Java: Intro Pemrogramman Java
- Tutorial ini menjelaskan tentang bahasa pemrogramman Java. Juga berisi "Contekan" dengan contoh untuk task standard saat pemrogramman.
- Tulisan ini tidak membahas tentang instalasi Java Development Kit (JDK).
Pendahuluan
Sejarah
Bahasa Pemrograman Java pertama kali di buat oleh James Gosling dari Sun Microsystems tahun 1991. Versi Java pertama yang tersedia untuk publik adalah Java 1.0 yang di release tahun 1995. Setelah melalui beberapa revisi dan perbaikan bahasa dan library. Versi Java sekarang adalah 1.6 yang juga di kenal sebagai Java 6.0.
Dari bahasa pemrogramman Java berevolusi Java platform. Java platform memungkinkan sebuah code yang di tulis dalam bahasa lain yang bukan bahasa pemrogramman Java dan masih dapat jalan di Java virtual machine.
Gambaran Umum
Bahasa pemrogramman Java terdiri dari Java compiler, Java virtual machine, dan Java class libraries. Java virtual machine (JVM) adalah software di komputer yang akan menjalankan program seperti mesin sesungguhnya.
Java compiler akan menterjemahkan source code Java menjadi byte-code. Java virtual machine akan menginterpretasikan byte-code dan menjalankan program.
Java virtual machine di tulis spesifik untuk sistem operasi yang spesifik.
Java runtime environment (JRE) terdiri dari JVM dan Java class libraries.
Karakteristik Java
Target Java adalah membuat sebuah program sekali saja, dan dapat di jalankan di banyak sistem operasi.
Jaka mempunyai karakteristik berikut:
- Platform independent: Program Java menggunakan Java virtual machine untuk abstraksi dan tidak mempunyai akses secara langsung ke sistem operasi. Hal ini menyebabkan program Java sangat portabel. Sebuah program Java yang memenuhi standard dan mengikuti aturan tertentu dapat di jalankan tanpa di modifikasi di beberapa platform sekaligus, seperti, Windows atau Linux.
- Bahasa Pemrogramman Object-orientated: Kecuali untuk tipe data primitif. Semua elemen di Java adalah objek.
- Bahasa Pemrogramman Strongly-typed: Java adalah strongly-typed, dimana type dari variable yang digunakan harus di definikan terlebih dulu dan konversi ke objek lain sangat ketat, dimana harus dilakukan pada umumnya oleh programmer.
- Bahasa Interpreted dan compiled: Java source code di transfer ke byte-code yang tidak tergantung pada platform targer-nya. Byte-code akan di interpretasikan oleh Java Virtual machine (JVM). JVM mempunyai Hotspot-Compiler yang akan menterjemahkan byte-code ke native code.
- Manajemen Memory Automatis: Java mengatur alokasi dan de-alokasi memory untuk membuat objek baru. Program tidak mempunyai akses langsung ke memory. Garbage collector akan mendelete secara automatis objek yang tidak ada pointer ke padanya.
Sintaks Java mirip dengan C++, Java case sensitif, artinya, variabel myValue dan myvalue akan dilihat sebagai variabel yang BERBEDA.
Development menggunakan Java
Programmer akan menulis source code Java menggunakan text editor yang mendukung text ASCII biasa. Biasanya programmer menggunakan IDE (integrated development environment) untuk melakukan pemrogramman. Sebuah IDE akan membantu programmer dalam berbagai tugas menulis source code, misalnya, dia akan memberikan auto-formatting dari source code, memberikan highlight pada keyword yang penting dll.
Pada suatu saat programmer (atau IDE) akan memanggil Java compiler (javac). Java compiler akan membuat platform independent code yang di sebut bytecode. byte-code tersebut akan di simpan di file ".class".
Bytecode dapat di jalankan oleh Java runtime environment. Java runtime environment (JRE) adalah sebuah program yang mengetahui bagaimana cara menjalankan bytecode di sistem operasi. JRE menterjemahkan bytecode menjadi native code dan mengekskusinya. Jadi native code untuk Linux akan berbeda dengann native code untuk Windows.
