03 basic syntax

7/31/2019 03 Basic Syntax

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2012 Marty Hall

Originals of Slides and Source Code for Examples:

-. . .

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2012 Marty Hall

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organization. Contact [email protected] for details.


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Topics in This Section

Basics rea ng, comp ng, an execu ng s mp e ava programs

Accessing arrays Loo in Indenting Code Using if statements ompar ng s r ngs Building arrays

- Two-step process

Using multidimensional arrays er orm ng as c ma ema ca opera ons Reading command-line input

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Basics




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Eclipse: Making Projects

Creating new project File New Project

Java Java Project Pick an name To simplify applets later,

choose Sources/Classesin same folder

Creating new class R-click New Class

You can have Eclipse makemain when class created,or use shortcut to insert it later

You can also copy/paste existingclass, then give it new name

Getting Started: Syntax Example

public class HelloWorld { public static void main(String[] args) {

System.out.println("Hello, world.");

}

Details Processing starts in main Eclipse can create main automatically

When creating class: choose main as option Eclipse shortcut inside class: type main then hit Control-space

Routines usually called methods, not functions. . . ...

System.out.println, System.out.print, System.out.printf Eclipse shortcut: type sysout then hit Control-space

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Command-line Arguments

Are useful for learning and testing Command-line args are helpful for practice

But, programs given to end users should almost never use-

They should pop up a GUI to collect input.

Eclipse has poor support Entering command-line args via Eclipse is more trouble

than it is worth , -

Save the file in Eclipse (causing it to be compiled) Navigate to folder on desktop (not within Eclipse) Open command window (Run cmd) Type java Classname arg1 arg2

Example: Command Line Args

File: ShowTwoArgs.java (nave version)

public class ShowTwoArgs {

System.out.println("First arg: " +args[0]);

ys em.ou .pr n n econ arg:args[1]);

}}

Oops! Crashes if there are less than two command-line arguments. The code should have checked the length field, like this:if (args.length > 1) {

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doThePrintStatements();} else {

giveAnErrorMessage();}


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Example (Continued)

Compiling ( automatic on save in Eclipse )DOS> javac ShowTwoArgs.java

Manual executionava ow wo rgs e o ass

First args Hello

DOS> java ShowTwoArgs[Error message]

Eclipse execution (cumbersome) To assign command line args: R-click, Run As,

Run Configurations, click on Arguments tab13

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Loops




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Looping Constructs

for/each

body;

} for

for(init; continueTest; updateOp) { body;

} while

while (continueTest) { body;

} do

do {o y;

} while (continueTest);

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For/Each Loops public static void listEntries(String[] entries) {

System.out.println(entry);

}}

ResultString[] test = {"This", "is", "a", "test"};listEntries(test);

Thisisatest

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For Loops

public static void listNums1(int max) {or n = ;


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Do Loops

public static void listNums3(int max) {n = ;

do {S stem.out. rintln "Number: " + i ;i++;

} while (i < max);Dont orget sem co on

}

ResultlistNums3(3);

Number: 0 Number: 1 Number: 219

2012 Marty Hall

Formattin




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Defining Multiple Methods in

public class LoopTest { pu c s a c vo ma n r ng args

String[] test ={ "This", "is", "a", "test"};listEntries(test);listNums1(5);

listNums3(7);}

These methods say static because they are called directly from main.In the next two sections on OOP, we will explain what static means andwh most re ular methods do not use static. But for now, ust note that

public static void listEntries(String[] entries) {} public static void listNums1(int max) {}

methods that are directly called by main must say static.

public static void listNums2(int max) {} public static void listNums3(int max) {}

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Indentation: blocks that are nested Yes No

blah; blah;

for(...) {

blah;

for(...) {

blah; blah;

for(...) { blah;

for(...) {a ;

blah;a ;

blah;

} }22


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2012 Marty Hall

Strin s



If Statements Single option

if (boolean-expression) {statement1 ;...statement N ;

} w

if (boolean-expression) {...

} else {...

} Multiple options

if (boolean-expression) {...

The value inside parens must strictly be boolean, unlikeC, C++, and JavaScript.

A widely accepted best practice is to use the braces evenif there is onl a sin le statement inside the if or else.

} else if (boolean-expression) {...

} else if (boolean-expression) {...

} else {...

}

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Example: If Statements

pu c stat c nt max nt n , nt n

if (n1 >= n2) {return n1

} else {return(n2);

}}

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Strings Basics

String is a real class in Java, not an array of characters as in Cand C++.

The Strin class has a shortcut method to create a new ob ect: just use double quotes

This differs from normal objects, where you use the new

construct to build an object Use equals to compare strings

Never use == to test if two Strings have same characters! any use u u n me o s

contains, startsWith, endsWith, indexOf,substring, split, replace, replaceAll

Note: can use regular expressions, not just static strings toUpperCase, toLowerCase, equalsIgnoreCase

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Common String Error:==

public static void main(String[] args) {r ng ma c = es ;

if (args.length == 0) {System.out.println("No args");} else if (args[0] == match) {

System.out.println("Match");

System.out.println("No match");}

}

Prints "No match" for all inputs Fix:

if (args[0] .equals (match))31

2012 Marty Hall

Arrays




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Building Arrays:-

Declare and allocate array in one fell swoop

type [] var = { val1, val2, ... , val N };

Examples:= , ,

String[] names = {"Joe", "Jane", "Juan"};Point[] points = { new Point(0, 0),

new Point(1, 2),new Point(3, 4) };

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Building Arrays:

Step 1: allocate an array of references: ype var = new ype s ze ;

E.g.:int[] primes = new int[7];String[] names = new String[someArray.length];

Step 2: populate the array= = " "

primes[1] = 3; names[1] = "Jane"; primes[2] = 5; names[2] = "Juan";

" " etc.

If you fail to populate an entry Default value is 0 for numeric arrays Default value is null for object arrays

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Two-Step Process: Examples public static Circle[] makeCircles1(int numCircles) {

Circle[] circles = new Circle[numCircles]; // Empty array of proper sizefor(int i=0; i


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Multidimensional Arrays

Multidimensional arrays Implemented as arrays of arrays

int[][] twoD = new int[64][32];

String[][] cats = {{ "Caesar", "blue-point" },{ "Heather", "seal-point" },

" " " - ",

Note: um er o e emen s n eac row nee no e equa

int[][] irregular = { { 1 }, , , ,{ 5 },{ 6, 7 } };

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TriangleArray: Example public class TriangleArray {

int[][] triangle = new int[10][];

for(int i=0; i


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TriangleArray: Result

> java TriangleArray

0000000000

000000000000000000000000000000

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Math and Input




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Wrap-Up



Summary Basics

oops, con t ona statements, an array access s s m ar to C and C++

But new for loop: for(String s: someStrings) { } Indent your code for readability String is a real class in Java

Use equals , not == , to compare strings Allocate arrays in one step or in two steps

If two steps, loop down array and supply values se a . a or s mp e ma opera ons Simple input from command window

Use Scanner to read values after prompts

Neither is very important for most real-life applications46


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2012 Marty Hall

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