Quick Learning Java for beginners

1- Introduction

First of all, you should see this document:

2- Create new Project

First we create a new Project, it will be used in this tutorial.
Enter the name of the project:
  • BasicJavaTutorial
Project has been created:
Note: To be able to write in languages other than English in the Project, you should switch to UTF8 encoding.
Right-click the Project and select Properties:

3- Primitive Data Types

There are eight primitive types in JAVA
  • For integer has 4 types: byte, short, int, long
  • Real data type we have: float, double
  • Character type: char
  • Boolean: returns true or false value (true or false)
Type Description bit Min value Max value
byte Số tự nhiên 8 bit 8  -128 (-2^7) 127 (2^7-1)
short Số tự nhiên 16 bit 16 -32,768 (-2^15) 32,767 (2^15 -1)
int Số tự nhiên 32 bit 32 - 2,147,483,648
(-2^31)
2,147,483,647
(2^31 -1)
long Số tự nhiên 64 bit 64 -9,223,372,036,854,775,808
(-2^63)
9,223,372,036,854,775,807
(2^63 -1)
float Số thực 32 bit 32 -3.4028235 x 10^38 3.4028235 x 10^38
double Số thực 64 bit 64 -1.7976931348623157 x 10^308 1.7976931348623157 x 10^308
boolean Kiểu logic   false true
char Kiểu ký tự 16 '\u0000' (0) '\uffff' (65,535).

4- Variable

Right-click the src select "New/Package":
Name the package are:
  • org.o7planning.tutorial.javabasic.variable
Enter the name of the class:
  • VariableExample1
VariableExample1 class is created:
Edit VariableExample1 class:
  • VariableExample1.java
package org.o7planning.tutorial.javabasic.variable;

public class VariableExample1 {

    public static void main(String[] args) {

        // Declare a variable of type int (integer 32-bit)
        int firstNumber;

        // Assigning values to firstNumber
        firstNumber = 10;

        System.out.println("First Number =" + firstNumber);

        // Declare a 32-bit real number (float)
        // This number is assigned a value of 10.2
        float secondNumber = 10.2f;

        System.out.println("Second Number =" + secondNumber);

        // Declare a 64-bit real numbers
        // This number is assigned a value of 10.2
        // character d at the end to tell with Java this is the type double.
        // Distinguished from a float.
        double thirdNumber = 10.2d;

        System.out.println("Third Number =" + thirdNumber);

        // Declare a character
        char ch = 'a';

        System.out.println("Char ch= " + ch);

    }

}
Run class VariableExample1:
Right-click on the class VariableExample1 select "Run As/Java Application":
Results of running the class to see on the console:
You can also declare multiple variables at once, the next example illustrates this:
Create a new class VariableExample2
  • VariableExample2.java
package org.o7planning.tutorial.javabasic.variable;

public class VariableExample2 {

    public static void main(String[] args) {

        // Declare three 64-bit integer (long)
        long firstNumber, secondNumber, thirdNumber;

        // Assign value to firstNumber
        // L at the end to tell java a long type, distinguished from type int.
        firstNumber = 100L;

        // Assign values to secondNumber
        secondNumber = 200L;

        // Assign values to thirdNumber
        thirdNumber = firstNumber + secondNumber;

        System.out.println("First Number = " + firstNumber);
        System.out.println("Second Number = " + secondNumber);
        System.out.println("Third Number = " + thirdNumber);
    }

}
Results of running the class VariableExample2:

5- Control flow

5.1- if-else statement

The structure of an if-else statement is:
if(condition1 true)  {
 // Do something here
}else if(condition2 true) {
 // Do something here
}else if(condition3 true) {
 // Do something here
}else  { // Other
 // Do something here
}
Create class ElseIfExample1:
  • ElseIfExample1.java
package org.o7planning.tutorial.javabasic.controlflow;

public class ElseIfExample1 {

    public static void main(String[] args) {
        
        // Declaring a integer number (int)        
        int score = 20;

        System.out.println("Your score =" + score);

        // If the score is less than 50
        if (score < 50) {
            System.out.println("You are not pass");
        }

        // Else if the score more than or equal to 50 and less than 80.
        else if (score >= 50 && score < 80) {
            System.out.println("You are pass");
        }