Secara default, compiler akan meletakan setiap file class di directory ytang sama dengan file source. Kita biasanya dapat menentukan directory yang beda dengan -d
Classpath
Classpath adalah sambungan antara Java compiler dan Java interpreter. Classpath mendefinisikan dimana compiler dan interpreter menjadi file .class untuk di load.
Classpath di Java mendefinisikan Java class yang mana yang tersedia untuk program Java kita. Contoh, jika kita untuk menggunakan external Java library kita harus menambahkan library tersebut ke classpath kita untuk dapat menggunakannya di program kita,
Program Java Yang Pertama
Menulis Source Code
Program Java berikut di kembangkan di Linux. Proses di sistem operasi lain harusnya tidak berbeda jauh jadi tidak akan di bicarakan disini. Pilih directory tempat kita menyimpan source code. Disini akan digunakan directory ~/java yang selanjutnya akan disebut sebagai "javadir". Sebaiknya kita buat dulu directory tersebut
mkdir ~/java
Buka text editor untuk mengedit text biasa. Kita dapat menggunakan vi, bagi yang terbiasa menggunakan GUI sangat di sarankan untuk mengggunakan gedit. Ketik
gedit ~/java/HelloWorld.java
Isi dengan program java berikut
// The smallest Java program possible
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World");
}
}
Simpan source code tersebut di directory "javadir" dengan nama "HelloWorld.java". Perhatikan bahwa nama file Java source HARUS SAMA dengan nama class (dalam source code) di tambahkan akhiran .java. Dalam kasus kita filename harus HelloWorld.java karena class yang dibuat adalah HelloWorld.
Compile Source Code
Untuk mengcompile source code. Masuk ke shell dan tulis
cd ~/java javac HelloWorld.java
Cek isi folder menggunakan perintah
ls ~/java
harusnya sekarang ada file "HelloWorld.class". Jika kita melihat file tersebut, berarti kita berhasil mengcompile source Java pertama kita menjadi byte-code.
Menjalankan Code
Untuk menjalankan / me-run code kita dapat melakukannya dari shell menggunakan perintah
cd ~/java java HelloWorld
Harusnya akan keluar kata-kata "Hello World".
Menggunakan classpath
Kita dapat menggunakan classpath untuk menjalankan program dari directory / tempat lain. Misalnya kita masuk ke shell kemudian masuk ke sembarang directory kemudian ketik
java HelloWorld
Jika kita tidak di directory dimana class yang sudah di compile berada maka sistem akan mengeluarkan error misalnya,
Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld Caused by: java.lang.ClassNotFoundException: HelloWorld at java.net.URLClassLoader$1.run(URLClassLoader.java:202) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:190) at java.lang.ClassLoader.loadClass(ClassLoader.java:307) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301) at java.lang.ClassLoader.loadClass(ClassLoader.java:248) Could not find the main class: HelloWorld. Program will exit.
Untuk memperbaiki ini kita dapat menulis di shell
java -classpath "mydirectory" HelloWorld
Ubah "mydirectory" dengan directory dimana terdapat HelloWorld.class. Dalam hal ini menjadi
java -classpath "/home/namauseranda/java" HelloWorld
Perhatikan harus lengkap /home/namauseranda/java tidak bisa menggunakan ~/java. Sekarang, harusnya kita dapat melihat keluaran "HelloWorld".
Integrated Development Environment
Pada bagian sebelumnya di terangkan bagaimana cara membuat dan mengcompile program java menggunakan command line. Java Integrated Development Environment (IDE) memberikan banyak kemudahan fungsi untuk membuat program java. Salah satu IDE yang sangat powerful adalah Eclipse.
Untuk pendahuluan cara penggunakan Eclipse IDE mohon membaca-baca Eclipse Tutorial .
In the following I will say "Create e.g. the types of the used variables must be pre-defined and conversion to other objects is relatively strict, e.g. must be done in most cases by the programmer. a Java project SomeName". This will refer to creating an Eclipse Java project. If you are using a different IDE please follow the required steps in this IDE.
Aplikasi Graphical User Internet (GUI) yang Pertama
Using Eclipse create a new Java project "JavaIntroductionUI".
Create the following class MyFirstUI.java in package ui.
package ui; import java.awt.BorderLayout; import java.awt.GridLayout;
e.g. the types of the used variables must be pre-defined and conversion to other objects is relatively strict, e.g. must be done in most cases by the programmer.
