Lecture
V
Language Basics III
About this Lecture
In This lecture, we will look at how you make decision and choices using if/else statement. In additon, we will also examine how to make your programs repeat a set of actions until a specific condition is met
Mary C, Kanthy W, Alison H The Java(TM) Tutorial:
A Short Course on the Basics (3rd Edition)?
Sun Microsystems Inc.
ISBN 0-201-70393-9.
Learning Objective
In this lecture, you will know how to analyze a sequence of statements, identify the flow of control, and determine the results of statements execution. In addtion you will also know the basic concepts of Exception Handling. On completion of this lecture, you will get familiar with a wide of Java programming statemnts as:
- The while and do-while Statements
- The for Statement
- The if/else Statements
- The switch Statement
- Exception Handling Statements
- Branching Statements
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About this Lecture
Learning Objective
Control Flow Statements
The while and do-while Statements
The for Statement
The if/else Statements
The switch Statement
Exception Handling Statements
Branching Statements
Summary
Review
Questions & Amswers
Semple Test
Assignment
Required Readings |
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Note:
The material in this lecture has been developed by Knowledge Systems Institute.
The lecture material and figures that are linked to from the lecture come
from material supplied in conjunction with the required text for the course. The
copyright for this material is held by Sun Microsystems Inc.
The slides are to be used only in conjunction with the required text for
the course:Mary C. , Kanthy W. , Alison H. The Java(TM) Tutorial: A Short Course on the Basics (3rd Edition.) Sun Microsystems Inc. ISBN 0-201-70393-9.
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Control Flow Statements
When you write a program, you type statements into a file. Without control flow statements, the interpreter executes these statements in the order they appear in the file from left to right, top to bottom. You can use control flow statements in your programs to conditionally execute statements, to repeatedly execute a block of statements, and to otherwise change the normal, sequential flow of control. For example, in the following code snippet, the if statement conditionally executes the System.out.println statement within the braces, based on the return value of
Character.isUpperCase(aChar):
char c;
...
if (Character.isUpperCase(aChar)) {
System.out.println("The character " + aChar + " is upper case.");
}
The Java programming language provides several control flow statements, which are listed in the following table.
| Statement Type |
Keyword |
| looping |
while, do-while , for |
| decision making |
if-else, switch-case |
| exception handling |
try-catch-finally, throw |
| branching |
break, continue, label:, return |
In the sections that follow, you will see the following notation to describe the general form of a control flow statement:
control flow statement details {
statement(s)
}
Technically, the braces, { and }, are not required if the block contains only one statement. However, we recommend that you always use { and }, because the code is easier to read and it helps to prevent errors when modifying code.
- The while and do-while Statements
- The for Statement
- The if/else Statements
- The switch Statement
- Exception Handling Statements
- Branching Statements
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The while and do-while Statements
You use a while statement to continually execute a block of statements while a condition remains true. The general syntax of the while statement is:
while (expression) { statement }
First, the while statement evaluates expression, which must return a boolean value. If the expression returns true, then the while statement executes the statement(s) associated with it. The while statement continues testing the expression and executing its block until the expression returns false.
The example program shown below, called WhileDemo, uses a while statement to step through the characters of a string, appending each character from the string to the end of a string buffer until it encounters the letter g.
public class WhileDemo {
public static void main(String[] args) {
String copyFromMe = "Copy this string until you " +
"encounter the letter 'g'.";
StringBuffer copyToMe = new StringBuffer();
int i = 0;
char c = copyFromMe.charAt(i);
while (c != 'g') {
copyToMe.append(c);
c = copyFromMe.charAt(++i);
}
System.out.println(copyToMe);
}
}
The value printed by the last line is: Copy this strin.
The Java programming language provides another statement that is similar to the while statement--the do-while statement. The general syntax of the do-while is:
do { statement(s) } while (expression);
Instead of evaluating the expression at the top of the loop, do-while evaluates the expression at the bottom. Thus the statements associated with a do-while are executed at least once.
