Lesson5 Objects_Strings

8/7/2019 Lesson5 Objects_Strings

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Lesson5: JAVA

INTRODUCTION TO JAVA PROGRAMMING,EHSAN CHAUHDRY

Objects and Strings

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Objects-I

• So far, we have seen:

methods, which represent behavior

variables, which represent data

types, which represent categories ofdata




Objects-II

• In Java and other "object-oriented" programminglanguages, it is possible to create new types that arecombinations of the existing primitive types.– Such types are called object types or reference

types.– An object is an entity that contains data and behavior.

• There are variables inside the object, storingits data.

• There are methods inside the object,

representing its behavior.

• how to communicate with certain objects that exist inJava.


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Constructing objects

• construct: To create a new object.– Objects are constructed with the new keyword.

– Most objects other than Strings must be constructed before they canbe used.

• Constructing objects, general syntax:<Object type> < Object name> = new < Object type> ( <parameters> );

– Examples:

– SimpleInput keyword = new SimpleInput();

DrawingPanel panel = new DrawingPanel(300,200);




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Reminder: primitive variables

• We now need to examine some important differencesbetween the behavior of objects and primitive values.

• We saw with primitive variables that modifying the value of one variable does not modify the value of another.

• When one variable is assigned to another, the value iscopied.– Example:

int x = 5;

int y = x; // x = 5, y = 5y = 17; // x = 5, y = 17

x = 8; // x = 8, y = 17




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Reference variables

• However, objects behave differently than primitives.– When working with objects, we have to understand the distinction

between an object, and the variable that stores it.

– Variables of object types are called reference variables.

– Reference variables do not actually store an object; they store the

address of an object's location in the computer memory.– If two reference variables are assigned to refer to the same object, the

object is not copied; both variables literally share the same object.Calling a method on either variable will modify the same object.

– Example:

DrawingPanel p1 = new DrawingPanel(80, 50);

DrawingPanel p2 = p1; // same window

p2.setBackground(Color.CYAN);




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References example

• When panel2 refers to the same object as panel1,modifying either variable's background color will affect thesame window:DrawingPanel panel1 = new DrawingPanel(80,50);

DrawingPanel panel2 = new DrawingPanel(80,50);

DrawingPanel panel3 = new DrawingPanel(80,50);

panel1.setBackground(Color.RED);panel2.setBackground(Color.GREEN);

panel3.setBackground(Color.BLUE);

panel2 = panel1;panel2.setBackground(Color.MAGENTA);




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Modifying parameters

• When we call a method and pass primitive variables' values as parameters, it islegal to assign new values to the parameters inside the method.

– But this does not affect the value of the variable that was passed, because its valuewas copied.

– Example:

public static void main(String[] args) {

int x = 1;

foo(x);

System.out.println(x); // output: 1

}

public static void foo(int x) {

x = 2;

}

value 1 is copied into parameter

parameter's value is changed to 2

(variable x in main is unaffected)INTRODUCTION TO JAVA PROGRAMMING,EHSAN CHAUHDRY



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Objects as parameters

• When an object is passed as a parameter, it is not copied.It is shared between the original variable and the method'sparameter.– If a method is called on the parameter, it will affect the original

object that was passed to the method.–

Example:public static void main(String[] args) {DrawingPanel p = new DrawingPanel(80,

50);p.setBackground(Color.YELLOW);foo(p);

}

public static void foo(DrawingPanel panel) {panel.setBackground(Color.CYAN);

}




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Strings

• One of the most common types of objects in Java is type String.– String: A sequence of text characters.– Object types' names are usually uppercase (String), unlike primitives

(int).

•

String variables can be declared and assigned, just like primitivevalues:– String <name> = "<text>";– String <name> = <expression that produces a String>;

– Examples:String name = "Marla Singer";

int x = 3, y = 5;String point = "(" + x + ", " + y + ")";




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Indexes

• The characters in a String are each internally numberedwith an index , starting with 0 for the first character:– Example:String name = "M. Stepp";

name -->

• Individual text characters are represented by a primitive

type called char. Literal char values are surrounded withapostrophe (single-quote) marks, such as 'a' or '4'.– An escape sequence can be represented as a char, such as'\n' (new-line character) or '\'' (apostrophe).

0 1 2 3 4 5 6 7

'M' '.' ' ' 'S' 't' 'e' 'p' 'p'




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Calling methods of Strings

• Strings are objects that contain methods.– A String contains code inside it that can manipulate or process the

String in several useful ways.– When we call a method of a String, we don't just write the method's

name. We also have to write which String we want to execute themethod. The results will be different from one String to another.