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JCheckBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JTextField;
public class MyFirstUI extends JFrame {
private JCheckBox checkbox;
private JTextField firstName;
private JTextField lastName;
public MyFirstUI() {
// Lets make it look nice
// This you can ignore / delete if you don't like it
// try {
// for (LookAndFeelInfo info : UIManager.getInstalledLookAndFeels()) {
// if ("Nimbus".equals(info.getName())) {
// UIManager.setLookAndFeel(info.getClassName());
// break;
// }
// }
// } catch (Exception e) {
// e.printStackTrace();
// }
setTitle("My First UI");
// We create a panel which will hold the UI components
JPanel pane = new JPanel(new BorderLayout());
// We always have two UI elements (columns) and we have three rows
int numberOfRows = 3;
int numberOfColumns = 2;
pane.setLayout(new GridLayout(numberOfRows, numberOfColumns));
// create and attach buttons
// create a label and add it to the main window
JLabel firstNamelabel = new JLabel(" Firstname: ");
pane.add(firstNamelabel);
firstName = new JTextField();
pane.add(firstName);
JLabel lastNamelabel = new JLabel(" Lastname: ");
pane.add(lastNamelabel);
lastName = new JTextField();
pane.add(lastName);
JButton sayHello = new JButton("Say something");
pane.add(sayHello);
checkbox = new JCheckBox("Nice");
pane.add(checkbox);
// Add the pane to the main window
getContentPane().add(pane);
// Pack will make the size of window fitting to the compoents
// You could also use for example setSize(300, 400);
pack();
// Set a tooltip for the button
sayHello
setToolTipText("This button will say something really nice of something bad");
// sayHello need to do something
sayHello.addActionListener(new MyActionListener());
}
private class MyActionListener implements ActionListener {
public void actionPerformed(ActionEvent e) {
if (!checkbox.isSelected()) {
JOptionPane.showMessageDialog(null, "I don't like you, "
+ firstName.getText() + " " + lastName.getText() + "!");
} else {
JOptionPane.showMessageDialog(null, "How are you, "
+ firstName.getText() + " " + lastName.getText() + "?");
}
}
}
}
Create also the following class MainTester.java in package test and start this class.
package test;
import ui.MyFirstUI;
public class MainTester {
public static void main(String[] args) {
MyFirstUI view = new MyFirstUI();
view.setVisible(true);
}
}
You should see the following. A mesage dialog should be seen if you press the button.
6. Statements
The following describes certain aspects of the software.
6.1. Boolean Operations
Use == to compare two primitives or to see if two references refers to the same object. Use the equals() method to see if two different objects are equal.
&& and || are both Short Circuit Methods which means that they terminate once the result of an evaluation is already clear. Example (true || .... ) is always true while (false && ...) always false is. Usage:
(var !=null && var.method1()..) ensures that var is not null before doing the real check.
Table 1. Boolean Operations Description == Is equal, in case of objects the system checks if the reference variable point to the same object, is will not compare the content of the objects! && And != is not equal, similar to the "==" a.equals(b) Checks if string a equals b a.equalsIgnoreCase(b) Checks if string a equals b while ignoring lower cases If (value ? false : true) {} Return true if value is not true. Negotiation
6.2. Switch Statement
The switch statement can be used to handle several alternatives if they are based on the same constant value.
switch (expression) {
case constant1:
command;
break; // Will prevent that the other cases or also executed
case constant2:
command;
break;
...
default:
}
Example:
switch (cat.getLevel()) {
case 0:
return true;
case 1:
if (cat.getLevel() == 1) {
if (cat.getName().equalsIgnoreCase(req.getCategory())) {
return true;
}
}
case 2:
if (cat.getName().equalsIgnoreCase(req.getSubCategory())) {
return true;
}
}
7. Working with Strings
The following lists the most common string operations.
