Java Control Structure 1

Java Control Structures I

There are three ways to process a computer program. 

In a sequence (executing statements in order)

By making a selection or choice (branching) based on certain conditions (conditional statements)

Repeating statements a certain number of times (looping)

Conditional statements can be created using the word if.  A condition is met if it evaluates to true and then certain statements are executed.  If it evaluates to false other statements are executed.

Relational Operators

In Java, a condition is represented by a logical (Boolean) expression (an expression that has a value of true or false when evaluated).  These expressions are created using relational operators (a binary operator consisting of two operands) and can be used to make comparisons.  Some relational operators in Java include:

==        equal to          

!=         not equal to

<          less than

<=        less than or equal to

>          greater than

>=        greater than or equal to

Relational Operators and Primitive Data Types

Relational operators can be used with integral and floating-point primitive data types.  For char values, whether an expression evaluates to true or false depends on the collating sequence of the Unicode character set.  When Java evaluates a logical expression, it returns the Boolean value true if the expression evaluates to true and false otherwise.

Comparing Strings

In Java, strings are compared character by character, starting with the first character and using the collating sequence. The character-by-character comparison continues until one of three conditions is met: a mismatch is found, the last characters have been compared and are equal, or one string is exhausted.

If two strings of unequal length are compared, and they are equal until the last character of the shorter string, then the shorter string is evaluated as less than the larger string. 

The method compareTo in the class String can be used to compare objects of the class.  This expression returns an integer value as follows:

Str1.compareTo(str2) = integer value less than 0 if string str1 less than string str2

Str1.compareTo(str2) = 0 if string str1 equal to string str2

            Str1.compareTo(str2) = integer value greater than 0 if string str1 greater than string str2

The method equals can be used to compare two strings and determine if they are equal.

Logical (Boolean) Operators and Logical Expressions

Logical (Boolean) operators enable you to combine logical expressions.  The three logical (Boolean) operators in Java are:

!           not       (unary)

&&      and      (binary)

||           or         (binary)

Logical operators take only logical values as operands and yield only logical values as results.

When you use the not operator, !true is false and !false is true.  Putting ! in front of a logical expression reverses the value of that logical expression.

&& and || are used to evaluate a combination of expressions.  An expression using && only returns true if ALL expressions are true.  || returns true as long as one expression in the combination is true.

Order of Precedence

Logical expressions have the following order of precedence when evaluating expressions:

Operators                                            Precedence

!  +  -  (unary operators)                      first

*  /  %                                                  second

+  -                                                       third

<  <=  >=  >                                         fourth

==  !=                                                  fifth

&&                                                      sixth

||                                                           seventh

= (assignment operator)                      last

Relational and logical operators are evaluated from left to right; their associativity is from left to right.

Short-Circuit Evaluation

Logical expressions in Java are evaluated using a high efficient algorithm known as short-circuit evaluation.  With short-circuit evaluation, the computer evaluates the logical expression from left to right. As soon as the value of the entire logical expression is known, the evaluation stops.

Teaching Tip

The operators & and | can be used in place of && and ||, respectively.  There is no short-circuit evaluation with these operators.

boolean Data Type and Logical (Boolean) Expressions

You can manipulate logical (Boolean) expressions using the boolean data type and Java reserved words boolean, true and false.

One-Way Selection

In Java, one-way selections are incorporated using the if statement. The syntax of one-way selection is:

if (expression)

    statement

The expression, which is a logical expression, is referred to as the decision maker because it decides whether to execute the statement (called the action statement).

Two-Way Selection

To choose between two alternatives Java provides the if...else statement. Two-way selection uses the following syntax:

if (expression)

    statement1

else

    statement2

If the value of the expression is true then statement1 executes; otherwise statement2 executes.  The else statement must follow an if statement; it does not exist on its own in Java.

Compound (Block of) Statements

The if and if...else structures control only one statement at a time. To permit more complex statements, Java provides a structure called a compound statement or block of statements.  Block statements are enclosed in curly braces {} and consist of a sequence of statements to be executed depending on the evaluation of the if and if…else expressions.

Multiple Selections: Nested if

You can include multiple selection paths in a program by using an if...else structure, if the action statement itself is an if or if...else statement. When one control statement is located within another, it is said to be nested. Nested statements help solve problems that require the implementation of more than two alternatives. 

In a nested if statement, Java associates an else with the most recent incomplete if—that is, the most recent if that has not been paired with an else.

Comparing if...else Statements with a Series of if Statements

A series of if statements can be used in place of if…else statements to complete a task.  However, the program may execute more slowly in this case because there are more evaluations to be made.

Conditional Operator (? :)

Certain if...else statements can be written more concisely by using Java’s conditional operator. The conditional operator, written as ?:, is a ternary operator, which means that it takes three arguments. The syntax for using the conditional operator is:

expression1 ? expression2 : expression3

If expression1 evaluated to true, the result of the conditional expression is expression2. Otherwise, the result of the conditional expression is expression3.

switch Structures

The second selection structure in Java does not require the evaluation of a logical expression. It is called the switch Structure and gives the computer the power to choose from many alternatives.  The switch statement has the following syntax:

            switch(expression)

            {

                        case value1: statements1

                                           break;

                        case value2: statements2

                                           break;

                        …

                        case valuen; statementsn

                                           break;

                        default: statements

            }

The expression (sometimes called the selector) is evaluated first and the value of the expression is then used to perform the actions specified in the statements that follow the reserved word case. 

The expression is usually an identifier and the value is always an integral.  The value of the expression determines which statement is selected for execution, therefore a particular case values must be unique. 

One or more statements may follow a case symbol, and curly braces are unnecessary to group them. 

The break statement may or may not appear after each statement. 

A switch statement executes according to the following rules:

When the value of the expression is matched against a case value (also called a label), the statements execute until either a break statement is found or the end of the switch structure is reached.

If the value of the expression does not match any of the case values, the statements following the default label execute. If the switch structure has no default label, and if the value of the expression does not match any of the case values, the entire switch statement is skipped.

A break statement causes an immediate exit from the switch structure