Chapter 5: Control Statements

Chapter 5: Control StatementsControl Flow: A control flow Statement

regulates the order in which statements get executed.

Flow-order in which the computer executes the lines of code in our program

Control flow blocks are basically blocks of code that control the way data flows, or else which statements are executed when. This can be useful if you only want certain areas of your code to be executed under a given condition.

The default flow of control in a program is TOP to BOTTOM, i.e. all the statements of a program are executed one by one in the sequence from top to Bottom. This execution order can be altered with the help of control instructions.

Java supports three types of control instructions -

1. Sequence Statement 2.Decision Making / Conditional / Selection

Statements 3..Looping / Iterative Statements

Program Structure

public class MyProgram

{

}

// comments about the class

public static void main (String[] args)

{

}

// comments about the method

method headermethod body

SEQUENCE The sequence means the statements are

being executed sequentially. This represents default flow of statement.

Statement 1

Statement 3

Statement 2

Statement 1

Statement 2

SELECTION

The sequence Statements means the execution of statement(s) depending upon a given condition.

Statement 1

Statement 2

Condition ?

true falseCondition ?

Statement 1

Condition ?

Statement 2

Statement 1

Condition ?

Statement 1

Statement 2

Statement 1

Condition ?

Statement 2

Statement 1

Statement 2

Statement 1

Condition ?

Statement 1

Statement 2

Statement 1

Condition ?

Statement 1

Statement 2

Statement 1

Condition ?

Statement 2

Statement 1

Condition ?

Statement 2

Statement 1

Condition ?

Statement 2

Statement 1

Condition ?

Statement 2

Statement 1

Condition ?

true

Statement 2

Statement 1

Condition ?

true

Statement 2

Body of iF

Condition ?

false

Statement 1

Statement 1

false

Statement 1

false

Statement 1

false

Statement 1

false

Body of else

falseCondition?

Statement 2

Statement 1

Statement 2

Conditional Statement - if Statement First Form of if Statement

if ( <conditional expression> ) {

execute statements -for-the-true-case;

}

=>Second form

if ( < conditional expression > ) {

execute statement1 -for-the-true-case

}

else {

execute statement2 -for-the-false-case

}Result of conditional expression : Logical Type(true or false)

There are certain points worth remembering about the if statement as outlined below:

The conditional expression is always enclosed in parenthesis.

The conditional expression may be a simple expression or a compound expression.

Each statement block may have a single or multiple statements to be executed.

In case there is a single statement to be executed then it is not mandatory to enclose it in curly braces ({}) but if there are multiple statements then they must be enclosed in curly braces ({})

The else clause is optional and needs to be included only when some action is to be taken if the test condition evaluates to false.

Example– if ( j<5 ) { // This is recommended

System.out.println(“j is less than 5”):

}

Else

{

System.out.println(“j is no less than 5”):

}

OR

if ( j<5 ) System.out.println(“j is less than 5”): //single statement

else

System.out.println(“j is no less than 5”):

if ( testScore < 70 )

JOptionPane.showMessageDialog(null, "You did not pass" );

else

JOptionPane.showMessageDialog(null, "You did pass " );

Syntax for the if Statementif ( <boolean expression> )

<then block>

else

<else block>

Then BlockThen Block

Else BlockElse Block

Boolean ExpressionBoolean Expression

Indentation is important!

Can be visualized as a flowchart

Nested if statementif (<cond. expr.>) if (<cond. expr.>) // . . . <statement>

if (<cond. expr.1>) <statement 1> else if (<cond. expr.2>) < statement 2> … else if (<cond. expr. n>) < statement n> else < statement>

Conditional Operator(?)if (x > 0)

{

y = 1

}else{ y = -1;}

is equivalent toy = (x > 0) ? 1 : -1;

Syntax for the switch Statement

switch ( gradeLevel ) {

case 1: System.out.print("Go to the Gymnasium");

break;

case 2: System.out.print("Go to the Science Auditorium");

break;

case 3: System.out.print("Go to Harris Hall Rm A3");

break;

case 4: System.out.print("Go to Bolt Hall Rm 101");

break;

}

switch ( <arithmetic expression> ) {

<case label 1> : <case body 1>

…

<case label n> : <case body n>

}

Case Body

Case Body

Arithmetic ExpressionArithmetic Expression

Case Label

Case Label

This is the general syntax rule for a switch statement. The case body may contain zero or more statements.

Exampleswitch(x) { case 1:

System.out.println(“go to 1”); break; // what happened if break is not here?case 2:case 3:

System.out.println(“go to 2 or 3”); break;

default : System.out.println(“not 1, 2 or 3”);

}

=>The expression of switch must not be long, float, double, or Boolean, it must be either byte, short, char, or int. (assignment compatible with int)

=>The arguments to case labels must be constants, or at least a constant expression that can be fully evaluated at compile time.

