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.class
files and can be executed by the Java Virtual Machine.
.class
files to
load. This instructions can be used in the Java program.
java -versionIf Java is correctly installed, you should see some information about your Java installation. If the command line returns the information that the program could not be found, you have to install Java. The central website for installing Java is the following URL:
http://java.comIf you have problems installing Java on your system, a Google search for How to install JDK on
YOUR_OS
should result
in
helpful links. Replace
YOUR_OS
with your
operating
system, e.g.
Windows,
Ubuntu, Mac OS X, etc.
c:\temp\java
which will be called
javadir
in the following description.
// A small Java program public class HelloWorld { public static void main(String[] args) { System.out.println("Hello World"); } }
HelloWorld.java
filename. The name of a Java source
file must always equals
the class
name (within the source code)
and end
with the
.java
extension. In this example
the
filename must be
HelloWorld.java
because
the
class is called
HelloWorld.
javac HelloWorld.javaCheck the content of the directory with the command "dir". The directory contains now a file "HelloWorld.class". If you see this file you have successfully compiled your first Java source code into bytecode.
java HelloWorldThe system should write "Hello World" on the command line.
java HelloWorldIf you are not in the directory in which the compiled class is stored then the system should result an error message Exception in thread "main" java.lang.NoClassDefFoundError: test/TestClass
java -classpath "mydirectory" HelloWorld
package ui; import java.awt.BorderLayout; import java.awt.GridLayout; 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 do not 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 do not like
you," +firstName.getText() + " " + lastName.getText() +
"!"); } else { JOptionPane.showMessageDialog(null, "How are
you, "
+firstName.getText() + " " + lastName.getText()
+ "?");
}
}
}
}
package test; import ui.MyFirstUI; public class MainTester { public static void main(String[] args) { MyFirstUI view = new MyFirstUI(); view.setVisible(true); } }
| 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 |
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;
}
}
| 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. |
// 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"
// 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);
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;
}
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();
}
}
| 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;
}
}
|
| 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>(); |
