Java Basics (continued) Mr. Smith AP Computer Science A.

Java Basics(continued)

Mr. SmithAP Computer Science

A

Basic Java Syntax and Semantics

Methods, Messages, and Signatures Classes implement methods, and objects are

instances of classes. Objects that share common behavior are

grouped into classes. An object responds to a message only if its

class implements a corresponding method. Messages are sometimes accompanied by

parameters and sometimes not:pen.move(); // No parameter expected

pen.moveDown(8); // One parameter expected

Java Concepts 2.3 (Objects, Classes, and Methods), 2.4 (Method Parameters and return values)

Basic Java Syntax and Semantics Some methods return a value and

others do not. To use a method successfully we must

know: What type of value it returns It’s name (identifier) The number and type of the parameters

it expects This information is called the method’s

signature.

Java Concepts 2.3 (Objects, Classes, and Methods), 2.4 (Method Parameters and return values)

Basic Java Syntax and Semantics

Programming Protocols: Use camelCase

When forming a compound variable name, programmers usually capitalize the first letter of each word except the first. (For example: taxableIncome)

All the words in a class’s name typically begin with a capital letter (EmployeePayroll).

Constant names usually are uppercase and words are separated with underlines (INCOME_TAX_RATE).

Java Concepts 2.1 (Types and Variables)

Comments Comments are explanatory sentences

inserted in a program in such a matter that the compiler ignores them.

There are two styles for indicating comments:

Single line comments:

These include all of the text following a double slash (//) on any given line; in other words, this style is best for just one line of comments

Multiline comments:

These include all of the text between an opening /* and a closing */

Java Concepts 1.6 (Compiling a Simple Program)

Additional OperatorsExtended Assignment Operators

The assignment operator = can be combined with the arithmetic and concatenation operators to provide extended assignment operators. For example:

int a = 9;

String s = "hi";

a += 3; // Equivalent to a = a + 3;

a -= 3; // Equivalent to a = a – 3;

a *= 3; // Equivalent to a = a * 3;

a /= 3; // Equivalent to a = a / 3;

a %= 3; // Equivalent to a = a % 3;

s += " there"; // Equivalent to s=s + " there";

Java Concepts 4.3 (Assignment, Increment, and Decrement)

Additional Operators

Increment and Decrement Java includes increment (++) and

decrement (--) operators that increase or decrease a variables value by one:

int m = 7;

double x = 6.4;

m++; // Equivalent to m = m + 1;

x--; // Equivalent to x = x – 1.0;

The precedence of the increment and decrement operators is the same as unary plus, unary minus, and cast.

Java Concepts 4.3 (Assignment, Increment, and Decrement)

Standard Classes and Methods

Eight methods in the Math Class:

Java Concepts 4.4 (Arithmetic Operations and Mathematical Functions)

static double random() Returns a double in the range [0.0, 1.0)


Using the Math classdouble absNum, powerNum, randomNum, sqrtNum;

absNum = Math.abs(-30);powerNum = Math.pow(-3, 3);randomNum = Math.random();sqrtNum = Math.sqrt(25.0);

Results:absNum has a value of 30.0 //Absolute value of -30powerNum has a value of -27.0 //-3 to the 3rd powerrandomNum has a value of ??? //Random number between

//0 (inclusive) and 1 (exclusive)sqrtNum has a value of 5.0 //Square root of 25.0

Java Concepts 4.4 (Arithmetic Operations and Mathematical Functions)


Random Numbers and Simulation The Random class of the Java library implements a

random number generator. To generate random numbers, you construct an object of

the Random class and then apply one of the following methods:

nextInt(n) – returns a random integer between 0 (inclusive) and n (exclusive)

nextDouble() – returns a random floating point number between 0 (inclusive) and 1 (exclusive)

For example, if a contestant on “Deal or No Deal” randomly selects their briefcase from the 26 initial briefcases:

import java.util.Random; //Include this at top of program Random generator = new Random();int briefcaseNum = generator.nextInt(26) + 1; Java Concepts 6.5 (Random Numbers and Simulation)

DiceRollerDiceRoller (object class) and DiceRollerViewer client class:

Write a DiceRoller class (similar to Yahtzee) to: Allow the person to initially roll five dice.

Each die has six faces representing numbers from 1 to 6, so the program should randomly pick a number between 1 and 6.

Print the results of each die to the console. After the first roll, the person should input the number of dice (between 0 and 5) that they

want to roll again. If they enter 0, then don’t roll the dice again. Else, roll that many dice and print the results of each die to the console again.

Repeat the prompt one more time to see if the person wants to roll some of the dice again (for a maximum of 3 rolls).

Write a DiceRollerGame client class to: Instantiate a DiceRoller object Run a method in DiceRoller (such as playGame() ) to play the game.

EXTRA CREDIT (5 points): Have the player continue to take a turn (3 rolls) until they want to stop.

HINT: You may want to use: the Scanner class to input data, the Random class to generate a random number to simulate rolling a die, a while loop for rolling each of the 6 dice, an if statement for checking the input, System.out.print for printing to the console. Java Concepts 6.5 (Random Numbers and Simulation)

Control Statements

while (some condition){ do stuff;}

Do stuff repeatedly as long as the condition holds true

if (some condition){ do stuff 1;}else{ do stuff 2;}

If some condition is true, do stuff 1, and if it is false, do stuff 2.

While and if-else are called control statements.