=>Can not use a variable or expression involving variables.

=>Default clause can be placed any where in, the block.

=> The fall of control to the following cses of matching case is called FALL –THROUGH.

Switch There are times in which you wish to check for a number

of conditions, and to execute different code depending on the condition. One way to do this is with if/else logic, such as the example below:

int x = 1; int y = 2; if (SOME_INT == x) { //DO SOMETHING } else if (SOME_INT == y) { //DO SOMETHING ELSE } else { //DEFAULT CONDITION }

This works, however another structure exists which allows us to do the same thing. Switch statements allow the programmer to execute certain blocks of code depending on exclusive conditions. The example below shows how a switch statement can be used:

int x = 1; int y = 2; switch (SOME_INT) { case x: method1(SOME_INT); break; case y: method2(SOME_INT); break; default: method3(); break; }

Switch takes a single parameter, which can be either an integer or a char. In this case the switch statement is evaluating SOME_INT, which is presumably an integer. When the switch statement is encountered SOME_INT is evaluated, and when it is equal to x or y, method1 and method2 are executed, respectively. The default case executes if SOME_INT does not equal x or y, in this case method3 is executed. You may have as many case statements as you wish within a switch statement.

Notice in the example above that "break" is listed after each case. This keyword ensures that execution of the switch statement stops immediately, rather than continuing to the next case. Were the break statement were not included "fall-through" would occur and the next case would be evaluated (and possibly executed if it meets the conditions).

Differences between the If-else and Switch ST: Switch can only test for equality whereas if can evaluate a

relational or logical expression that is multiple condition. The switch statement select its branches by testing the value of

same variable whereas the if-else construction allow you use a series of expression that may

involve unrelated variables and complex expressions. The if-else is more versatile of the two statements. The if-else statement can handle floating-point tests also apart

from handling integer and character test whereas a switch cannot handle floating-point test. the case labels of switch must be an integer byte,short,int or a char.

The switch case label value must be a constant. so if two or more variables are to be compared ,use if-else.

The switch statement is more efficient choice in terms of code used in a situation that supports the nature of switch operation( testing a value against a se of constant).

Repetitions for Loops while Loops

do Loops break and continue

Looping / Repetitive Statement Some time some portion of the program (one or more statement) needs to be executed repeatedly for fixed no. of time or till a particular condition is being satisfied. This repetitive operation is done through a looping statement.

A loop is repetition of one or more instructions, which continues till certain condition is met.

Definition

Statement 1

Statement 2

Statement n

ITERATION / LOOP

Looping structures are the statements that execute instructions repeatedly until a certain condition is fulfilled(true).

Condition?

false

True

In an Entry-Controlled loop/Top-Tested/Pre-Tested loop the test expression is evaluated before entering into a loop.

for Examples: For loop and While loop. In an Exit-Controlled loop/Bottom-Tested/Post-

Tested loop the test expression is evaluated before exiting from the loop.

for Examples: do-While loop. Do not put a semicolon after the right parenthesis. If

you do, the for loop would think that there no statements to execute. it would continue looping, doing nothing each time until the test expression becomes false.

for Loopsfor (initialization; Test-condition; update-statement){ //loop body;}

Example:

int i;for (i = 0; i<100; i++) { System.out.println("Welcome to Java! ” + i); }

In for loop contains three parts separated by semicolons(;) Initialization Before entering in a loop, its variables

must be initialized. The initialization expression is executed only once in the beginning of the loop.

Test Expression It decides whether the loop-body will be executed or not. if the test expression evaluates to true the loop-body gets executed, otherwise the loop is terminated. Test expression gets checked every time before entering in the body of the loop.

Update Expression(s) The update expressions change the values of loop variables. The update expression is executed every time after executing the loop-body .

The body of the loop The statements that are executed repeatedly (as long as the test-expression is nonzero) from the body of the loop.

for Loop Flow Chart

for// print 1 2 3 4 5 6 7 8 9 10public class ClassXI {

public static void main(String args[]) {

int n; for(n=1; n<=10; n++)

{ System.out.println(“ ” + n);

}// end for loop }// end main} // end class

INFINITE LOOP: An infinite loop can be created by omitting the test expression as shown below:

1. for( int j=25; ; --j)

{

System.out.println(“ An infinite for Loop”);

}

2. for ( ; ; ) // infinite loop

{

System.out.println(“ An infinite for Loop”);

}

3.Empty Loop: If a loop does not contain any statement in its loop-body, it is said to be an empty loop.