• Calling a method of an object, general syntax:<name> . <methodName> ( <parameters> )

– Examples:String name = "Marty";

System.out.println(name.toUpperCase()); //MARTY

String name2 = "Marty Stepp";System.out.println(name2.length()); //11




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String methods

• Here are several of the most useful String methods:

Method name Description

charAt(index ) character at a specific index

indexOf(String) index where the start of the given Stringappears in this String (-1 if it is not there)

length() number of characters in this String

substring(index1, index2) the characters from index1 to just before

index2toLowerCase() a new String with all lowercase letters

toUpperCase() a new String with all uppercase letters




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String method examples

// index 012345678901

String s1 = "Stuart Reges";

String s2 = "Marty Stepp";

System.out.println(s1.length()); // 12

System.out.println(s1.indexOf("e")); // 8

System.out.println(s1.substring(1, 4)); // tua

String s3 = s2.toUpperCase();

System.out.println(s3.substring(6, 10)); // STEP

String s4 = s1.substring(0, 6);System.out.println(s4.toLowerCase()); // stuart




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Methods that return values

• The methods of String objects do not print their resultsto the console.– Instead, a call to one of these methods can be used as an

expression or part of an expression.

• Recall: return value: A value that is produced by a call to amethod, and can be used in an expression.– Return values are the opposite of parameters. Parameters pass

information inward into a method from the caller. Return valuesgive information outward from the method to the caller.

– The methods of String objects produce (or return) a result whichis either a new String or a number, depending on the method.

– The result can be used in a larger expression, stored in avariable, or printed to the console.




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Return values example

String str = "Janet Smith";int len = 2 * str.length() + 1;System.out.println(len);// 23

String first = str.substring(0, 5);

System.out.println("first name is " + first);// Janet

• What expression would produce the first letter of theString? The last letter?

• What expression would trim any String, not just the oneabove, to its first word?– Does our answer assume anything about the letters in the

String?




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Modify and reassign

• The various methods that modify Strings return a new String withthe new contents.– They don't modify the existing String.

• Just like int or double variables, String variables do notchange when used in an expression unless you reassign them:–

Bad Example:String s = "I get it";s.toUpperCase();System.out.println(s); // I get it

– Better Code:

String s = "I get it";s = s.toUpperCase();

System.out.println(s); // I GET IT

Equivalent with an int:

int x = 3;

x + 1;

System.out.println(x); // 3

Equivalent with an int:

int x = 3;

x = x + 1;

System.out.println(x); // 4INTRODUCTION TO JAVA PROGRAMMING,EHSAN CHAUHDRY



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Strings vs. other objects

• Strings are extremely useful objects, but they behave differentlythan most objects in Java.– Strings are created differently than most objects.

We don't use the new keyword when constructing Strings.(This is because Sun felt that Strings were so important, they shouldbe integrated into the language with a shorter syntax.)

– Strings can't be modified without reassigning them.• Color objects are the same way.

• An object that cannot be changed after construction is sometimes called animmutable object.

– It is harder to visualize Strings as having data and behavior, but aString's data is its characters, and its behavior is the methods liketoUpperCase and length that manipulate or examine thosecharacters.




Selected Methods of String Class


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More Useful Methods


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Some Comparison Methods


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String Example-1public class FindCharacters {

public static void main(String[] args) {// Text string to be analyzed

String text = "To be or not to be, that is the question;"

+ " Whether 'tis nobler in the mind to suffer"

+ " the slings and arrows of outragous fortune,"

+ " or to take arms against a sea of troubles,"

+ " and by opposing end them?";

int andCount = 0; // Number of and's

int theCount = 0; // Number of the's


22Code is taken from WROX



Example-1 Cont.int index = -1; // Current index position

String andStr = "and"; // Search substringString theStr = "the"; // Search substring

// Search forwards for "and"

index = text.indexOf(andStr); // Find 1st 'and'while(index >= 0) {

++andCount;

index += andStr.length(); // Step to position after last 'and'index = text.indexOf(andStr, index);

}





Example – 1 Cont.// Search backwards for "the"

index = text.lastIndexOf(theStr); // Find last 'the'while(index >= 0) {

++theCount;

index -= theStr.length(); // Step to position before last 'the'

index = text.lastIndexOf(theStr, index);

}

System.out.println("The text contains " + andCount + " ands\n"

+ "The text contains " + theCount + " thes\n");}

}





Example-2 Cont.String firstString = "Peter";

String secondString = “David";if (firstString.compareTo(secondString) < 0)

System.out.println("firstString comes beforesecondString");

else if (firstString.compareTo(secondString) > 0)System.out.println("firstString comes after

secondString");

else

System.out.println("firstString and secondString areequal");



Lesson5 Objects_Strings

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