john hurley cal state la cs 201 lecture 4. if if statements do just what you expect test whether a...
TRANSCRIPT
John HurleyCal State LA
CS 201 Lecture 4
If
If statements do just what you expecttest whether a condition is true and if so, execute
some statementssyntax:if(test) {
// statements to execute is condition is true}
Example: if(x < y){
System.out.println(“x < y”);}
If
Boolean Expression
false true
Statement(s) for the false case Statement(s) for the true case
If
If there is only one statement in the block, you can omit the brackets: if(x < y)
System.out.println(“x < y”);
Note that there is no semicolon between the condition and the statements to execute!
Ifpublic class IfDemo{
public static void main(String[] args){int firstInt = 0;int secondInt = 1;
if(firstInt < secondInt) System.out.println(firstInt + " < " + secondInt);
if(firstInt == secondInt) System.out.println(firstInt + " = " + secondInt);
if(firstInt > secondInt) System.out.println(firstInt + " > " + secondInt);}
}
If / elseIf can be followed by else:
if(condition) {// statements to execute if
condition is true}
else {// statements to execute if
condition is false}
If / elseif / elseWe can also use else if, with or without a final else:
if(condition 1) {// statements to execute if condition 1 is true
}else if(condition 2) {
// statements to execute if condition 1 is false but condition 2 is true}
// can use any number of else if blocks
else {// statements to execute if condition none of
the conditions are true}
8
Multiple Alternative if Statements
if (score >= 90.0) grade = 'A'; else if (score >= 80.0) grade = 'B'; else if (score >= 70.0) grade = 'C'; else if (score >= 60.0) grade = 'D'; else grade = 'F';
Equivalent
if (score >= 90.0) grade = 'A'; else if (score >= 80.0) grade = 'B'; else if (score >= 70.0) grade = 'C'; else if (score >= 60.0) grade = 'D'; else grade = 'F';
If / elseif / elsepublic class IfElseDemo{
public static void main(String[] args){int firstInt = 5;
for(int secondInt = 0; secondInt < 10; secondInt++){
char compare;if(firstInt < secondInt) compare = '<';else if(firstInt > secondInt) compare = '>';else compare = '='; System.out.println(firstInt + " " + compare +
" " + secondInt);} // end for
} // end main()} // end class
If / else shorthandSyntax:
condition?value if condition is true:value if condition is false;
System.out.println(a<b?"true!":"false!");
This is equivalent to the following: if(a < b) System.out.println(“true”); else System.out.prinltn(“false”);
If / else shorthandpublic class ShorthandDemo{
public static void main(String[] args){int firstInt = 5;
for(int secondInt = 0; secondInt < 10; secondInt++){String comp = null;int a = 0;a=(firstInt < secondInt)?1:0;System.out.println(a);System.out.println(firstInt <
secondInt?"true!":"false!");} // end for
} // end main()} // end class
Logical OperatorsAND: &&
if(condition 1 && condition 2) {}example: if(a < b && b < 1) c = 1;example 2: if(a < b && a < 0) System.out.println(“a is
negative and less than b”);OR: ||
if(condition 1 || condition 2) {}example: if(a < b || b < 1) c = 1;
NOT: !!(a == b) is equivalent to (a != b)Can also use with more complex conditions
!((a == b) && (a == 1))!(a || b) !(a &&(b||c))
Can string any number of conditions together, but be careful not to write code that is hard to understand
Multiple Conditionspublic class MultipleConditionsDemo{
public static void main(String[] args){boolean trueCondition = true;boolean falseCondition = false;
System.out.println("True is " + (trueCondition?"true":"false"));
System.out.println("False is " + (falseCondition?"true":"false"));
System.out.println("(True and False) is "+(trueCondition&& falseCondition?"true":"false"));
System.out.println("(True or False) is "+(trueCondition||falseCondition?"true":"false"));
} // end main()} // end class
Negations
a b c b||c a &&(b||c) !(a &&(b||c))T T T T T FT T F T T FT F T T T FT F F F F TF T T T F TF T F T F TF F T T F TF F F F F T
Negationspublic class BangDemo{
public static void main(String[] args){
boolean a = true;
boolean b = true;
boolean c = true;
System.out.println(!(a &&(b||c)));
c = false;
System.out.println(!(a &&(b||c)));
b = false;
c = true;
System.out.println(!(a &&(b||c)));
b = false;
c = false;
System.out.println(!(a &&(b||c)));
Negationsa = false;
b = true;
c = true;
System.out.println(!(a &&(b||c)));
c = false;
System.out.println(!(a &&(b||c)));
b = false;
c = true;
System.out.println(!(a &&(b||c)));
b = false;
c = false;
System.out.println(!(a &&(b||c)));
} // end main()
} // end class
Nested If Blocks
One if block can be inside another oneThe inner if only runs if the condition in the
outer one evaluates to trueCan be nested to any depth, but this quickly
gets confusingWe will soon see nested loops, too!