Table 2. Command Description text1.equals("Testing"); return true if text1 is equal to "Testing". The test is case sensitive. text1.equalsIgnoreCase("Testing"); return true if text1 is equal to "Testing". The test is not case sensitive. For example it would also be true for "testing" StringBuffer str1 = new StringBuffer(); Define a new String with a variable length. str.charat(1); Return the character at position 1. (Strings starting with 0) str.substring(1); Removes the first characters. str.substring(1, 5); Gets the substring from the second to the fifths character. str.indexOf(String) Find / Search for String Returns the index of the first occurrence of the specified string. str.lastIndexOf(String) Returns the index of the last occurrence of the specified string. StringBuffer does not support this method. Hence first convert the StringBuffer to String via method toString. str.endsWith(String) Returns true if str ends with String str.startsWith(String) Returns true if str starts with String str.trim() Removes spaces str.replace(str1,str2) Replaces all occurrences of str1 by str2 str.concat(str1); Concatenates str1 at the end of str. str.toLowerCase() str.toUpperCase() Converts string to lower- or uppercase str1 + str2 Concatenate str1 and str2 String[] zeug = myString.split("-"); String[] zeug = myString.split("\\."); Spits myString at / into Strings. Attention: the split string is a regular expression, so if you using special characters which have a meaning in regular expressions you need to quote them. In the second example the . is used and must be quoted by two backslashes.
8. Collection
A collection is a data structure which is used to contain and process sets of data. The data is encapsulated and the access to the data is only possible via predefined methods.
For example if your applications saves data in the object People you can store different People objects about people in a collection.
Tip While arrays are fixed sized, collections have a dynamic size, e.g. a collection can contain a flexible number of object.
When you need elements of various types or a dynamically changing number of them you use Java Collections.
Typical collections are: stacks, queues, deques, lists and trees.
As of Java 5 collections should get parameterized with an object declaration to enable the compiler to check if objects which are added to the collection have the correct type.
package collections;
import java.util.ArrayList;
public class MyArrayList {
public static void main(String[] args) {
// Declare the ArrayList
ArrayList<String> var = new ArrayList<String>();
// Add a few Strings to it
var.add("Lars");
var.add("Jennifer");
// Loop over it and print the result to the console
for (String s : var) {
System.out.println(s);
}
}
}
java.util.Collections is the basis class which provides you useful functionality.
Table 3. Sample Table Collections.copy(list, list) Copy a collection to another Collections.reverse(list) Reserve the order of the list Collections.shuffle(list) Shuffles the list Collections.sort(list) Sort the list
9. Type Conversion
If you use variables of different types Java requires for certain types an explicit conversion. The following gives examples for this conversion. 9.1. Conversion to String
Use the following to convert from other types to Strings
// Convert from int to String String s1 = String.valueOf ( 10 ); // "10" String s2 = // Convert from double to String String.valueOf ( Math.PI ); // "3.141592653589793" // Convert from boolean to String String s3 = String.valueOf ( 1 < 2 ); // "true" // Convert from date to String String s4 = String.valueOf ( new Date() ); // "Tue Jun 03 14:40:38 CEST 2003"
9.2. Conversion from String to Number
// Conversion from String to int int i = Integer.parseInt(String); // Conversion from float to int float f = Float.parseFloat(String); // Conversion from double to int double d = Double.parseDouble(String);
The conversion from string to number is independent from the Locale settings, e.g. it is always using the English notification for number. In this notification a correct number format is "8.20". The German number "8,20" would result in an error.
To convert from a German number you have to use the NumberFormat class. The challenges is that if the value is for example 98.00 then the NumberFormat class would create a Long which cannot be casted to Double. Hence the following complex conversion class.
private Double convertStringToDouble(String s) {
Locale l = new Locale("de", "DE");
Locale.setDefault(l);
NumberFormat nf = NumberFormat.getInstance();
Double result = 0.0;
try {
if (Class.forName("java.lang.Long").isInstance(nf.parse(s))) {
result = Double.parseDouble(String.valueOf(nf.parse(s)));
} else {
result = (Double) nf.parse(new String(s));
}
} catch (ClassNotFoundException e1) {
e1.printStackTrace();
} catch (ParseException e1) {
e1.printStackTrace();
}
return result;
}
9.3. Double to int
int i = (int) double;
9.4. SQL Date conversions
Use the following to convert a Date to a SQL date
package test;
import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
public class ConvertDateToSQLDate {
private void convertDateToSQL(){
SimpleDateFormat template =
new SimpleDateFormat("yyyy-MM-dd");
java.util.Date enddate =
new java.util.Date("10/31/99");
java.sql.Date sqlDate =
java.sql.Date.valueOf(
template.format(enddate));
}
public static void main(String[] args) {
ConvertDateToSQLDate date = new ConvertDateToSQLDate();
date.convertDateToSQL();
}
}
JAR files - Java Archive
What is a jar
A JAR file is a Java Archive based on the pkzip file format. A jar files can contain java classes and other resources (icons, property files) and can be executable.
