Learning Java

Statements and Expressions

Although the method declaration syntax of Java is quite different from that of C++, Java statement and expression syntax is like that of C. Again, the 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 classes; they describe all activities of a Java program. Variable declarations and assignments, such as those in the previous section, are statements, as are the basic language structures like conditionals and loops. Expressions describe values; an expression is evaluated to produce a result, to be used as part of another expression or in a statement. Method calls, object allocations, and, of course, mathematical expressions are examples of expressions. Technically, since variable assignments can be used as values for further assignments or operations (in somewhat questionable programming style), they can be considered to be both statements and 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 us doing this once or twice when we 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 series 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, the if clause has the familiar (to C/C++ programmers) functionality of taking two different forms:

if ( condition )  
    statement;

or:

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

Here the condition is a boolean expression. You can’t use an integer expression or a reference type, as in C. In other words, while i==0 is legitimate, i is not (unless i itself is boolean).

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; their conditional test is also a boolean expression:

while ( condition )
    statement;  
  
do  
    statement;  
while ( condition );

The for statement also looks like it does in C:

for ( initialization; condition; 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 does not 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 automatically promoted to an integer type) and selects among a number of alternative case branches:

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 this 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 both 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 the remaining justification for the evil goto statement in C/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 8, for a discussion of thread synchronization.

Unreachable statements

On a final note, we should mention that the Java compiler flags “unreachable” statements as compile-time errors. An unreachable statement is one that the compiler determines won’t be called at all. Of course there may be many methods that are actually never called in your code, but the compiler will only detect those that it can “prove” will never be called simply by checking at compile time. For example, a method with an unconditional return statement in the middle of it will cause a compile-time error. So will a method with something like this:

if (1 < 2)
	return;
// unreachable statements

Expressions

An expression produces a result, or value, when it is 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 is the declared type of 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 runtime is either of this type or, in the case of a reference type, a compatible (assignable) subtype.

Operators

Java supports almost all standard C operators. These operators also have the same precedence in Java as they do in C, as shown 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 doesn’t support the reference (&), dereference (*), and sizeof operators that are familiar to C/C++ programmers.

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-arithmetic-shift operation with sign extension. The >>> operator treats the operand as an unsigned number and performs a right-arithmetic-shift with no sign extension. The new operator, as in C++, 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, with no resulting value, variable assignment alone is also an expression:

int i, j;          // statement
i = 5;             // both expression and statement

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.

The null value

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 reference anything and attempting to do so generates a NullPointerException at runtime.

Variable access

The dot (.) operator has multiple meanings. It can retrieve the value of an instance variable (of some object) or a static variable (of some class). It can also specify a method to be invoked on an object or class. Using the dot (.) to access a variable in an object is an 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.

Method invocation

A method invocation is essentially a function call: an expression that results in a value. The value’s type is the return type of the method. Thus far, we have seen methods invoked by name:

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

Selecting which method to invoke is more complicated than it appears, because Java allows method overloading and overriding; the details are discussed in Chapter 5.

Like the result of any expression, the result of a method invocation can be used in further evaluations, as we saw earlier. You can allocate intermediate variables to make it absolutely clear what your code is doing, or you can opt for brevity where appropriate; it’s all a matter of coding style. The following are equivalent:

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

and:

String temp1 = myObject.name;
String temp2 = temp1.substring(5, 10);
int initialLen = temp2.length( );

Object creation

Objects in Java are allocated with the new operator:

Object o = new Object( );

The argument to new is the constructor for the class. The constructor is a method which always has the same name as the class. The constructor specifies any required parameters to create an instance of the object. The value of the new expression is a reference of the type of the created object. Objects always have one or more constructors.

We’ll look at object creation in detail in Chapter 5. For now, just note 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 dot (.), you can easily create a new object and invoke a method in it, without assigning the object to a reference type variable:

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, combining operations like this is common.

The instanceof operator

The instanceof operator can be used to determine the type of an object at run- time. It is used to compare an object against a particular type. instanceof returns a boolean value that indicates whether an object is an instance of a specified 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 the object is of the type of an array or a specified interface:

if ( foo instanceof byte[] )
    ...

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 instanceof String ) 
    // won't happen