Here's the previous program rewritten to use do-while and renamed to DoWhileDemo:
public class DoWhileDemo {
public static void main(String[] args) {
String copyFromMe = "Copy this string until you " +
"encounter the letter 'g'.";
StringBuffer copyToMe = new StringBuffer();
int i = 0;
char c = copyFromMe.charAt(i);
do {
copyToMe.append(c);
c = copyFromMe.charAt(++i);
} while (c != 'g');
System.out.println(copyToMe);
}
}
The value printed by the last line is: Copy this strin.
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The for Statement
The for statement provides a compact way to iterate over a range of values. The general form of the for statement can be expressed like this:
for (initialization; termination; increment) { statement }
The initialization is an expression that initializes the loop-it's executed once at the beginning of the loop. The termination expression determines when to terminate the loop. This expression is evaluated at the top of each iteration of the loop. When the expression evaluates to false, the loop terminates. Finally, increment is an expression that gets invoked after each iteration through the loop. All these components are optional. In fact, to write an infinite loop, you omit all three expressions:
for ( ; ; ) { // infinite loop ... }
Often for loops are used to iterate over the elements in an array, or the characters in a string. The following sample, ForDemo, uses a for statement to iterate over the elements of an array and print them:
public class ForDemo {
public static void main(String[] args) {
int[] arrayOfInts = { 32, 87, 3, 589, 12, 1076,
2000, 8, 622, 127 };
for (int i = 0; i < arrayOfInts.length; i++) {
System.out.print(arrayOfInts[i] + " ");
}
System.out.println();
}
}
The output of the program is: 32 87 3 589 12 1076 2000 8 622 127.
Note that you can declare a local variable within the initialization expression of a for loop. The scope of this variable extends from its declaration to the end of the block governed by the for statement so it can be used in the termination and increment expressions as well. If the variable that controls a for loop is not needed outside of the loop, it's best to declare the variable in the initialization expression. The names i, j, and k are often used to control for loops; declaring them within the for loop initialization expression limits their life-span and reduces errors.
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The if/else Statements
The if statement enables your program to selectively execute other statements, based on some criteria. For example, suppose that your program prints debugging information, based on the value of a boolean variable named DEBUG. If DEBUG is true, your program prints debugging information, such as the value of a variable, such as x. Otherwise, your program proceeds normally. A segment of code to implement this might look like this:
if (DEBUG) {
System.out.println("DEBUG: x = " + x);
}
This is the simplest version of the if statement: The block governed by the if is executed if a condition is true. Generally, the simple form of if can be written like this:
if (expression) {
statement(s)
}
What if you want to perform a different set of statements if the expression is false? You use the else statement for that. Consider another example. Suppose that your program needs to perform different actions depending on whether the user clicks the OK button or another button in an alert window. Your program could do this by using an if statement along with an else statement:
. . .
// response is either OK or CANCEL depending
// on the button that the user pressed
. . .
if (response == OK) {
// code to perform OK action
} else {
// code to perform Cancel action
}
The else block is executed if the if part is false. Another form of the else statement, else if, executes a statement based on another expression. An if statement can have any number of companion else if statements but only one else. Following is a program, IfElseDemo, that assigns a grade based on the value of a test score: an A for a score of 90% or above, a B for a score of 80% or above, and so on:
public class IfElseDemo {
public static void main(String[] args) {
int testscore = 76;
char grade;
if (testscore >= 90) {
grade = 'A';
} else if (testscore >= 80) {
grade = 'B';
} else if (testscore >= 70) {
grade = 'C';
} else if (testscore >= 60) {
grade = 'D';
} else {
grade = 'F';
}
System.out.println("Grade = " + grade);
}
}
The output from this program is:
Grade = C
You may have noticed that the value of testscore can satisfy more than one of the expressions in the compound if statement: 76 >= 70 and 76 >= 60. However, as the runtime system processes a compound if statement such as this one, once a condition is satisfied, the appropriate statements are executed (grade = 'C';), and control passes out of the if statement without evaluating the remaining conditions.
