CSE 201 Java Lecture 2 Tc

7/30/2019 CSE 201 Java Lecture 2 Tc

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CSE 201

Object Oriented Programming Language

Java: Basics


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Notes on Packages

` Java classes are arranged in a hierarchy or packagestructure` We can place our classes into packages, but it isoptional`

When we do not specify any package for our class,

the class is placed in a default unnamed package` To place a class in a package we use thepackagestatement

` A class without any access modifier has packagescope only` A public class from a package can be used acrosspackages (i.e., can be used from a class of anotherpackage => the key is import declaration)


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Notes on Packages (Contd.)

` Classes in the same package are implicitlyimported into the source code files of otherclasses in the same package

` Thus, an import declaration is not required whenone class in a package uses another in the samepackage


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An Example/* /*

Filename: A.java Filename: JavaTest.java

*/ */

package my_package; import my_package.A;

public class A{ public class JavaTest{

private int a; public static void main(String[] args){

public A(int a){ A a = new A(10);

this.a = a; System.out.println(a.getA());

} }

public void setA(int a){ }

this.a = a;

}

public int getA(){return this.a;

}

}


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Examining the Example

`The declaration package my_package; in A.java putsthe classes in this file into the package named

my_package

` Class A is made public so it is accessible from outside thepackage my_package` No package declaration at the top of JavaTest.java =>class JavaTest is put under default unnamed package

` The import declaration import my_package.A; inJavaTest.java indicates that class A from my_package willbe used in this file => if this declaration is missing thenCompilation error at A a = new A(10);

` Then why no import for System class ??


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Why no import is required for System class` Because System resides in the packagejava.lang, which is automatically importedto every Java application.

`So, we can use all classes from java.lang inour program without explicitly using theimport statement.


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More on import

`The use of import is similar to the use ofusing namespace in C++.

` To import all the classes from a package (e.g.java.util) we have to use import java.util.*;

`

Then what will we write to import all classes

from my_package??


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Taking input: A Sample Program(InputTest.java)

import java.util.Scanner;

public class InputTest{

public static void main(String args[ ]){

Scanner input = new Scanner(System.in);System.out.print(Enter an integer: );int num1 = input.nextInt();System.out.printf(You entered: %d\n, num1);

// No need to use delete or free type of things.// Java manages memory allocation/deallocation automatically.

} // end method main} // end class InputTest


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Examining InputTest.java

`inof System.in is a static member of classSystem. It represents the standard input (similar tostdin or cin)

`Scanner is a class inside the Java API and is locatedin the java.util package`Scanner contains some useful methods to read

different types of data from the standard input (e.g.the keyboard)

`

We have to use the import statement to tell thecompiler that we will use Scanner from java.utilin our program. Otherwisejavac will not recognizeScanner


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Examining InputTest.java (Contd.)

` We use a Scanner object to wrap the standardinput object System.in

`We call the method nextInt( ) on the Scannerobject input to read an integer from thekeyboard

` If we enter a non-integer value, an exception isthrown and the program terminates abnormally.Exception Handling is a major topic in Java andwill be discussed elaborately later in the course


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Primitive (built-in) Datatypes


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Primitive (built-in) Datatypes

` Integers` byte 8 bit integer

` short 16 bit integer

` int 32 bit integer

` long 64 bit integer

` Real Numbers` float 32 bit floating point number

` double 64 bit floating point number

` Others` char 16 bit, Unicode 2.1 character` boolean true or false. false is not zero (0) in Java


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Non-Primitive Datatypes

` Non-primitive datatypes` Class Variables

` Array

` Non-primitive types are also called Reference types` Object Example

class Cuboid{private int l, w, h;

public Cuboid(int a, int b, int c){l=a;w=b;h=c;}public static void main(String args[ ]){

Cuboid ob; // ob is a reference pointing to nullob = new Cuboid(10,20,5); // now the actual object is created

}

}


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Primitive vs. Non-Primitive Types

Primitive Types

` Can store exactly one value of itsdeclared type at a time. Whenanother value is assigned to thatvariable initial value is replaced

` Primitive type instance variables

are initialized by default

` Instance variables of type byte,short, int, long, float, double and

char initialized to 0` Instance variables of type boolean

initialized to false

` Note: Local variables are notinitialized by default

Reference Types

` Stores locations of objects inmemory and are said to refer toobjects. Objects that arereferenced may contain manyinstance variables and methods

` Reference type instance variables

are by default initialized to null

` Note: Local variables are notinitialized by default


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Primitive vs. Non-Primitive Types (Contd.)

