Java Concepts - Java Basics

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

Basic Java Syntax and Semantics Methods, Messages, and Signatures 

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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. To correspond the method must have the same name as the message. 

Messages are sometimes accompanied by parameters and sometimes not: pen.move(); pen.moveDown(8);

// No parameter expected // One parameter expected

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

Basic Java Syntax and Semantics 

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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 program’s name

typically begin with a capital letter (ComputeEmployeePayroll).

 Constant names usually are all

uppercase (TAX_RATE).

Java Concepts 2.1 (Types and Variables)

Comments 

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Comments are explanatory sentences inserted in a program in such a matter that the compiler ignores them. There are two styles for indicating comments:  End of 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 Operators Extended Assignment Operators 

The assignment operator can be combined with the arithmetic and concatenation operators to provide extended assignment operators. For example: int a = 17; String s = "hi"; a += 3; a -= 3; a *= 3; a /= 3; a %= 3; s += " there"; there”;

// Equivalent to a = a + 3; // Equivalent to a = a – 3; // Equivalent to a = a * 3; // Equivalent to a = a / 3; // Equivalent to a = a % 3; // Equivalent to s = s + “ 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;

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

Java Concepts 4.3 (Assignment, Increment, and Decrement)

Standard Classes and Methods Eight methods in the Math Class

static double random()

Returns a double in the range [0.0, 1.0) Java Concepts 4.4 (Arithmetic Operations and Mathematical Functions)

Standard Classes and Methods Using the Math class double absNum, powerNum, randomNum; absNum = Math.abs(-30); powerNum = Math.pow(-3, 3); randomNum = Math.random(); Results: absNum = 30 powerNum = -27 randomNum = ????? and 1

//Absolute value of -30 //-3 to the 2nd power //Random number between 0

Java Concepts 4.4 (Arithmetic Operations and Mathematical Functions)

Standard Classes and Methods Random Numbers and Simulation 

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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:  

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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” is randomly selecting their briefcase from the 26 initial briefcases: import java.util.Random; //Include this at top of program Random generator = new Random(); int briefcaseNum = 1 + generator.nextInt(26); Java Concepts 6.5 (Random Numbers and Simulation)

Classwork/Homework DiceRoller program (similar to Yahtzee): Write a program to:  Allow the person to initially roll five dice. Each die has six faces representing numbers from 1 to 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 stop. 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). Note: You may want to use the Scanner class to input data, the Random class to generate a random number, a for loop for rolling the dice, an if statement for checking the input and a break command to exit the 

Java Concepts 6.5 (Random Numbers and Simulation)

Control Statements While and if-else are called control statements. while (some condition) { do stuff; }

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

Do stuff repeatedly as long as the condition holds true

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

The if and if-else Statements Relational Operators The complete list of relational operators available for use in Java.

Java Concepts 5.2 (Comparing values)

The if and if-else Statements Relational Operators Which of the following if statements are incorrect? int x;

     

if if if if if if

( ( ( ( ( (

x x x x x x

> 10 ) = 10 ) >< 10 ) == 10 ) >= 10 ) != 10 )

Incorrect Incorrect

Java Concepts 5.2 (Comparing values)

The while Statement 

The while statement provides a looping mechanism that executes statements repeatedly for as long as some condition remains true. while (condition) statement; the loop body

// loop test //one statement inside

while (condition) { statement; statement; ... }

// loop test // many statements // inside the // loop body

Java Concepts 6.1 (while Loops)

The while Statement Common Structure 

Loops typically adhere to the following structure: initialize variables

//initialize //test

while (condition)

{ perform calculations or do something and

//loop

body change variables involved in the condition

//loop

body } 

In order for the loop to terminate, each iteration through the loop must move

Java Concepts 6.1 (while Loops)

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

Java Concepts 6.1 (while Loops)

The while Statement int sum = 0; int count = 1; while (count <= 100) { sum += count; count++; } System.out.println (sum); Java Concepts 6.1 (while Loops)

The for Statement 

The for statement combines counter initialization, condition test, and update into a single expression.

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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 Statement Declaring the Loop Control Variable in a for Loop. 

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The for loop allows the programmer to declare the loop control variable inside of the loop header. The following are equivalent loops that show Use this technique if these two alternatives: int i; loop

you need to //Declare control variable above reference the value of i later

for (i = 1; i <= 10; i++) System.out.println(i);

Use this technique if you only use variable i within the loop Java Concepts (for Loops) for (int i = 1; i<= 10; i++) //Declare variable in6.2loop

Nested Control Statements and the break Statement 

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Control statements can be nested inside each other in any combination that proves useful. The break statement can be used for breaking out of a loop early, that is before the loop condition is false. If you are nesting loops, the break statement only breaks out of the loop the program is currently in (the inside loop). break statements can be used similarly with both for loops and while loops (break terminates the loop immediately). Java Concepts 6.4 Advanced Topic (break and continue Statements)

Sentinel 

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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. Example: Enter your age or type “Q” to quit. Note: if you do it this way then the value you enter needs to be a string. Java Concepts 6.4 (Processing Sentinel Values)

Sentinel Write a segment of code that will have the user input data values and then do something until the user indicates there are no more data values.

Java Concepts 6.4 (Processing Sentinel Values)

Sentinel Write a segment of code that will read in data values and do something until there are no more. OPTION 1: int num; boolean done = false; while ( ! done) { System.out.print(“enter a value, 99 to end “); num = in.nextInt(); if (num == 99) { done = true; } else { // rest of code to do something goes here } } Java Concepts 6.4 (Processing Sentinel Values)

Sentinel Write a segment of code that will read in data values and do something until there are no more. OPTION 2: int num; while (true) { System.out.print(“enter a value, 99 to end “); num = in.nextInt(); if (num == 99) { break; } // rest of code to do something goes here } Java Concepts 6.4 (Processing Sentinel Values)

Classwork/Homework CircleCalc program: Write a program to:  Have the user input the diameter of a circle and the program outputs the:  Radius (which is the diameter divided by 2)  Circumference (which is PI times the diameter)  Area (which is PI times the radius squared)  Note: you can use Math.PI to get the value of PI 

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Use a for loop to prompt the user 4 times (4 circle diameters) and print the radius, circumference, and area each time. Then use a sentinel controlled loop (sentinel value should be 99999) to do the same thing. Test input for validity.

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