The Java programming language supports an operator, ?:, that is a compact version of an if statement. Recall this statement from the MaxVariablesDemo program:
if (Character.isUpperCase(aChar)) {
System.out.println("The character " + aChar + " is upper case.");
} else {
System.out.println("The character " + aChar + " is lower case.");
}
Here's how you could rewrite that statement using the ?: operator:
System.out.println("The character " + aChar + " is " +
(Character.isUpperCase(aChar) ? "upper" : "lower") +
"case.");
The ?: operator returns the string "upper" if the isUpperCase method returns true. Otherwise, it returns the string "lower". The result is concatenated with other parts of a message to be displayed. Using ?: makes sense here because the if statement is secondary to the call to the println method. Once you get used to this construct, it also makes the code easier to read.
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The switch Statement
Use the switch statement to conditionally perform statements based on an integer expression. Following is a sample program, SwitchDemo, that declares an integer named month whose value supposedly represents the month in a date. The program displays the name of the month, based on the value of month, using the switch statement:
public class SwitchDemo {
public static void main(String[] args) {
int month = 8;
switch (month) {
case 1: System.out.println("January"); break;
case 2: System.out.println("February"); break;
case 3: System.out.println("March"); break;
case 4: System.out.println("April"); break;
case 5: System.out.println("May"); break;
case 6: System.out.println("June"); break;
case 7: System.out.println("July"); break;
case 8: System.out.println("August"); break;
case 9: System.out.println("September"); break;
case 10: System.out.println("October"); break;
case 11: System.out.println("November"); break;
case 12: System.out.println("December"); break;
}
}
}
The switch statement evaluates its expression, in this case the value of month, and executes the appropriate case statement. Thus, the output of the program is: August. Of course, you could implement this by using an if statement:
int month = 8;
if (month == 1) {
System.out.println("January");
} else if (month == 2) {
System.out.println("February");
}
. . . // and so on
Deciding whether to use an if statement or a switch statement is a judgment call. You can decide which to use, based on readability and other factors. An if statement can be used to make decisions based on ranges of values or conditions, whereas a switch statement can make decisions based only on a single integer value. Also, the value provided to each case statement must be unique.
Another point of interest in the switch statement is the break statement after each case. Each break statement terminates the enclosing switch statement, and the flow of control continues with the first statement following the switch block. The break statements are necessary because without them, the case statements fall through. That is, without an explicit break, control will flow sequentially through subsequent case statements. Following is an example, SwitchDemo2, which illustrates why it might be useful to have case statements fall through:
public class SwitchDemo2 {
public static void main(String[] args) {
int month = 2;
int year = 2000;
int numDays = 0;
switch (month) {
case 1:
case 3:
case 5:
case 7:
case 8:
case 10:
case 12:
numDays = 31;
break;
case 4:
case 6:
case 9:
case 11:
numDays = 30;
break;
case 2:
if ( ((year % 4 == 0) && !(year % 100 == 0))
|| (year % 400 == 0) )
numDays = 29;
else
numDays = 28;
break;
}
System.out.println("Number of Days = " + numDays);
}
}
The output from this program is:
Number of Days = 29
Technically, the final break is not required because flow would fall out of the switch statement anyway. However, we recommend using a break for the last case statement just in case you need to add more case statements at a later date. This makes modifying the code easier and less error-prone.
You will see break used to terminate loops in Branching Statements.