JAR files are the deployment format for Java. You can distribute your program in a jar file or you can use exiting java code via jars by putting them into your classpath.
Executable jar
An executable JAR means the end-user doesn't have to pull the class files out before running the program. This is done via a manifest.txt file which tells the JVM which class has the main() method. The content of the manifest.txt file:
Manifest-Version: 1.0 Main-Class: MyApp Class-Path:
. lib/jcommon-1.0.6.jar lib/itext-1.4.6.jar "Empty Line"
An empty line is required otherwise the jar won't be executable. Space after a new line is also required
To create one executable JAR file run on the command line
jar -cvmf mainfest.txt app1.jar *.class
Cheat Sheets
The following can be used as a reference for certain task which you have to do.
Working with classes
While programming Java you have to create several classes, methods, instance variables. The following uses the package test.
Table 4. What to do How to do it Create a new class called "MyNewClass".
package test;
public class MyNewClass {
}
Create a new attribute (instance variable) "var1" in MyNewClass with type String
package test;
public class MyNewClass {
private String var1;
}
Create a Constructor for "MyNewClass which has a String parameter and assigns the value of it to the "var1" instance variable.
package test;
public class MyNewClass {
private String var1;
public MyNewClass(String para1) {
var1 = para1;
// or this.var1= para1;
}
}
Create a new method "doSomeThing" in class which do not return a value and has no parameters
package test;
public class MyNewClass {
private String var1;
public MyNewClass(String para1) {
var1 = para1;
// or this.var1= para1;
}
public void doSomeThing() {
}
}
Create a new method "doSomeThing2" in class which do not return a value and has two parameters, a int and a Person
package test;
public class MyNewClass {
private String var1;
public MyNewClass(String para1) {
var1 = para1;
// or this.var1= para1;
}
public void doSomeThing() {
}
public void doSomeThing2(int a, Person person) {
}
}
Create a new method "doSomeThing2" in class which do return an int value and has three parameters, two Strings and a Person
package test;
public class MyNewClass {
private String var1;
public MyNewClass(String para1) {
var1 = para1;
// or this.var1= para1;
}
public void doSomeThing() {
}
public void doSomeThing2(int a, Person person) {
}
public int doSomeThing3(String a, String b, Person person) {
return 5; // Any value will do for this example
}
}
Create a class "MyOtherClass" with two instance variables. One will store a String, the other will store a Dog. Create getter and setter for these variables.
package test;
public class MyOtherClass {
String myvalue;
Dog dog;
public String getMyvalue() {
return myvalue;
}
public void setMyvalue(String myvalue) {
this.myvalue = myvalue;
}
public Dog getDog() {
return dog;
}
public void setDog(Dog dog) {
this.dog = dog;
}
}
11.2. Working with local variable
A local variable must always be declared in a method.
Table 5. What to do How to do it Declare a (local) variable of type string. String variable1; Declare a (local) variable of type string and assign "Test" to it. String variable2 = "Test"; Declare a (local) variable of type Person Person person; Declare a (local) variable of type Person, create a new Object and assign the variable to this object. Person person = new Person(); Declare a array of type String String array[]; Declare a array of type Person and create an array for this variable which can hold 5 Persons. Person array[]= new Person[5]; Assign 5 to the int variable var1 (which was already declared); var1 = 5; Assign the existing variable pers2 to the exiting variable pers1; pers1 = pers2; Declare a ArrayList variable which can hold objects of type Person ArrayList<Person> persons; Create a new ArrayList with objects of type Person and assign it to the existing variable persons persons = new ArrayList<Person>(); Declare a ArrayList variable which can hold objects of type Person and create a new Object for it. ArrayList<Person> persons = new ArrayList<Person>();