Java Concepts 5.1 (The if Statement), 6.1 (while Loops)

The if and if-else Statements

Principal Forms In Java, the if and if-else statements allow for

the conditional execution of statements.if (condition){ statement1; statement2;}

if (condition){ statement1; statement2;}else{ statement3; statement4;}

Java Concepts 5.1 (The if Statement)


Relational Operators

The complete list of relational operators available for use in Java:

Java Concepts 5.2 (Comparing values)


Relational Operators

Which of the following if statements are incorrect?

int x;

1. if ( x > 10 )

2. if ( x = 10 )

3. if ( x >< 10 )

4. if ( x == 10 )

5. if ( x >= 10 )

6. if ( x != 10 )

7. if ( !x = 10 )

Incorrect

Incorrect

Java Concepts 5.2 (Comparing values)

Incorrect

The while Statement The while statement provides a looping mechanism that

executes statements repeatedly for as long as some condition remains true.

while (condition) // loop teststatement; // one statement inside the loop body

while (condition) // loop test{ statement; // many statements statement; // inside the ... // loop body}

Java Concepts 6.1 (while Loops)

The while Statement

Common Structure Loops typically adhere to the following

structure:1) initialize variables2) while (condition) // perform test {3) Loop body - perform calculations or do something4) Loop body - change variables involved in the

condition }

In order for the loop to terminate, each iteration through the loop must move variables involved in the condition closer to satisfying the condition. Java Concepts 6.1 (while Loops)

The while Statement

1. Write a while loop that will sum the numbers from 1 to 100 (i.e. 1+2+3+… +100).

2. Print the answer to the console.


The while Statement

int sum = 0;

int count = 1;

while (count <= 100)

{

sum += count;

count++;

}

System.out.println (sum);


The for Statement The for statement combines counter

initialization, condition test, and update into a single expression.

The form for the statement:for (initialize counter; test counter; update counter)

statement; // one statement inside the loop body

for (initialize counter; test counter; update counter){

statement; // many statements statement; // inside the

. . .; // loop body }

Java Concepts 6.2 (for Loops)

The for StatementDeclaring the Loop Control Variable in a for Loop.

The for loop allows the programmer to declare the loop control variable inside or outside of the loop header.

The following are equivalent loops that show these two alternatives:

int i; //Declare control variable above loop

for (i = 1; i <= 10; i++){

System.out.println(i); }

for (int i = 1; I <= 10; i++) //Declare var. in loop {

System.out.println(i);}

Use this technique if you need to

reference the value of i later

Use this technique if you only use variable

i within the loopJava Concepts 6.2 (for Loops)

the break Statement

The break statement can be used for breaking out of a loop early (before the loop condition is false).

break statements can be used similarly with both for loops and while loops (break terminates the loop immediately).

int sum = 0;

for (int i = 1; i<=50; i++){ sum += (i * 2);

if (sum > 100) break;}

Java Concepts 6.4 Advanced Topic (break and continue Statements)

Nested Control Statements and the break Statement Control statements can be nested inside each other

in any combination that proves useful. If you are nesting loops, the break statement only

breaks out of the loop the program is currently in (the inside loop).

int sum= 0;for (int i = 1; i<=100; i++){ for (int j = 1; j<=100; j++)

{ sum += (i * j);

if (sum > 100) break;

} System.out.println(“The sum is “ + sum);

sum = 0;}

Java Concepts 6.4 Advanced Topic (break and continue Statements)

Breaks out of this loop (not the outside loop)

Sentinel

A sentinel is a data value that is used to denote the end of a data list or data input.

The value cannot be a valid data value Usually 99999 or an unreachable high value or it could be a character

such as “Q” for Quit. Note: if you want the sentinel value to be “Q” then the value being entered needs to be a string.

Example: Sum up the ages of your family members:

int ageSum = 0;while (true){ System.out.println(“Enter age of family member (or 999 to quit)”); int age = in.nextInt(); if (age == 999) break; ageSum += age;}System.out.println(“Total age of all family members is “ + ageSum);

Java Concepts 6.4 (Processing Sentinel Values)

Sentinel

Write a segment of code that will have the user input their grades and print the average afterwards. The user should enter a sentinel value when there are no more grades to enter.


Sentinel

Write a segment of code that will have the user input their grades and print the average afterwards. The user should enter a sentinel value when there are no more grades to enter.

POSSIBLE OPTION:

int sum = 0, numGrades = 0, grade;while (true)

{ System.out.print(“Enter a grade or 999 to end “);

grade = in.nextInt(); if (grade == 999)

break;sum += grade;numGrades += 1;

}System.out.println(“Average is “ + (sum / numGrades );


• Missing parentheses• Need to convert to a double• Possible Divide by Zero error

What bugs do you see in this line of code?

CircleCalc

CircleCalc and CircleCalcViewer classes:Write a class to: Have the user input the diameter of a circle (floating point

number) and the program outputs the: Radius (how do you get the radius if you know the diameter?) Circumference Area Note: you should use Math.PI in these calculations

Use a sentinel controlled loop (sentinel value should be 99999) to keep asking the user the diameter of different circles.

Test input for validity. EXTRA CREDIT (5 points): See next page

What is the advantage of using a sentinel controlled loop?It allows the user to stop the looping whenever they want

CircleCalc Extra Credit(5 extra points)

CircleComponent and CircleGraphicalViewer classes:Write classes to: In the client class, have the user input the diameter of a

circle (floating point number) and instantiate a component object to:

Pass the diameter to a Component object

Calculate radius, circumference, and area (Note: you should use Math.PI in these calculations)

Draw the circle in a frame and print the calculated values beside it. The diameter of the circle (in pixels) should match what the user entered.

Use a sentinel controlled loop (sentinel value should be 99999) to keep asking the user the diameter of different circles.

Test input for validity.

Java Basics (continued) Mr. Smith AP Computer Science A.

Documents

Transcript of Java Basics (continued) Mr. Smith AP Computer Science A.