For(int j=20;j>=0;--j); 4. Declaration of variable inside the loops

and if: A variable declared within an if or for/while/do-while statement cannot be accessed after the statement is over, because its scope becomes the body of the statement(if/for/while/ do-while). It cannot be accessed outside the statement body.

if( ch= =‘a’){ int ans=1; }else {int b=2; } System.out.print(ans);// invalid

for( int ans=1; ans<100; ans+=20) { System.out.print(“ ” +ans);//valid }System.out.print(ans);//invalid

=>A variable scope is the part of program within which you can access the variable . A variable scope is the block of code( that is the part of code enclosed within{}) where the variable has been declared.

while Loopswhile (condition) { // loop-body;} Note: A while loop is pre-test loop. It first tests a

specified conditional expression and as long as the conditional expression is evaluated to non zero (true), action taken (i.e. statements or block after the loop is executed).

while Loop Flow ChartSTART

while// Demonstrate the while loop.public class While {

public static void main(String args[]) {int n = 1;while(n<=1 0) {

System.out.println(“ " + n);n++;

} //end while} // end main

} // end class

do Loops

do{ // Loop body;} while (continue-condition);

The Do-while loop is an POST-TEST or bottom test loop.that is, it executes its body at least once without testing specified conditional expression and then makes test. the do-while loop terminates when the text expression is evaluated to be False(0).

do Loop Flow Chart

do-while

// Demonstrate the do-while loop.public class DoWhile {

public static void main(String args[]) {int n =1;do { System.out.println(" " + n); n++; } while(n <= 0);

} // main} // class

Jump

Java supports three jump statements: break, continue, and return.

These statements transfer control to another part of your program.

breakIn Java, the break statement has three

uses. First, as you have seen, it terminates a

statement sequence in a switch statement. Second, it can be used to exit a loop. Third, it can be used as a "civilized" form

of goto. java does not support goto statement

break statement Alter flow of control– Causes immediate exit from control structure

Used in while, for, do…while or switch statements

Labeled break Labeled block– Set of statements enclosed by { }– Preceded by a label Labeled break statement – Exit from nested control structures– Proceeds to end of specified labeled block

breakThe general form of the labeled break

statement is shown here:break label;

The break Keyword

break// Using break to exit from for loop.class BreakLoop {

public static void main(String args[]) {for(int i=0; i<100; i++)

{if(i = = 10)

break; // terminate loop if i is 10System.out.println(“ i: " + i);

}System.out.println("Loop complete.");

}}

break// Using break to exit from while loop.class BreakLoop2 {

public static void main(String args[]) {int i = 0;while(i < 100) { if(i == 10) break; // terminate loop if i is 10 System.out.println("i: " + i); i++;}System.out.println("Loop complete.");

}}

break// Using break to exit from do while loop.class BreakLoop2 {

public static void main(String args[]) {int i = 0;

do {

if(i == 10) break; // terminate loop if i is 10

System.out.println("i: " + i); i++;

} while(i < 100);}System.out.println("Loop complete.");

}

Branch Statement - break Statement

To move control to the out of the block From of break statement

break [label] ;

int i = 1; while (true) { if (i = = 3) break; System.out.println("This is a " + i + " iteration"); ++i; }

continue you might want to continue running the loop,

but stop processing the remainder of the code in its body for particular iteration. This is, in effect, a goto just past the body of the loop, to the loop's end.

In while and do-while loops, a continue statement causes control to be transferred directly to the conditional expression that controls the loop.

In a for loop, control goes first to the iteration portion of the for statement and then to the conditional expression.

continue statement As with the break statement, continue may

specify a label to describe which enclosing loop to continue.– Skips remaining statements in loop body– Proceeds to next iteration

Used in while, for or do…while statements

Labeled continue statement – Skips remaining statements in nested-loop body– Proceeds to beginning of specified labeled block

The continue Keyword

// Demonstrate continue.class Continue {

public static void main(String args[]) {for(int i=0; i<10; i++) {

System.out.print(i + " ");if (i%2 == 0)

continue;System.out.println("");

}}

}

// Using continue with a label.class ContinueLabel {

public static void main(String args[]) {outer: for (int i=0; i<10; i++) {

for(int j=0; j<10; j++) { if(j > i) {

System.out.println();continue outer;

} System.out.print(" " + (i * j));}

}System.out.println();

}}

Branch Statement – continue Statement

To move control to the start of next repeatation From of continue statement

continue [Label] ;

When used in for statement

for (i=0; i<=5; ++i) { if (i % 2 == 0) continue; System.out.println("This is a " + i + " iteration"); }

Branch Statement – continue Statement

When used in while statement

i = 0; while (i <= 5) { ++i; if (i % 2) == 0) continue; System.out.println("This is a odd iteration - " + i); }

Branch Statement – continue Statement

Label continue statement

[LabeledContinue.java]

labelName:labelName: Rep. St. 1 { Rep. St. 2 { // ... continue; // ... continue labelName;continue labelName; } }

returnThe return statement is used to

explicitly return from a method. That is, it causes program control to transfer back to the caller of the method.

