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Exploring Java

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The Java Language
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4.4 Statements and Expressions

Although the method declaration syntax of Java is quite different from that of C++, Java statement and expression syntax is very much like that of C. Again, the design intention was to make the low-level details of Java easily accessible to C programmers, so that they can concentrate on learning the parts of the language that are really different. Java statements appear inside of methods and class and variable initializers; they describe all activities of a Java program. Variable declarations and initializations like those in the previous section are statements, as are the basic language structures like conditionals and loops. Expressions are statements that produce a result that can be used as part of another statement. Method calls, object allocations, and, of course, mathematical expressions are examples of expressions.

One of the tenets of Java is to keep things simple and consistent. To that end, when there are no other constraints, evaluations and initializations in Java always occur in the order in which they appear in the code--from left to right. We'll see this rule used in the evaluation of assignment expressions, method calls, and array indexes, to name a few cases. In some other languages, the order of evaluation is more complicated or even implementation dependent. Java removes this element of danger by precisely and simply defining how the code is evaluated. This doesn't, however, mean you should start writing obscure and convoluted statements. Relying on the order of evaluation of expressions is a bad programming habit, even when it works. It produces code that is hard to read and harder to modify. Real programmers, however, are not made of stone, and you may catch me doing this once or twice when I can't resist the urge to write terse code.

Statements

As in C or C++, statements and expressions in Java appear within a code block. A code block is syntactically just a number of statements surrounded by an open curly brace ({) and a close curly brace (}). The statements in a code block can contain variable declarations:

{ 
    int size = 5; 
    setName("Max"); 
    ... 
} 

Methods, which look like C functions, are in a sense code blocks that take parameters and can be called by name.

setupDog( String name ) { 
    int size = 5; 
    setName( name ); 
    ... 
} 

Variable declarations are limited in scope to their enclosing code block. That is, they can't be seen outside of the nearest set of braces:

{ 
    int i = 5; 
} 
 
i = 6;           // compile time error, no such variable i 

In this way, code blocks can be used to arbitrarily group other statements and variables. The most common use of code blocks, however, is to define a group of statements for use in a conditional or iterative statement.

Since a code block is itself collectively treated as a statement, we define a conditional like an if/else clause as follows:

if ( condition ) 
    statement; 
[ else 
    statement; ] 

Thus, if/else in Java has the familiar functionality of taking either of the forms:

if ( condition ) 
    statement; 

or:

if ( condition )  { 
    [ statement; ] 
    [ statement; ] 
    [ ... ] 
} 

Here the condition is a boolean expression. In the second form, the statement is a code block, and all of its enclosed statements are executed if the conditional succeeds. Any variables declared within that block are visible only to the statements within the successful branch of the condition. Like the if/else conditional, most of the remaining Java statements are concerned with controlling the flow of execution. They act for the most part like their namesakes in C or C++.

The do and while iterative statements have the familiar functionality, except that their conditional test is also a boolean expression. You can't use an integer expression or a reference type; in other words you must explicitly test your value. In other words, while i==0 is legitimate, i is not, unless i is boolean. Here are the forms of these two statements:

while ( conditional ) 
    statement; 
 
do 
    statement; 
while ( conditional ); 

The for statement also looks like it does in C:

for ( initialization; conditional; incrementor ) 
    statement; 

The variable initialization expression can declare a new variable; this variable is limited to the scope of the for statement:

for (int i = 0; i < 100; i++ ) { 
    System.out.println( i ) 
    int j = i; 
    ... 
} 

Java doesn't support the C comma operator, which groups multiple expressions into a single expression. However, you can use multiple, comma-separated expressions in the initialization and increment sections of the for loop. For example:

for (int i = 0, j = 10; i < j; i++, j-- ) { 
   ... } 

The Java switch statement takes an integer type (or an argument that can be promoted to an integer type) and selects among a number of alternative case branches[2] :