Nested If Blocksif(condition 1) {
// statements execute if condition 1 is trueif (condition 2){
// statements execute if condition 1 and condition 2 are both true}
else {// statements execute if condition 1 is true but condition 2 is false
}
}
else{
// statements execute if condition 1 is false
}
Nested If Blockspublic class NestedIfDemo{
public static void main(String[] args){
boolean trueCondition = true;
boolean falseCondition = false;
if(trueCondition){
if(falseCondition)
System.out.println("true and false");
else
System.out.println("true and ~false");
}
else{
if(falseCondition)
System.out.println("~true and false");
else
System.out.println("~true and ~false");
}
} // end main()
} // end class
Equality with Floating Point Types
Due to the limited precision of floating point types, it is unwise to test floats and doubles with == operator:public static void main(String[] args) {
double d = 1.23456789;double e = d;e = e + 1;e = e - 1;System.out.println("d = " + d + "; e = " + e);
} // end main()
Equality with Floating Point Types
Instead of using equality test, use Math.abs as follows:Math.abs(a-b) < tolerance;
For example, if(Math.abs(grade - 3.7) < .02) {}
SwitchChooses one statement or block to execute from among several options,
based on the value of a variable
switch (status) { case 0: compute taxes for single filers; break; case 1: compute taxes for married file jointly; break; case 2: compute taxes for married file separately; break; case 3: compute taxes for head of household; break; default: System.out.println("Errors: invalid status"); System.exit(0);}
23
switch Statement Flow Chart
status is 0 Compute tax for single filers break
Compute tax for married file jointly break status is 1
Compute tax for married file separatly break status is 2
Compute tax for head of household break status is 3
Default actions default
Next Statement
24
switch Statement Rules
switch (switch-expression) {
case value1: statement(s)1;
break;
case value2: statement(s)2;
break;
…
case valueN: statement(s)N;
break;
default: statement(s)-for-default;
}
The switch-expression must yield a value of char, byte, short, or int type and must always be enclosed in parentheses.
The value1, ..., and valueN must have the same data type as the value of the switch-expression. The resulting statements in the case statement are executed when the value in the case statement matches the value of the switch-expression. Note that value1, ..., and valueN are constant expressions, meaning that they cannot contain variables in the expression, such as 1 + x.
25
switch Statement Rules
The keyword break is optional, but it should be used at the end of each case in order to terminate the remainder of the switch statement. If the break statement is not present, the next case statement will be executed.
switch (switch-expression) {
case value1: statement(s)1;
break;
case value2: statement(s)2;
break;
…
case valueN: statement(s)N;
break;
default: statement(s)-for-default;
}
The default case, which is optional, can be used to perform actions when none of the specified cases matches the switch-expression. The case statements are executed in sequential
order, but the order of the cases (including the default case) does not matter. However, it is good programming style to follow the logical sequence of the cases and place the default case at the end.
26
Trace switch statement
switch (ch) { case 'a': System.out.println(ch); case 'b': System.out.println(ch); case 'c': System.out.println(ch);}
Suppose ch is 'a':
animation
27
Trace switch statement
switch (ch) { case 'a': System.out.println(ch); case 'b': System.out.println(ch); case 'c': System.out.println(ch);}
ch is 'a':
animation
28
Trace switch statement
switch (ch) { case 'a': System.out.println(ch); case 'b': System.out.println(ch); case 'c': System.out.println(ch);}
Execute this line
animation
29
Trace switch statement
switch (ch) { case 'a': System.out.println(ch); case 'b': System.out.println(ch); case 'c': System.out.println(ch);}
Execute this line
animation
30
Trace switch statement
switch (ch) { case 'a': System.out.println(ch); case 'b': System.out.println(ch); case 'c': System.out.println(ch);}
Execute this line
animation
31
Trace switch statement
switch (ch) { case 'a': System.out.println(ch); case 'b': System.out.println(ch); case 'c': System.out.println(ch);}
Next statement;
Execute next statement
animation
32
Trace switch statement
switch (ch) { case 'a': System.out.println(ch); break; case 'b': System.out.println(ch); break; case 'c': System.out.println(ch);}
Suppose ch is 'a':
animation
33
Trace switch statement
switch (ch) { case 'a': System.out.println(ch); break; case 'b': System.out.println(ch); break; case 'c': System.out.println(ch);}
ch is 'a':
animation
34
Trace switch statement
switch (ch) { case 'a': System.out.println(ch); break; case 'b': System.out.println(ch); break; case 'c': System.out.println(ch);}
Execute this line
animation
35
Trace switch statement
switch (ch) { case 'a': System.out.println(ch); break; case 'b': System.out.println(ch); break; case 'c': System.out.println(ch);}
Execute this line
animation
36
Trace switch statement
switch (ch) { case 'a': System.out.println(ch); break; case 'b': System.out.println(ch); break; case 'c': System.out.println(ch);}
Next statement;
Execute next statement
animation
Command Line Input
There are several ways to get input from a command line
In production, you will usually write programs that use GUIs, not command line I/O
In school most programming classes focus on functionality, not user interface, so you need to know how to use command line I/O
Command Line Input
The simplest command line input class is Scanner
import java.util.Scanner;Scanner has a variety of methods to get input of different data types
Scanner Input MethodsWe describe methods using in this format:
Class.method()If there are any parameters, their type goes inside
the parenthesesYou have already seen
System.out.println(String)You will often replace the class name with the
name of an instance of the class:Scanner input = new Scanner(System.in);… stuff deleted…name = input.next();
In the example above, input is an instance of Scanner.We set up a Scanner and called it input!
