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Relational Expressions

• Relational expressions:

– Expressions that compare operands

– Sometimes called conditions

– Evaluated to yield a result

– Typically used as part of a selection statement

• A simple relational expression consists of a relational operator connecting two variable and/or constant operands

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Logical Operators

• Complex conditions can be created using Boolean logical operations:– AND

– OR

– NOT

• AND operator– &&

– True only if both individual expressions are true by themselves

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Logical Operators (continued)

• OR operator– ||

– True if either one or both of two expressions are true

• NOT operator– !

– Changes expression to opposite state

• Relational operators have higher precedence than logical operators

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Logical Operators (continued)

• && and || operators:

– Can only be used with Boolean operands

– Second operand not evaluated if evaluation of first is sufficient to determine final value of logical operation

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A Numerical Accuracy Problem

• Tests for equality of numbers using relational operator == should be avoided– Applies to:

• Floating-point

• Double-precision

• Many decimal numbers cannot be represented exactly in binary using a finite number of bits

• Require that absolute value of difference between operands be less than an extremely small value

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The if-else Statement

• Directs computer to select a sequence of instructions based on the result of a comparison

• Condition is evaluated first

– If the value of condition is true statement1 is executed

– If the value is false the statement after reserved word else is executed

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The if-else Statement (continued)

if (condition) <-----------------no semicolon here

statement1;

else <-----------------no semicolon here

statement2;

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Compound Statements

• Any number of single statements contained between braces

• Takes place of single statement

• Semicolon is not placed after braces that define compound statement

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The Boolean Data Type

• Tested condition in if-else statement must always evaluate to a Boolean value

• Value of condition must be either true or false• The boolean data type is restricted in its usage as

the value of a relational expression• Boolean values can be:

– Displayed

– Compared

– Assigned

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One-Way Selection

• No else expression

• Syntax:

if (condition)

statement;

• The statement following if (condition) is only executed if condition has a true value

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Placement of Braces in a Compound Statement

• Common practice for some programmers:– Place opening brace of compound statement on same

line as if and else statements

if (condition) {

statement1;

} else {

statement2;

}

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Nested if Statements

• One or more if-else statements can be included within either part of if-else statement

• Last else is with closest unpaired if

– Unless braces alter default pairing

• Process of nesting if statements can be extended indefinitely

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The if-else Chain

• Else part of an if statement contains another if-else statement

• Syntax:if (expression1)

statement1;

else

if (expression2)

statement2;

else

statement3;

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The if-else Chain (continued)

• Alternate syntax:

if (expression1)

statement1;

else if (expression2)

statement2;

else

statement3;

• Used extensively in programming applications

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The switch Statement

• Provides alternative to if-else chain

– For cases that compare the value of an integer expression to a specific value

• Reserved words:

– Switch

– Case

– Default

– Break

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Switch Syntaxswitch (expression){ // start of compound statement

case value-1: <------------terminated with a colonstatement1;statement2;break;

case value-2: <-------------terminated with a colonstatementm;statementn;break;

default: <-------------------terminated with a colonstatementaa;statementbb;

} // end of switch and compound statement

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The switch Statement (continued)

• Once entry point has been located:

– All further case evaluations are ignored

– Execution continues through end of compound statement

– Unless break statement is encountered

• The break statement causes an immediate exit from the switch statement

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Program Design and Development:Introduction to UML

• Explicit design

– Should always be undertaken before coding begins

– Referred to as program modeling

• Unified Modeling Language (UML)

– Program modeling language with its own set of rules and notations

– Not part of Java language

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Program Design and Development:Introduction to UML

(continued)

• Must understand and specify:

– What the objects in the system are

– What can happen to these objects

– When it can happen

• Each item is addressed by a number of individual and separate views and diagrams

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UML Diagram Types

• Class• Object • State• Sequence • Activity • Use-case

• Component• Deployment• Collaboration

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Class and Object Diagrams

• Class diagrams – Used to describe classes and their relationships

• Object diagrams – Describe specific objects and relationships

• Attribute– Characteristic that each object in class must have

• Attribute type– Primitive type

– Class type

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Class and Object Diagrams (continued)

• Visibility

– Defines where the attribute can be seen

– Whether the attribute can be used in other classes or is restricted to the class defining it

• Operations

– Transformations that can be applied to attributes

– Ultimately coded as Java methods

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Relationships

• The three basic relationships include:

– Association

– Aggregation

– Generalization

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Application: A Date Class

• Need to develop a date class

• Extremely important in financial programs

• Design stages:

– Identify type of objects

– Decide how Date object is internally represented

• Identify attributes

– Identify initial set of operations

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User Interfaces, Definitions, and Information Hiding

• User interface consists of: – Class’s public member methods’ header lines

– Supporting comments

• Definition consists of:– Class’s definition section

– Class’s private data members

• Information hiding refers to the principle that how a class is internally constructed is not relevant to any programmer who wishes to use the class

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Simplifying the Code

• Once a program is working, simplify the code:

– Remove print statements

– Use methods such as setDate() to initialize values in constructors

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Adding Class Methods

Leap Year Algorithm

If the year is divisible by 4 with a remainder of 0 AND the year is divisible by 100 with a nonzero remainder

OR the year is divisible by 400 with no remainder

then the year is a leap year

Else

the year is not a leap year

EndIf

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Common Programming Errors• Assuming the if-else statement is selecting an

incorrect choice when the problem is really the values being tested

• Using nested if statements without including braces to clearly indicate the desired structure

• Mistakenly using the assignment operator = instead of the relational operator == when comparing Boolean data

• Forgetting to use a break statement to close off a case within a switch statement

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Summary

• Relational expressions:

– Referred to as conditions

– Used to compare operands

• if-else statements:

– Used to select between two alternative statements

– Based on a value of a relational or logical expression

– Can contain other if-else statements

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Summary (continued)

• A compound statement consists of a number of individual statements enclosed within the brace pair { and }

• The switch statement is a multiway selection statement