Finally, you can use the default statement at the end of the switch to handle all values that aren't explicitly handled by one of the case statements.
int month = 8;
. . .
switch (month) {
case 1: System.out.println("January"); break;
case 2: System.out.println("February"); break;
case 3: System.out.println("March"); break;
case 4: System.out.println("April"); break;
case 5: System.out.println("May"); break;
case 6: System.out.println("June"); break;
case 7: System.out.println("July"); break;
case 8: System.out.println("August"); break;
case 9: System.out.println("September"); break;
case 10: System.out.println("October"); break;
case 11: System.out.println("November"); break;
case 12: System.out.println("December"); break;
default: System.out.println("Hey, that's not a valid month!"); break;
}
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Exception Handling Statements
The Java programming language provides a mechanism known as exceptions to help programs report and handle errors. When an error occurs, the program throws an exception. What does this mean? It means that the normal flow of the program is interrupted and that the runtime environment attempts to find an exception handler--a block of code that can handle a particular type of error. The exception handler can attempt to recover from the error or, if it determines that the error is unrecoverable, provide a gentle exit from the program.
Three statements play a part in handling exceptions:
-
The try statement identifies a block of statements within which an exception might be thrown.
-
The catch statement must be associated with a try statement and identifies a block of statements that can handle a particular type of exception. The statements are executed if an exception of a particular type occurs within the try block.
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The finally statement must be associated with a try statement and identifies a block of statements that are executed regardless of whether or not an error occurs within the try block.
Here's the general form of these statements:
try {
statement(s)
}
catch (exceptiontype name) {
statement(s)
}
finally {
statement(s)
}
This has been a brief overview of the statements provided by the Java programming language used in reporting and handling errors. However, other factors and considerations, such as the difference between runtime and checked exceptions and the hierarchy of exceptions classes, which represent various types of exceptions, play a role in using the exception mechanism. The Handling Errors with Exceptions lesson provides a complete discussion on this subject.
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Branching Statements
The Java programming language supports three branching statements:
-
The break statement
-
The continue statement
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The return statement
The break statement and the continue statement, which are covered next, can be used with or without a label. A label is an identifier placed before a statement. The label is followed by a colon (:):
statementName: someJavaStatement;
You'll see an example of a label within the context of a program in the next section.
The break statement has two forms: unlabeled and labeled. You saw the unlabeled form of the break statement used with switch earlier. As noted there, an unlabeled break terminates the enclosing switch statement, and flow of control transfers to the statement immediately following the switch. You can also use the unlabeled form of the break statement to terminate a for, while, or do-while loop. The following sample program, BreakDemo, contains a for loop that searches for a particular value within an array:
public class BreakDemo {
public static void main(String[] args) {
int[] arrayOfInts = { 32, 87, 3, 589, 12, 1076,
2000, 8, 622, 127 };
int searchfor = 12;
int i = 0;
boolean foundIt = false;
for ( ; i < arrayOfInts.length; i++) {
if (arrayOfInts[i] == searchfor) {
foundIt = true;
break;
}
}
if (foundIt) {
System.out.println("Found " + searchfor + " at index " + i);
} else {
System.out.println(searchfor + "not in the array");
}
}
}
The break statement terminates the for loop when the value is found. The flow of control transfers to the statement following the enclosing for, which is the print statement at the end of the program.
The output of this program is:
Found 12 at index 4
The unlabeled form of the break statement is used to terminate the innermost switch, for, while, or do-while; the labeled form terminates an outer statement, which is identified by the label specified in the break statement. The following program, BreakWithLabelDemo, is similar to the previous one, but it searches for a value in a two-dimensional array. Two nested for loops traverse the array. When the value is found, a labeled break terminates the statement labeled search, which is the outer for loop:
public class BreakWithLabelDemo {
public static void main(String[] args) {
int[][] arrayOfInts = { { 32, 87, 3, 589 },
{ 12, 1076, 2000, 8 },
{ 622, 127, 77, 955 }
};
int searchfor = 12;
int i = 0;
int j = 0;
boolean foundIt = false;
search:
for ( ; i < arrayOfInts.length; i++) {
for (j = 0; j < arrayOfInts[i].length; j++) {
if (arrayOfInts[i][j] == searchfor) {
foundIt = true;
break search;
}
}
}
if (foundIt) {
System.out.println("Found " + searchfor + " at " + i + ", " + j);
} else {
System.out.println(searchfor + "not in the array");
}
}
}
The output of this program is:
Found 12 at 1, 0
This syntax can be a little confusing. The break statement terminates the labeled statement; it does not transfer the flow of control to the label. The flow of control transfers to the statement immediately following the labeled (terminated) statement.