Scanner Input Methods
Scanner.next() reads the next parsable String
Scanner.nextLine() reads up to the next line break and puts the result in a String
Scanner.nextDouble() reads the next parseable string and tries to convert it to a Double double d = Scanner.nextDouble();
There are equivalent methods for nextInteger(), nextBoolean(), etc.
Scanner.next Examplepackage demos;
import java.util.Scanner;
public class InputDemo {public static void main(String[] args) {
Scanner input = new Scanner(System.in);String name = null;
System.out.println("Guess my name:");name = input.next();if (name.equals("Rumpelstiltskin"))
System.out.println("Correct! Continue spinning straw into gold!");
elseSystem.out.println("Wrong! I will now eat your
children!");}
}
Scanner.nextDouble Exampleimport java.util.Scanner;
public class ReadDouble {
public static void main(String[] args) {Scanner input = new Scanner(System.in);double myDouble = 0.0;
do {System.out.print("Input a double:");myDouble = input.nextDouble();System.out.println("\nYou entered: " +
myDouble);
} while (stuff != 0.0);
}}
Switchpublic static void main(String[] args) {
Scanner sc = new Scanner(System.in);
char ageCat = 'y';
while (ageCat != 'q') {
System.out
.println("Welcome to John's bar. Please enter your age category: \ny for under 21, m for 21-29, o for 30 and older. Enter q to quit.");
String input = sc.next();
ageCat = (char) input.charAt(0);
switch (ageCat) {
case 'y':
System.out.println("Enjoy your Shirley Temple, Junior");
break;
case 'm':
System.out.println("You'd better stick to beer");
break;
case 'o':
System.out.println("There's whiskey in the jar.");
break;
case 'q':
break;
default:
System.out.println("Invalid input");
} // end switch
} // end while
} // end main()
Eclipse IDE
44
IDE = Integrated Development EnvironmentAn IDE provides services that make it easier for you to program
Editor with syntax checking, automatic formatting, etcOne-step compile and runDebuggingOrganization
Eclipse IDE
45
• The IDE most often used for Java programming is called Eclipse. Eclipse is the standard IDE at CSULA.
• Eclipse supports many different programming languages with available plug-ins, but it is mostly used for Java
• Eclipse is open-source; you can get it at www.eclipse.org• Get the “Eclipse IDE for Java Developers”
• Others often used with Java include NetBeans, JBuilder, many others
Eclipse IDE
46
IDE = Integrated Development EnvironmentAn IDE provides services that make it easier for
you to programEditor with syntax checking, automatic formatting,
etcOne-step compile and runDebuggingOrganization
Eclipse IDE
47
The most widely used IDE at CSULA is Eclipse. Eclipse supports many different programming
languages with available plug-ins, but it is mostly used for Java
Eclipse is open-source; you can get it at www.eclipse.orgGet the “Eclipse IDE for Java Developers”
Others often used with Java include NetBeans, JBuilder, many others
Eclipse IDE
48
The current general-release version of Eclipse predates the most recent update to the JDK. If you don’t already have Eclipse working on your home computer, you may deal with this in one of two ways:Get the JDK 7 instead of 8, ORGet the JDK 8 and then follow the easy
instructions at https://wiki.eclipse.org/JDT/Eclipse_Java_8_Support_For_Kepler
Eclipse IDE
49
Create a new project in Eclipse and write a class
Eclipse: Create a Project
Add a Code Package
Add a class
Write code in the class
Run the project
At least one class must have a main()!
“Run Configurations” is critical
“Window/Preferences” contains many settings