        // Remaining cases (that is greater than or equal to 80)
        else {
            System.out.println("You are pass, good student!");
        }

    }
}
Results of running the class ElseIfExample1:
Change the value of the variable "score" in the above example and rerun the class ElseIfExample1:
int score = 20;

5.2- The normal operator

  • > Greater Than
  • < Less Than
  • >= Greater Than or Equal To
  • <= Less Than or Equal To
  • && AND
  • || OR
  • == HAS A VALUE OF
  • != Not Equal To
  • ! NOT
Create class ElseIfExample2
  • ElseIfExample2.java
package org.o7planning.tutorial.javabasic.controlflow;

public class ElseIfExample2 {

    public static void main(String[] args) {

        // Declare a variable int simulate your age.
        int age = 20;

        // Test age less than or equal 17
        if (age <= 17) {
            System.out.println("You are 17 or younger");
        }

        // Test age equals 18
        else if (age == 18) {
            System.out.println("You are 18 year old");
        }

        // Test age, greater than 18 and less than 40
        else if (age > 18 && age < 40) {
            System.out.println("You are between 19 and 39");
        }

        // Remaining cases (Greater than or equal to 40)
        else {
            // Nested if statements
            // Test age not equals 50.
            if (age != 50) {
                System.out.println("You are not 50 year old");
            }

            // Negative statements
            if (!(age == 50)) {
                System.out.println("You are not 50 year old");
            }

            // If age is 60 or 70
            if (age == 60 || age == 70) {
                System.out.println("You are 60 or 70 year old");
            }

        }

    }
}
You can change the value of "age" and rerun ElseIfExample2 class and see the results

5.3- Boolean value

boolean is a data type, it only has two values true or false.
Create class BooleanExample:
  • BooleanExample.java
package org.o7planning.tutorial.javabasic.controlflow;

public class BooleanExample {

    public static void main(String[] args) {

        // Declare a variable of type boolean
        boolean value = true;

        // If value is true
        if (value == true) {
            System.out.println("It's true");
        }
        // Else
        else {
            System.out.println("It's false");
        }

        // With boolean values you can also write
        if (value) {
            System.out.println("It's true");
        }
        // Else
        else {
            System.out.println("It's false");
        }
    }
}

5.4- switch- case -default statement

This is also a branch statement similar to the If-Else is introduced above
switch ( variable_to_test ) {
  case value:
   // code_here;
   break;
  case value:
   // code_here;
   break;
  default:
   // values_not_caught_above;
}
  • SwitchExample1.java
package org.o7planning.tutorial.javabasic.controlflow;

public class SwitchExample1 {

    public static void main(String[] args) {

        // Declare a variable age
        int age = 20;

        // Check the value of age
        switch (age) {

        // Case age = 18
        case 18:
            System.out.println("You are 18 year old");
            break;

        // Case age = 20
        case 20:
            System.out.println("You are 20 year old");
            break;

        // Remaining cases
        default:
            System.out.println("You are not 18 or 20 year old");
        }

    }

}
The results of running the class SwitchExample1:
Note that the case statement is a specific value you can not do the following:
// This is not allowed !!
case (age < 18) :

// case only accept a specific value eg:
case 18:
  // Do something here
  break;
See an example:
  • SwitchExample2.java
package org.o7planning.tutorial.javabasic.controlflow;

public class SwitchExample2 {

    public static void main(String[] args) {

        // Declare a variable age
        int age = 30;

        // Check the value of age
        switch (age) {

        // Case age = 18
        case 18:
            System.out.println("You are 18 year old");

        // Case age in 20, 30, 40
        case 20:
        case 30:
        case 40:
            System.out.println("You are " + age);
            break;

        // Remaining case:
        default:
            System.out.println("Other age");
        }

    }

}
The results run:

5.5- for Loop

Syntax:
for ( start_value; end_value; increment_number ) {
  // Code here
}
Consider an illustration:
  • ForLoopExample1.java
package org.o7planning.tutorial.javabasic.loop;

public class ForLoopExample1 {

    public static void main(String[] args) {

        // Declare a variable, step in loop
        int step = 1;

        // Declare a variable value with the start value is 0
        // After each iteration, value will increase 3
        // And the loop will end when the value greater than or equal to 10
        for (int value = 0; value < 10; value = value + 3) {

            System.out.println("Step =" + step + "  value = " + value);