The continue Statement
You use the continue statement to skip the current iteration of a for, while , or do-while loop. The unlabeled form skips to the end of the innermost loop's body and evaluates the boolean expression that controls the loop, basically skipping the remainder of this iteration of the loop. The following program, ContinueDemo, steps through a string buffer checking each letter. If the current character is not a p, the continue statement skips the rest of the loop and proceeds to the next character. If it is a p, the program increments a counter, and converts the p to an uppercase letter.
public class ContinueDemo {
public static void main(String[] args) {
StringBuffer searchMe = new StringBuffer(
"peter piper picked a peck of pickled peppers");
int max = searchMe.length();
int numPs = 0;
for (int i = 0; i < max; i++) {
//interested only in p's
if (searchMe.charAt(i) != 'p')
continue;
//process p's
numPs++;
searchMe.setCharAt(i, 'P');
}
System.out.println("Found " + numPs + " p's in the string.");
System.out.println(searchMe);
}
}
Here is the output of this program:
Found 9 p's in the string.
Peter PiPer Picked a Peck of Pickled PePPers
The labeled form of the continue statement skips the current iteration of an outer loop marked with the given label. The following example program, ContinueWithLabelDemo, uses nested loops to search for a substring within another string. Two nested loops are required: one to iterate over the substring and one to iterate over the string being searched. This program uses the labeled form of continue to skip an iteration in the outer loop:
public class ContinueWithLabelDemo {
public static void main(String[] args) {
String searchMe = "Look for a substring in me";
String substring = "sub";
boolean foundIt = false;
int max = searchMe.length() - substring.length();
test:
for (int i = 0; i <= max; i++) {
int n = substring.length();
int j = i;
int k = 0;
while (n-- != 0) {
if (searchMe.charAt(j++) != substring.charAt(k++)) {
continue test;
}
}
foundIt = true;
break test;
}
System.out.println(foundIt ? "Found it" : "Didn't find it");
}
}
Here is the output from this program:
Found it
The return Statement
The last of Java's branching statements is the return statement. You use return to exit from the current method. The flow of control returns to the statement that follows the original method call. The return statement has two forms: one that returns a value and one that doesn't. To return a value, simply put the value (or an expression that calculates the value) after the return keyword:
return ++count;
The data type of the value returned by return must match the type of the method's declared return value. When a method is declared void, use the form of return that doesn't return a value:
return;
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Summary
1. The if statement is a basic decision making tool in Java. It enables You to choose to execute a block of statements if a given logical expression has the value true. You can optionally execute another block of statements if the logical expression is false by using the else keyword.
2. You have three operations fo repeating a block of statements, a for loop, a while loop or a do-while loop.
3. You can use the switch statment to choose from a fixed number of alternatives.
4. The continue statement enables you to skip to the next iteration in the loop containing the continue statement.
5. The labeled continue statement enables you to skip to the next iteration in a loop enclosing the labeled continue that is identified by the label. The labeled loop need not be that immediately enclosing the labeled continue.
6. The break statement enables you to break out of a loop or block of statement in which it appers.
7. Three statements play a part in handling exceptions:
-
The try statement identifies a block of statements within which an exception might be thrown.
-
The catch statement must be associated with a try statement and identifies a block of statements that can handle a particular type of exception. The statements are executed if an exception of a particular type occurs within the try block.
-
The finally statement must be associated with a try statement and identifies a block of statements that are executed regardless of whether or not an error occurs within the try block.
