Expression, Statements and Blocks

Introduction
Expressions
Statements
Blocks

1. Introduction

Operators may be used in building expressions, which compute values.

Expressions are the core components of statements.

Statements may be grouped into blocks.

2. Expressions

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

Examples of expressions, illustrated in bold below:

    int speed = 0;

The data type of the value returned by an expression depends on the elements used in the expression.

The expression speed = 0 returns an int because the assignment operator returns a value of the same data type as its left-hand operand; in this case, speed is an int.

    // Expression Returns String
    System.out.println("Element 1 at index 0: " + anArray[0]);

    //Expression Returns Boolean
    if (value1 == value2)

Compound Expression.

The Java programming language allows you to construct compound expressions 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:

    1 * 2 * 3

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 an expression will be evaluated using balanced parenthesis: ( 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 for the operations 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.

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

3. Statements

Statements are roughly equivalent to sentences in natural languages. A statement forms a complete unit of execution.

There are three types of statements,

Expression Statement
Declaration Statement
Control Flow Statements

Expression Statement

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.

    // assignment statement
    aValue = 8933.234;

    // increment statement
    aValue++;

    // method invocation statement
    System.out.println("Hello World!");

    // object creation statement
    Bicycle myBike = new Bicycle();

Declaration Statement

A declaration statement declares a variable.

    // declaration statement
    double aValue = 8933.234;

Control Flow Statement

Finally, control flow statements regulate the order in which statements get executed. You'll learn about control flow statements in the next post.

4. Blocks

A block is a group of zero or more statements between balanced braces and can be used anywhere a single statement is allowed.

Example

                /**
                 * A Class used to demonstrate Blocks.
                 * 
                 * @author vasanth
                 *
                 */
                public class BlogDemo {

                    /**
                     * Main.
                     * 
                     * @param args
                     */
                    public static void main(String[] args) {
                        boolean condition = true;
                        if (condition) { // begin block 1
                            System.out.println("Condition is true.");
                        } // end block one
                        else { // begin block 2
                            System.out.println("Condition is false.");
                        } // end block 2
                    }
                }