Learning Basics of JAVA

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Lesson 3

Learning Basics

Strings

A string is commonly considered to be a sequence of characters stored in memory and accessible as a

unit. Strings in java are represented as objects.

String Concatenation

"+" operator is used to concatenate strings

System.out.pritln("Hello" + "World") will print Hello World on console

String concatenated with any other data type such as int will also convert that datatype to

String and the result will be a concatenated String displayed on console. For example,

int i = 4;

int j = 5;

System.out.println ("Hello" + i)

will print Hello 4 on screen

However

System,.out..println( i+j) ;

will print 9 on the console because both i and j are of type int.

Comparing Strings

For comparing Strings never use == operator, use equals method of String class.

== operator compares addresses (shallow comparison) while equals compares values (deep

comparison)

E.g string1.equals(string2)

Example Code: String concatenation and comparison

public class StringTest {

public static void main(String[] args) {

int i = 4;

int j = 5;

System.out.println("Hello" + i); // will print Hello4

System.out.println(i + j); // will print 9

String s1 = new String ("pakistan");

String s2 = "pakistan";

if (s1 == s2) {

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System.out.println("comparing string using == operator");

}

if (s1.equals( s2) ) {

System.out.println("comparing string using equal method");

}

On execution of the above program, following output will produce

Taking in Command Line Arguments

In Java, the program can be written to accept command-line-arguments.

Example Code: command-line arguments

/* This Java application illustrates the use of Java command-line

arguments. */

public class CmdLineArgsApp {

public static void main(String[] args){ //main method

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

System.out.println("Second argument " + args[1]);

}//end main

}//End class.

To execute this program, we pass two arguments as shown below:

public void someMethod( ) {

int x; //local variable

System.out.println(x); // compile time error

These parameters should be separated by space. .

The parameters that we pass from the command line are stored as Strings inside the "args"

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array. You can see that the type of "args" array is String.

Example Code: Passing any number of arguments

In java, array knows their size by using the length property. By using, length property we can determine

how many arguments were passed. The following code example can accept any number of arguments

* This Java application illustrates the use of Java

command-line arguments. */

public class AnyArgsApp {

public static void main(String[] args){ //main method

for(int i=0; i < args.length; i++)

System.out.println("Argument:" + i + "value" +args[i]);

}//end main

}//End class.

Output

C:\java AnyArgsApp i can pass any number of arguments

Argument:0 value i Argument:1 value

can Argument:2 value pass

Argument:3 value any Argument:4

value number Argument:5 value of

Argument:6 value arguments

Primitives vs Objects

Everything in Java is an "Object", as every class by default inherits from class

"Object" , except a few primitive data types, which are there for efficiency reasons.

Primitive Data Types

Primitive Data types of java

boolean, byte

1 byte

char, short

2 bytes

int, float

4 bytes

long, double

8 bytes

Primitive data types are generally used for local variables, parameters and instance

variables (properties of an object)

Primitive datatypes are located on the stack and we can only access their value, while objects

are located on heap and we have a reference to these objects

Also primitive data types are always passed by value while objects are always passed by

reference in java. There is no C++ like methods

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void someMethod(int &a, int & b ) // not available in java

Stack vs. Heap

Stack and heap are two important memory areas. Primitives are created on the stack while objects

are created on heap. This will be further clarified by looking at the following diagram that is

taken from Java Lab Course.

int num = 5;

Student s = new Student();

Stack

Heap

num

0F59

name ali

0F59

Wrapper Classes

Each primitive data type has a corresponding object (wrapper class). These wrapper classes

provides additional functionality (conversion, size checking etc.), which a primitive data type cannot

provide.

Wrapper Use

You can create an object of Wrapper class using a String or a primitive data type

Integer num = new Integer(4); or

Integer num = new Integer("4");

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Note: num is an object over here not a primitive data type

You can get a primitive data type from a Wrapper using the corresponding value function

int primNum = num.intValue();

Converting Strings to Numeric Primitive Data Types

To convert a string containing digits to a primitive data type, wrapper classes can help. parseXxx

method can be used to convert a String to the corresponding primitive data type.

String value = "532";

int d = Integer.parseInt(value);

String value = "3.14e6";

double d = Double.parseDouble(value);

The following table summarizes the parser methods available to a java programmer.

Data Type

Convert String using either ...

byte

Byte.parseByte(string )

new Byte(string ).byteValue()

short

Short.parseShort(string )

new Short(string ).shortValue()

int

Integer.parseInteger(string )

new Integer(string ).intValue()

long

Long.parseLong(string )

new Long(string ).longValue()

float

Float.parseFloat(string )

new Float(string ).floatValue()

double

Double.parseDouble(string )

new Double(string ).doubleValue()

Example Code: Taking Input / Output

So far, we learned how to print something on console. Now the time has come to learn how to print

on the GUI. Taking input from console is not as straightforward as in C++. Initially we'll study how to

take input through GUI (by using JOPtionPane class).

The following program will take input (a number) through GUI and prints its square on the console as

well on GUI.

1. import javax.swing.*;

2. public class InputOutputTest {

3. public static void main(String[] args) {

4. //takes input through GUI

5. String input = JOptionPane.showInputDialog("Enter number");

6. int number = Integer.parseInt(input);

7. int square = number * number;

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8. //Display square on console

9. System.out.println("square:" + square);

10. //Display square on GUI

11. JOptionPane.showMessageDialog(null, "square:"+ square);

12. System.exit(0);

13. }

14. }

On line 1, swing package was imported because it contains the JOptionPane class that will be used

for taking input from GUI and displaying output to GUI. It is similar to header classes of C++.

On line 5, showInputDialog method is called of JOptionPane class by passing string argument that

will be displayed on GUI (dialog box). This method always returns back a String regardless of whatever

you entered (int, float, double, char) in the input filed.

Our task is to print square of a number on console, so we first convert a string into a number by

calling parseInt method of Integer wrapper class. This is what we done on line number 6.

Line 11 will display square on GUI (dialog box) by using showMessageDialog method of JOptionPane

class. The first argument passed to this method is null and the second argument must be a String.

Here we use string concatenation.

Line 12 is needed to return the control back to command prompt whenever we use

JoptionPane class.

Compile & Execute

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Selection & Control Structure

The if-else and switch selection structures are exactly similar to we have in C++. All relational

operators that we use in C++ to perform comparisons are also available in java with same behavior.

Likewise for, while and do-while control structures are alike to C++.

Reference:

1- Java tutorial:

http://www.dickbaldwin.com/java

2- Example code, their explanations and corresponding figures for this handout are taken from the book

JAVA A Lab Course by Umair Javed. This material is available just for the use of VU students of the

course Web Design and Development and not for any other commercial purpose without the consent of

author.

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