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Chapter 2

Introductory Programs

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Getting started

• To create and run a Java program– Create a text file with a .java extension for the

source code. For instance, source code for a first program might reside in the file Hi.java.

– Write the source code.– Compile the source code into binary code.– Execute the binary code.

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Compiling

• Suppose that the Java source code resides in the text file Hi.java. Using the $ as the system prompt, the source code is compiled with the command

$ javac Hi.java

The javac utility comes with the JDK (Java Development Kit).

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Compiling

• Compiling a source file such as Hi.java generates one or more binary files with a .class extension, e.g., Hi.class.– If fatal errors occur during compilation, the

process ends with appropriate error messages.– If nonfatal errors occur during compilation, the

process continues with appropriate warnings.

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Executing

• Once the program has been compiled, it can executed with the java utility. For instance, if the file Hi.class contains a compiled standalone program, the command

$ java Hi

executes the program.

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Compiling versus executing

• Note that the .java extension is included in the compilation but that the .class extension is not included in the execution. Note the constrast:

$ javac Hi.java

$ java Hi

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A first program

// This program prints

// Hello, world!

// to the standard output.

class Hi {

public static void main( String[ ] a ){

System.out.println( “Hello, world!” );

}

}

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A first program

• The source code for the Hi program resides in a text file with a .java extension, e.g., Hi.java. (The name Hi.java is appropriate because the program has a single class named Hi.)

• Every Java program requires at least one source code class, as the Hi example illustrates.

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A first program

• The Hi program is a Java application rather than, for instance, an applet or a bean, which are other program types. A Java application is the most general and basic program type.

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A first program

• The source code begins with three comments, each introduced with a double slash //. A double-slash comment runs until the end of the line.

• The program’s single class is named Hi and its body is enclosed in curly braces { }.

• The class Hi encapsulates a single static method named main.

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A first program

• Java applications require that main be defined with the attributes public and static.– main has void in replace of a return type such

as int to signal that the method does not return a value.

– main expects one argument, a reference to an array that can hold references to Strings.

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A first program

• The syntax

String[ ] a

indicates that a (for “array”) refers to an array. In general, the square brackets indicate an array. In this case, the array can hold references to Strings, which are any command-line arguments passed to the program.

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A first program

• In the print statement

System.out.println( “Hello, world!” );– System is a standard Java class that represents

the system on which the program executes.– out is a static field in the System class

and its type is PrintStream, another standard Java class.

– println is a PrintStream method.

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A first program

• In the Hi program’s print statement, the parts System, out, and println are separated by the member operator, the period.

• The print statement is terminated by a semicolon.– In Java, the semicolon terminates statements.

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Identifiers

• The Hi program has a single class named Hi. A programmer-defined class can have any name except a Java keyword such as int.

• Java names are case sensitive. A class named Hi is distinct from one named hi.

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The import statement

• Java programs often include import statements such as

import java.util.Date;

for convenience. If a program requires the standard Date class, for instance, then the fully qualified name java.util.Date must be used if the import is omitted.

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The import statement

• The * can be used to import all classes in a package but not to import subpackages. The statements

import java.util.*; import java.awt.*;

import java.awt.event.*;

illustrate.

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Fields and local variables

• A class can encapsulate fields and constructors and methods can have local variables.

• In Java, all fields and local variables must be declared before they are used.– A variable declaration specifies the variable’s

name, data type, and any attributes such as public.

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Fields and local variables

• Fields have default values but local variables do not. In the code segment

class C { void m() { int x; }

int n; // default value is 0

}

the field n has a default val but local variable x does not have a default value.

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Initializing local variables

• Because local variables do not have default values, they must be initialized before being used, for example, in function calls or as the source in an assignment operation.– A local variable can be initialized in its

declaration

int x = -1;

for convenience.

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The if statement

• The if statement is used for tests. Its simplest for is

int n1 = 1, n2 = 3; if ( n1 < n2 )

System.out.println( n1 );

• The if condition is enclosed in parentheses and must evaluate to a boolean value.

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The if statement

• If the body of an if statement contains multiple statements, the body must be enclosed in curly braces:

int n1 = 1, n2 = 3, n3; if ( n1 < n2 ) {

n3 = n1;

System.out.println( n1 );

} // end of if

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The if statement

• The if construct can be used for multiway decisions through the use of else and else if clauses:

int n1 = 1, n2 = 3, min; if ( n1 < n2 )

min = n1;

else

min = n2;

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The while statement

• The while statement is one of three loop constructs.

• The while loop is suited for conditional looping, that is, looping until a specified condition is satisfied.

• The while statement is similar in syntax to the simple if statement.

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The while statement

• The code segment

int n = 100, i = 0; while ( i < n ) {

System.out.println( i );

i = i + 1;

} // end of while loop

illustrates. The loop body prints the integers 0 through 99.

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Arrays

• Primitive types such as int and class types such as String can be aggregated in arrays.

• Arrays are fixed size. Once storage for an array has been allocated, the array cannot grow or shrink in size.

• Arrays of one, two, and even more dimensions can be constructed.

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Arrays

• The code segment int[ ] nums = new int[ 100 ];

declares an array of int named nums and allocates storage for 100 ints. – The array’s cells are initialized to 0, the default

value for an int, regardless of whether nums is a field or a local variable.

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Arrays

• Array elements are accessed through an index expression. The code segment

int[ ] nums = new int[ 100 ];

int i = 0;

while ( i < nums.length )

nums[ i ] = i + 1;

illustrates by populating the array with the integers 1, 2,…,100.

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Arrays

• Every array has a convenient length member that specifies the number of elements in the array. The code segment

int[ ] nums = new int[ 100 ];

int i = 0;

while ( i < nums.length ){

nums[ i ] = i + 1; i = i + 1;

}

illustrates.

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Array utilities

• Java provides utility classes for filling, searching, sorting, and performing other standard operations on arrays. If nums refers to an array of integers, the statement

Arrays.sort( nums );

sorts the array in ascending order.

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Strings

• Java provides two standard classes, String and StringBuffer, for string processing. – Strings are immutable but StringBuffers are not.

• A double quoted string such as “hi” is a reference to a String object.

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The toString method

• Every class has a toString()instance method that returns an appropriate string representation. For example, the statement

System.out.println( new Date() );

prints a Date as a human-readable string.• toString() is an example of a

polymorphic method.

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Wrapper classes

• Java provides “wrapper” classes such as Integer and Boolean for primitive types such as int and boolean, respectively.

• Wrapper classes have various uses, including data conversion.

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Wrapper classes

• The code segment

String num = “123”; int n = Integer.parseInt( num );

illustrates data conversion with the Integer wrapper class.

• Wrapper classes also can be used to store primitive types in “object-only” constructs such as Vectors and Hashtables.

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Standard utility classes

• The java.util package has an assortment of utility classes such as Date for representing dates, Random for generating random numbers of various types, Vector for dynamically sized aggregates of class types, StringTokenizer for tokenizing strings, and so on.