CMSC 202 Java Primer 2.

CMSC 202 Java Primer 2

Copyright © 2008 Pearson Addison-Wesley.
The Class String There is no primitive type for strings in Java The class String is a predefined class in Java that is used to store and process strings Objects of type String are made up of strings of characters that are written within double quotes Any quoted string is a constant of type String "Live long and prosper." A variable of type String can be given the value of a String object String blessing = "Live long and prosper."; July 24, 2007 Copyright © 2008 Pearson Addison-Wesley. All rights reserved 2

Concatenation of Strings
Concatenation: Using the + operator on two strings in order to connect them to form one longer string If greeting is equal to "Hello ", and javaClass is equal to "class", then greeting + javaClass is equal to "Hello class" Any number of strings can be concatenated together When a string is combined with almost any other type of item, the result is a string "The answer is " evaluates to "The answer is 42“ Java Strings also support the += operator If greeting is equal to ”Hello”, then greeting += “ Bob”; changes greeting to “Hello Bob” July 24, 2007 Copyright © 2008 Pearson Addison-Wesley. All rights reserved 3

Classes, Objects, and Methods
A class is the name for a type whose values are objects Objects are entities that store data and take actions Objects of the String class store data consisting of strings of characters The actions that an object can take are called methods Methods can return a value of a single type and/or perform an action All objects within a class have the same methods, but each can have different data values July 24, 2007 Copyright © 2008 Pearson Addison-Wesley. All rights reserved 4

Classes, Objects, and Methods
Invoking or calling a method: a method is called into action by writing the name of the calling object, followed by a dot, followed by the method name, followed by parentheses This is sometimes referred to as sending a message to the object The parentheses contain the information (if any) needed by the method This information is called an argument (or arguments) July 24, 2007 Copyright © 2008 Pearson Addison-Wesley. All rights reserved 5

String Methods The String class contains many useful methods for string-processing applications A String method is called by writing a String object, a dot, the name of the method, and a pair of parentheses to enclose any arguments If a String method returns a value (e.g. an int), then it can be placed anywhere that a value of its type can be used String greeting = "Hello"; int count = greeting.length(); System.out.println("Length is " + greeting.length()); Always count from zero when referring to the position or index of a character in a string July 24, 2007 Copyright © 2008 Pearson Addison-Wesley. All rights reserved 6

Some Methods in the Class String (1 of 8)
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Some Methods in the Class String ( 8 of 8)

Copyright © 2008 Pearson Addison-Wesley
String Indexes The characters within the String may be accessed (but not changed) using the charAt( int index) method. July 24, 2007 Copyright © 2008 Pearson Addison-Wesley All rights reserved 15

Escape Sequences A backslash (\) immediately preceding a character (i.e., without any space) denotes an escape sequence or an escape character The character following the backslash does not have its usual meaning Although it is formed using two symbols, it is regarded as a single character July 24, 2007 Copyright © 2008 Pearson Addison-Wesley All rights reserved 16

String Processing A String object in Java is considered to be immutable, i.e., the characters it contains cannot be changed There is another class in Java called StringBuffer that has methods for editing its string objects However, it is possible to change the value of a String variable by using an assignment statement String name = "Soprano"; name = "Anthony " + name; July 24, 2007 Copyright © 2008 Pearson Addison-Wesley All rights reserved 18

Naming Constants Instead of using "anonymous" numbers in a program, always declare them as named constants, and use their name instead public static final int INCHES_PER_FOOT = 12; public static final double RATE = 0.14; The “final” modifier prevents a value from being changed inadvertently. We’ll talk more about public and static later. It has the added advantage that when a value must be modified, it need only be changed in one place Note the naming convention for constants: Use all uppercase letters, and designate word boundaries with an underscore character July 24, 2007 Copyright © 2008 Pearson Addison-Wesley All rights reserved 19

Comments A line comment begins with the symbols //, and causes the compiler to ignore the remainder of the line This type of comment is used for the code writer or for a programmer who modifies the code A C-style block comment begins with the symbol pair /*, and ends with the symbol pair */ The compiler ignores anything in between This type of comment can span several lines This type of comment provides documentation for the users of the program July 24, 2007 Copyright © 2008 Pearson Addison-Wesley. All rights reserved 20

Program Documentation
Java comes with a program called javadoc that will automatically extract documentation from block comments in the classes you define As long as their opening has an extra asterisk (/**) Ultimately, a well written program is self-documenting Its structure is made clear by the choice of identifier names and the indenting pattern When one structure is nested inside another, the inside structure is indented one more level We’ll discuss javadoc in lab July 24, 2007 Copyright © 2008 Pearson Addison-Wesley All rights reserved 21

Comments & Named Constant
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Comments and Coding Standards
Be sure to check the course website regarding comment requirements in projects undergraduate/202/fall07/Projects/ codingstd.shtml July 24, 2007

Java Flow Control Java supports the usual flow control constructs with the same basic syntax as C/C++. We assume you are familiar with these constructs. Decisions if, if-else, switch Loops for, while, do-while Boolean expressions Like C/C++, Java flow control constructs evaluate boolean expressions July 24, 2007

Boolean Expressions A Boolean expression is an expression (or Boolean variable) that is either true or false The simplest Boolean expressions compare the value of two expressions time < limit yourScore == myScore Note that Java, like C, uses two equal signs (==) to perform equality testing: A single equal sign (=) is used only for assignment A Boolean expression does not need to be enclosed in parentheses, unless it is used in an if-else statement July 24, 2007 Copyright © 2008 Pearson Addison-Wesley. All rights reserved 25

Java Comparison Operators

Building Boolean Expressions
When two Boolean expressions are combined using the "and" (&&) operator, the entire expression is true provided both expressions are true Otherwise the expression is false When two Boolean expressions are combined using the "or" (||) operator, the entire expression is true as long as one of the expressions is true The expression is false only if both expressions are false Any Boolean expression can be negated using the ! operator Place the expression in parentheses and place the ! operator in front of it Unlike mathematical notation, strings of inequalities must be joined by && (the “and” operator) Use (min < result) && (result < max) rather than min < result < max July 24, 2007 Copyright © 2008 Pearson Addison-Wesley. All rights reserved 27

Pitfall: Using == with Strings
The equality comparison operator (==) can correctly test two values of a primitive type However, when applied to two objects such as objects of the String class, == tests to see if they are stored in the same memory location, not whether or not they have the same value (more on this later) In order to test two strings to see if they have equal values, use the method equals, or equalsIgnoreCase string1.equals(string2) string1.equalsIgnoreCase(string2) July 24, 2007 Copyright © 2008 Pearson Addison-Wesley. All rights reserved 28

Variable Scope The scope of a variable is that set of code statements in which the variable is known to the compiler; where is can be referenced in your program. Most commonly, the scope of a variable is limited to the code block in which it is defined. A code block is a set of code enclosed in braces ({, }). One interesting application of this principle allowed in Java involves the for-loop construct. July 24, 2007

for-loop index Java gives us ability to define variables in the heading of a for loop. Most commonly the loop index is declared and initialized in the for loop heading. These variables are considered local to the for-loop and so may be reused in other loops. String s = “hello world”; int count = 1; for (int i = 0; i < s.length(); i++) { count *= 2; } // using 'i' here generates a compiler error July 24, 2007

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