Primitive Types

` Primitive type variablesare handled by value -

the actual values arestored in variables andpassed to methods

Reference Types

` All objects and arrays arehandled by reference -

the references are storedin variables and passed tomethods


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Java References

` Java references are used to point to Java objectscreated by new

`Java objects are always passed by reference to otherfunctions

` In Java you can never pass an object to anotherfunction by-value` Java references act as pointers but does not allowpointer arithmetic

` We cannot read the value of a reference and hencecannot find the address of a Java object` We cannot take the address of a Java reference


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Java References (Contd.)`

`

`

But, we can make a Java reference point to a new object` by copying one reference to another

ClassName ref1 = new ClassName();ClassName ref2 = ref1;

`by creating a new object using new and placing the returnedaddress to the reference in question.

ClassName ref1 = new ClassName(); // first objectref1 = new ClassName(); // another object, address of first object is lost !!!

We cannot place arbitrary values to a reference except the specialvalue null which means that the reference is pointing to nothing.

ClassName ref1 = 100; // compiler errorClassName ref2 = null; // no problem

Trying to use a null reference causes unexpected results orExceptions in many cases

ClassName ref2 = null;ref2.methodName();` Exception in thread "main" java.lang.NullPointerException


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Type boolean

` Java boolean type variables cannot be cast toanother datatype

` In Java, 0 and null are not the same as false andnon-zero and non-null are not the same as true

` Both true and false are keywords in Java

` Does not have any relation with int` boolean b = false;` int a = b; // compiler error` int a = (int)b; // compiler error


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Operators (Precedence & Associativity)


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Operators (Precedence & Associativity)(Contd.)


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Operators (Precedence & Associativity)(Contd.)


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Using Dialog Boxes

import javax.swing.JOptionPane;

public class NameDialog{

public static void main(String args[ ]){

String name = JOptionPane.showInputDialog(What is your name?);

String message = String.format(Welcome, %s, name);

JOptionPane.showMessageDialog(null, message);

}

}


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Executing NameDialog


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SumOfInt.java

/*FileName: SumOfInt.java*/

import javax.swing.*;

class SumOfInt

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

String s1=JOptionPane.showInputDialog(null,"Enter 1st number:");String s2=JOptionPane.showInputDialog(null,"Enter 2nd Number:");int num1=Integer.parseInt(s1);

int num2=Integer.parseInt(s2);JOptionPane.showMessageDialog(null,"Sum is : " + (num1+num2));

}

}


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Executing SumOfInt


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Creating Your First Window(FirstWindow.java)

import java.awt.*;

import javax.swing.*;

public class FirstWindow{

public static void main(String args[]){

JFrame frame = new JFrame("My First Window");

JLabel label1 = new JLabel("I like Java", SwingConstants.CENTER);label1.setFont(new Font("ARIAL", Font.PLAIN, 72));Font fontForButtons = new Font("ARIAL", Font.ITALIC, 24);JButton topButton = new JButton("Click me");topButton.setFont(fontForButtons);JButton bottomButton = new JButton("Click me too");bottomButton.setFont(fontForButtons);


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FirstWindow.Java (Contd.)

Container c = frame.getContentPane();c.add(label1);

c.add(topButton, BorderLayout.NORTH);

c.add(bottomButton, BorderLayout.SOUTH);frame.setSize(800, 600);

frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

frame.setVisible(true);

}

}


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Output of FirstWindow.java


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Lecture Content

` Java: How to Program` Chapter 2, 3, 4

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CSE 201 Java Lecture 2 Tc

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