The following example illustrates this point. Here, return causes execution to return to the Java run-time system, since it is the run-time system that calls main( ).

Branch Statement – return Statement

To terminate the execution of method, then pass the method of caller that control

Forms of return statement – return; return;

– return <expr.>;return <expr.>;

return

// Demonstrate return.class Return {

public static void main(String args[]) {boolean t = true;System.out.println("Before the return.");if(t) return; // return to callerSystem.out.println("This won't

execute.");}

}

END

Objects and Classes

Object = active program unit containing both data and procedures

Class = a template for all objects of the same type

An Object is often called an instance of the class.

Components of an object

Instance variable = variable within an object

Method = function or procedure within an object– Can manipulate the object’s instance variables

Constructor = special method to initialize a new object instance

Encapsulation

Encapsulation = a way of restricting access to the internal components of an object– Private– Public

Additional object-oriented concepts

Inheritance: allows new classes to be defined in terms of previously defined classes

Polymorphism: allows method calls to be interpreted by the object that receives the call

Figure 6.22 The structure of a class describing a laser weapon

in a computer game

Figure 6.23 A class with a constructor

Figure 6.24 Our LaserClass definition using encapsulation as it would appear in

a Java or C# program

Programming concurrent activities

Parallel or concurrent processing = simultaneous execution of multiple processes– True concurrent processing requires multiple

CPUs– Can be simulated using time-sharing with a

single CPU

Interaction between processes

Mutual exclusion = a method for ensuring that data can be accessed by only one process at a time

Monitor = a data item augmented with the ability to control access to itself

Figure 6.25 Spawning processes

Comparing Objects

String str1 = new String("Java");String str2 = new String("Java");

if (str1 == str2) {System.out.println("They are equal");

} else {

System.out.println("They are not equal");} Solution : They are not equal

Discussion of some string methods

With primitive data types, we have only one way to compare them, but with objects (reference data type), we have two ways to compare themWe can test whether two variables point to the same object (use ==), or We can test whether two distinct objects have the same contents.

Explanation of the above Program: When we use the equality operator (==), we can

comparing the contents of the variables str1 and str2. So str1 == str2 being true means the contents are the same, which in turn, means they are pointing to the same object because the content of a reference data type is an address. Therefore, if there are two distinct objects, even the values hold by these objects are the same, the equality testing by the equality operator will always result in false.

PROGRAM import javax.swing.*; // To support simple input public class Control { // Quadratic formula public static void main(String[] args) { final double TOL= 1E-15; // Constant(use ‘final’) String input; input= JOptionPane.showInputDialog("Enter a"); double a= Double.parseDouble(input); input= JOptionPane.showInputDialog("Enter b"); double b= Double.parseDouble(input); input= JOptionPane.showInputDialog("Enter c"); double c= Double.parseDouble(input);

double discriminant= b*b - 4.0*a*c; if ( discriminant < 0) System.out.println("Sorry, no real root"); else if (Math.abs(discriminant) <= TOL) { double root= -0.5 * b / a; System.out.println("Root is " + root); } else { // Redefine ‘root’; blocks have own scopes double root=(-b + Math.sqrt(discriminant))/ (2.0*a); double root2=(-b- Math.sqrt(discriminant))/ (2.0*a); System.out.println("Roots" + root + “," + root2); } System.exit(0); } }

Arithmetic Operators

Intro to OOP with Java,=> C. Thomas Wu

The precedence table shows the order of operator evaluation. Instead of memorizing this table, it is more convenient and the code more readable if you use the parentheses judiciously.

Operator Precedence Rules

Selection Statements–Using if and if...else–Nested if Statements–Using switch Statements–Conditional Operator

Repetition Statements–Looping: while, do, and for–Nested loops–Using break and continue

Note: Java does NOT support goto

Java Statement

Assign Statement. : var = exp

Compound Statement. : { }

Condition Statement. : if St., switch St.

Control Loop Statement. : for St., while St., do-while St.

Branch Statement. : break St., continue St., return St.

Exception Handling St.: try-catch-finally

Synchronization St. : synchronized

Standard I/O : System.in.read(), System.out.println()Assignment Statement: Assignment Statement: Assign the value to variableForm : <variable> = <expression> remainder = dividend % divisor; i = j = k = 0; x *= y;

Statement