, as shown in the following table.

Some Expressions from MaxVariablesDemo

Expression	Action	Value Returned
aChar = 'S'	Assigns the character S to the character variable aChar	The value of aChar after the assignment ('S')
"The largest byte value is " + largestByte	Concatenates the string "The largest byte value is " and the value of largestByte converted to a string	The resulting string: The largest byte value is 127
Character.isUpperCase(aChar)	Invokes the method isUpperCase	The return value of the method: true

The data type of the value returned by an expression depends on the elements used in the expression. The expression aChar = 'S' returns a character because the assignment operator returns a value of the same data type as its operands and aChar and 'S' are characters. As you can see from the other expressions, an expression can return a boolean value, a string, and so on.

The Java programming language allows you to construct compound expressions and statements from various smaller expressions as long as the data type required by one part of the expression matches the data type of the other. Here's an example of a compound expression.

x * y * z

In this particular example, the order in which the expression is evaluated is unimportant because the result of multiplication is independent of order; the outcome is always the same, no matter in which order you apply the multiplications. However, this is not true of all expressions. For example, the following expression gives different results, depending on whether you perform the addition or the division operation first.

x + y / 100     //ambiguous

You can specify exactly how you want an expression to be evaluated using balanced parentheses: ( and ). For example, to make the previous expression unambiguous, you could write the following.

(x + y)/ 100    //unambiguous, recommended

If you don't explicitly indicate the order in which you want the operations in a compound expression to be performed, the order is determined by the precedence assigned to the operators in use within the expression. Operators that have a higher precedence get evaluated first. For example, the division operator has a higher precedence than does the addition operator. Thus, the following two statements are equivalent.

x + y / 100
x + (y / 100) //unambiguous, recommended

When writing compound expressions, you should be explicit and indicate with parentheses which operators should be evaluated first. This practice makes code easier to read and to maintain.

The following table shows the precedence assigned to the operators in the Java platform. The operators in this table are listed in precedence order: The closer to the top of the table an operator appears, the higher its precedence. Operators with higher precedence are evaluated before operators with relatively lower precedence. Operators on the same line have equal precedence. When operators of equal precedence appear in the same expression, a rule must govern which is evaluated first. All binary operators except for the assignment operators are evaluated from left to right; assignment operators are evaluated right to left.

Operator Precedence

Operators	Precedence
postfix	expr++ expr--
unary	++expr --expr +expr -expr ~ !
multiplicative	* / %
additive	+ -
shift	<< >> >>>
relational	< > <= >= instanceof
equality	== !=
bitwise AND	&
bitwise exclusive OR	^
bitwise inclusive OR	|
logical AND	&&
logical OR	||
conditional	? :
assignment	= += -= *= /= %= &= ^= |= <<= >>= >>>=

Statements

Statements are roughly equivalent to sentences in natural languages. A statement forms a complete unit of execution. The following types of expressions can be made into a statement by terminating the expression with a semicolon (;).

• Assignment expressions
• Any use of ++ or --
• Method invocations
• Object creation expressions

Such statements are called expression statements. Here are some examples of expression statements.

aValue = 8933.234;                      //assignment statement
aValue++;                               //increment         "
System.out.println(aValue);             //method invocation "
Integer integerObject = new Integer(4); //object creation   "

In addition to these kinds of expression statements, there are two other kinds of statements. A declaration statement declares a variable. You've seen many examples of declaration statements.

double aValue = 8933.234; //declaration statement

A control flow statement regulates the order in which statements get executed. The for loop and the if statement are both examples of control flow statements. You'll learn about control flow statements in the Control Flow Statements section.

Blocks

A block is a group of zero or more statements between balanced braces and can be used anywhere a single statement is allowed. The listing that follows shows two blocks from the MaxVariablesDemo program, each containing a single statement.

if (Character.isUpperCase(aChar)) {
    System.out.println("The character " + aChar
                       + " is uppercase.");
} else {
    System.out.println("The character " + aChar
                       + " is lowercase.");
}

Summary of Expressions, Statements, and Blocks

An expression is a series of variables, operators, and method invocations, which is constructed according to the syntax of the language, that evaluates to a single value. You can write compound expressions by combining expressions as long as the types required by all the operators involved in the compound expression are correct. When writing compound expressions, be explicit and indicate with parentheses which operators should be evaluated first.

If you choose not to use parentheses, then the Java platform evaluates the compound expression in the order dictated by operator precedence. The table in section Expressions, Statements, and Blocks shows the relative precedence assigned to Java platform operators.

A statement forms a complete unit of execution and is terminated with a semicolon (;). There are three kinds of statements: expression statements, declaration statements, and control flow statements.

You can group zero or more statements together into a block with braces: { and }. Even though not required, we recommend using blocks with control flow statements even if only one statement is in the block.

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