[2] An object-based switch statement is desirable and could find its way into the language someday.

switch ( int expression ) { 
    case int expression : 
        statement; 
    [ case int expression 
        statement; 
    ... 
    default : 
        statement;  ] 
} 

No two of the case expressions can evaluate to the same value. As in C, an optional default case can be specified to catch unmatched conditions. Normally, the special statement break is used to terminate a branch of the switch:

switch ( retVal ) { 
    case myClass.GOOD : 
        // something good 
        break; 
    case myClass.BAD : 
        // something bad 
        break; 
    default :
        // neither one 
        break; 
} 

The Java break statement and its friend continue perform unconditional jumps out of a loop or conditional statement. They differ from the corresponding statements in C by taking an optional label as an argument. Enclosing statements, like code blocks and iterators, can be labeled with identifier statements:

one: 
    while ( condition ) { 
        ... 
        two: 
            while ( condition ) { 
                ... 
                // break or continue point 
            } 
        // after two 
    } 
// after one 

In the above example, a break or continue without argument at the indicated position would have the normal, C-style effect. A break would cause processing to resume at the point labeled "after two"; a continue would immediately cause the two loop to return to its condition test.

The statement break two at the indicated point would have the same effect as an ordinary break, but break one would break two levels and resume at the point labeled "after one." Similarly, continue two would serve as a normal continue, but continue one would return to the test of the one loop. Multilevel break and continue statements remove much of the need for the evil goto statement in C and C++.

There are a few Java statements we aren't going to discuss right now. The try, catch, and finally statements are used in exception handling, as we'll discuss later in this chapter. The synchronized statement in Java is used to coordinate access to statements among multiple threads of execution; see Chapter 6, Threads for a discussion of thread synchronization.

On a final note, I should mention that the Java compiler flags "unreachable" statements as compile-time errors. Of course, when I say unreachable, I mean those statements the compiler determines won't be called by a static look at compile-time.

Expressions

As I said earlier, expressions are statements that produce a result when they are evaluated. The value of an expression can be a numeric type, as in an arithmetic expression; a reference type, as in an object allocation; or the special type void, which results from a call to a method that doesn't return a value. In the last case, the expression is evaluated only for its side effects (i.e., the work it does aside from producing a value). The type of an expression is known at compile-time. The value produced at run-time is either of this type or, in the case of a reference type, a compatible (assignable) type.

Operators

Java supports almost all standard C operators. These operators also have the same precedence in Java as they do in C, as you can see in Table 4.3.

Table 4.3: Java Operators
Precedence Operator Operand Type Description
1 ++, -- Arithmetic Increment and decrement
1 +, - Arithmetic Unary plus and minus
1 ~ Integral Bitwise complement
1 ! Boolean Logical complement
1

( type )

Any Cast
2 *, /, % Arithmetic Multiplication, division, remainder
3 +, - Arithmetic Addition and subtraction
3 + String String concatenation
4 << Integral Left shift
4 >> Integral Right shift with sign extension
4 >>> Integral Right shift with no extension
5

<, <=, >, >=

Arithmetic Numeric comparison
5 instanceof Object Type comparison
6 ==, != Primitive Equality and inequality of value
6 ==, != Object Equality and inequality of reference
7 & Integral Bitwise AND
7 & Boolean Boolean AND
8 ^ Integral Bitwise XOR
8 ^ Boolean Boolean XOR
9 | Integral Bitwise OR
9 | Boolean Boolean OR
10 && Boolean Conditional AND
11 || Boolean Conditional OR
12 ?: NA Conditional ternary operator
13 = Any Assignment
13

*=, /=, %=, +=, -=, <<=, >>=, >>>=, &=, ^=, |=

Any Assignment with operation

There are a few operators missing from the standard C collection. For example, Java doesn't support the comma operator for combining expressions, although the for statement allows you to use it in the initialization and increment sections. Java doesn't allow direct pointer manipulation, so it does not support the reference (*), dereference (&), and sizeof operators.

