The Java Language: Statements and Expressions

Java statements appear inside 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 basic language structures such as if/then conditionals and loops.

 int size = 5;  
 if ( size > 10 )  
 doSomething();  
 for( int x = 0; x < size; x++ ) { ... }  

Expressions produce values; an expression is evaluated to produce a result that is 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, because 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.

 new Object();  
 Math.sin( 3.1415 );  
 42 * 64;  

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, top to bottom. 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 mean you should start writing obscure and convoluted statements, however. Relying on the order of evaluation of expressions in complex ways is a bad programming habit, even when it works. It produces code that is hard to read and harder to modify.