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Review
Questions
- What is the purpose of a statement block?
- What restrictions are palced on the values of each case of a switch statement?
-
What is the purpose of the finally clause of a try-catch-finally statement?
- What Java statement is used to completely abort the execution of a loop?
a. The continue statement
b. The goto statement
c. The exit statement
d. The break statement
Answers to Questions
1. A statement block is used to organize a sequence of statements as a single statement group.
2. during compilation, the values of each case of a switch statement must evalue that can be promoted to an int value.
3. The finally clause is used to provide the capability to execute code no matter whether or not an exception is thrown or caught.
4. (d) The break statement cause a loop's execution to be terminated.
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| Sample Test
1. The following method takes a char input and returns an int value.
1. public int maze( char d ){
2. if d <= 'N' ){
3. if( d == 'E' ) return 2;
4. return 1;
5. } e1se if( d == 'S' ) return 3;
6. e1se if( d == 'W' ) return 4;
7. return 0;
8. }
Check all of the following statements about this method that are true:
a. Input of A produces output of 1.
b. Input of X produces output of 0.
c. Input of D produces output of 0.
d. The method fails to compile due to syntax errors.
2. Given the following method that will be called with various input values:
1. public void soundOff( int x ){
2. switch(x){
3. case 1: System.out.print("One ");
4. case 2: System.out.print("Two "); break ;
5. case 3: System.out.print("Three ");
6. default: System.out.print("Oo what?");
7. }
8. }
Which of these input and output pairs will be observed?
a. Input = 1, Output = "One"
b. Input = 0, Output = "One Two"
c. Input = 3, Output = "Three Do What?"
d. Input = 4, Output = "Do What?"
3. Given the following code fragment with a break to a labeled statement, predict the printed output.
1. int i, j;
2. 1ab: for( i = 0 ; i< 6 ; 1i+ ){
3. for( j = 5 ; j > 2 ; j--){
4. if( i == j){
5. System.out.print(" "+ j ); break lab ;
6. }
7. }
8. }
a. Output will be 3 4 5
b. Output will be 3 4
c. Output will be 3
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Assignment 2
Date of Sbumission :
On site class due by 02/13/2007.
On line class due by 02/27/2007.
1. What is the difference between the boolean & opetator and the && operator?
2. What is the difference between a break statement and a continue statement?
3. Write a Java statement or a set of Java statements to accomplish each of the following:
a. Sum the odd integers between 1 and 99 using a for structure. Assume the integer variables sum and count have been declared.
b. Calculate the value of 2.5 rasied to the power of 3 using the pow method.
c. Print the integers from 1 to 20 using a while loop and the counter variable x. Assume that variable x has been declared, but not initialized. Print only 5 integers per line.
Hint: Use the calculation x % 5. When the value of this is 0, print a nweline character, otherwise print a tab character. Assume this is an application - use the System.out.println( ) method to output the newline character and use the System.out.print('\t') method to output the tab character.
d. Repeat 1.(c) using a for structure.
4. The switch statement is Java's multiway branch statement. It provides an easy way to dispatch execution to different parts of your code base on the value of an expression. To sample its more realistic usage, write a console program using both switch and break statements to determine which season a particular month is in.
Hint: A String variable season is used to store each the matched season and then display it to the users. An integer variable month is used to accept the value the user given month.
PS: The Java program is to be run on an IBM PC or clone such as are available in the computing labs, using Sun Microsystems J2SDK v1.4.2_13. NetBeans IDE 5.0 (Sun Microsystems) may be acquired if desired. The Java program file with extension .java and a compiled file with .class are to be submitted as email attachments for this program. A backup copy of each program should be kept by the student. As you are aware, you are required to submit this assignment on time to avoid grade deduction for lateness.
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Required Readings
Read Chapter 3 ( PP. 99 - 117) in the text and Chapter 5 ( PP. 99 - 127) in the reference.
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