Java also adds some new operators. As we've seen, the + operator can be used with String values to perform string concatenation. Because all integral types in Java are signed values, the >> operator performs a right-shift operation with sign extension. The >>> operator treats the operand as an unsigned number and performs a right shift with no extension. The new operator is used to create objects; we will discuss it in detail shortly.

Assignment

While variable initialization (i.e., declaration and assignment together) is considered a statement, variable assignment alone is an expression:

int i, j; 
i = 5;                           // expression 

Normally, we rely on assignment for its side effects alone, but, as in C, an assignment can be used as a value in another part of an expression:

j = ( i = 5 ); 

Again, relying on order of evaluation extensively (in this case, using compound assignments in complex expressions) can make code very obscure and hard to read. Do so at your own peril.

null

The expression null can be assigned to any reference type. It has the meaning of "no reference." A null reference can't be used to select a method or variable and attempting to do so generates a NullPointerException at run-time.

Variable access

Using the dot (.) to access a variable in an object is a type of expression that results in the value of the variable accessed. This can be either a numeric type or a reference type:

int i; 
String s; 
i = myObject.length; 
s = myObject.name; 

A reference type expression can be used in further evaluations, by selecting variables or calling methods within it:

int len = myObject.name.length(); 
int initialLen = myObject.name.substring(5, 10).length(); 

Here we have found the length of our name variable by invoking the length() method of the String object. In the second case, we took an intermediate step and asked for a substring of the name string. The substring method of the String class also returns a String reference, for which we ask the length. (Chapter 7, Basic Utility Classes describes all of these String methods in detail.)

Method invocation

A method invocation is basically a function call, or in other words, an expression that results in a value, the type of which is the return type of the method. Thus far, we have seen methods invoked via their name in simple cases:

System.out.println( "Hello World..." ); 
int myLength = myString.length();  

When we talk about Java's object-oriented features in Chapter 5, Objects in Java, we'll look at some rules that govern the selection of methods.

Like the result of any expression, the result of a method invocation can be used in further evaluations, as we saw above. Whether to allocate intermediate variables and make it absolutely clear what your code is doing or to opt for brevity where it's appropriate is a matter of coding style.

Object creation

Objects in Java are allocated with the new operator:

Object o = new Object(); 

The argument to new is a constructor that specifies the type of object and any required parameters to create it. The return type of the expression is a reference type for the created object.

We'll look at object creation in detail in Chapter 5, Objects in Java. For now, I just want to point out that object creation is a type of expression, and that the resulting object reference can be used in general expressions. In fact, because the binding of new is "tighter" than that of the dot-field selector, you can easily allocate a new object and invoke a method in it for the resulting expression:

int hours = new Date().getHours(); 

The Date class is a utility class that represents the current time. Here we create a new instance of Date with the new operator and call its getHours() method to retrieve the current hour as an integer value. The Date object reference lives long enough to service the method call and is then cut loose and garbage collected at some point in the future.

Calling methods in object references in this way is, again, a matter of style. It would certainly be clearer to allocate an intermediate variable of type Date to hold the new object and then call its getHours() method. However, some of us still find the need to be terse in our code.

instanceof

The instanceof operator can be used to determine the type of an object at run-time. instanceof returns a boolean value that indicates whether an object is an instance of a particular class or a subclass of that class:

Boolean b; 
String str = "foo"; 
b = ( str instanceof String );     // true 
b = ( str instanceof Object );     // also true 
b = ( str instanceof Date );       // false--not a Date or subclass 

instanceof also correctly reports if an object is of the type of an arry or a specified interface.

        if ( foo instanceof byte[] )
            ...

(See Chapter 5, Objects in Java for a full discussion of interfaces.)

It is also important to note that the value null is not considered an instance of any object. So the following test will return false, no matter what the declared type of the variable:

String s = null;
if ( s istanceof String )
    // won't happen


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