            // Increase 1
            step = step + 1;

        }

    }

}
Results of running the class ForLoopExample1:
See another example, sum the numbers from 1 to 100:
  • ForLoopExample2.java
package org.o7planning.tutorial.javabasic.loop;


public class ForLoopExample2 {

    
    // This is an example to sum the numbers from 1 to 100,
    // and print out the results to the console.
    public static void main(String[] args) {
           
        // Declare a variable
        int total = 0;
        
        // Declare a variable value
        // Initial value is 1
        // After each loop increases the 'value' by adding 1
        // Note: value++ equivalent to the statement: value = value + 1;
        // value-- equivalent to the statement: value = value - 1;
        for(int value = 1; value <= 100; value++)  {
            
            // Increase 'total' by adding value
            total = total + value;        
        }
        
        System.out.println("Total = "+ total);

    }

}
Result:

5.6- while Loop

This is while loop structure:
// While the condition is true, then do something.
while ( condition ) {
 // Do something here.
}
See illustration
  • WhileExample1.java
package org.o7planning.tutorial.javabasic.loop;

public class WhileExampe1 {

    
    public static void main(String[] args)  {
        
        int value = 3;
        
        // While the value is less than 10, the loop is working.
        while( value < 10)  {
            
            System.out.println("Value = "+ value);
            
            // Increase value by adding 2
            value = value + 2;
        }
    }
}

5.7- do-while Loop

This is the structure do-while loop:
// The do-while loop to work at least one round
// and while the condition is true, it also works to
do {
  // Do something here.
}
while ( condition );
Example:
  • DoWhileExample1.java
package org.o7planning.tutorial.javabasic.loop;

public class DoWhileExample1 {

   public static void main(String[] args) {

       int value = 3;

        // do-while loop will execute at least once
       do {

           System.out.println("Value = " + value);

           // Increase 3
           value = value + 3;

       } while (value < 10);

   }
}
Result:

6- Array in Java

6.1- What is Array?

An array is a list of the elements are arranged adjacent to each other in memory.
Let's see, a array with 5 elements, int type.

6.2- Working with arrays

How to declare an array in Java.
// Declare an array, not a specified number of elements.
int[] array1;


// Initialize the array with 100 elements
// The element has not been assigned a specific value
array1 = new int[100];

// Declare an array specifies the number of elements
// The element has not been assigned a specific value
double[] array2 = new double[10];

// Declare an array whose elements are assigned specific values.
// This array with 4 elements
long[] array3= {10L, 23L, 30L, 11L};
Let's look at an example:
  • ArrayExample1.java
package org.o7planning.tutorial.javabasic.array;

public class ArrayExample1 {

    
    
    public static void main(String[] args) {
        
        // Declare an array with 5 elements
        int[] myArray= new int[5];
        
        // Note: the first element of the array index is 0:
        
        // Assigning values to the first element (index 0)
        myArray[0] = 10;
        
        // Assigning values to the second element (index 1)
        myArray[1] = 14;
        
        myArray[2] = 36;
        myArray[3] = 27;
        
        // Value for the 5th element (the last element in the array)
        myArray[4] = 18;
        
        // Print out element count.
        System.out.println("Array Length="+ myArray.length);
        
        // Print to Console element at index 3 (4th element in the array)
        System.out.println("myArray[3]="+ myArray[3]);
        
        // Use a for loop to print out the elements in the array.
        for(int index=0; index < myArray.length; index++) {
            System.out.println("Element "+ index+" = "+ myArray[index]);    
        }
    }
}
Result:
An illustrative example to use a for loop to assign values to the elements:
  • ArrayExample2.java
package org.o7planning.tutorial.javabasic.array;

public class ArrayExample2 {

    public static void main(String[] args) {

        // Declare an array with 5 elements
        int[] myArray = new int[5];

        // Print out element count
        System.out.println("Array Length=" + myArray.length);

        // Using loop assign values to elements of the array.
        for (int index = 0; index < myArray.length; index++) {
            myArray[index] = 100 * index * index + 3;
        }
        
        // Print out the element at index 3
        System.out.println("myArray[3] = "+ myArray[3]);
    }
}
Result:

7- Class, Instance, Constructor

You need to have a distinction between three concepts:
  • Class
  • Constructor
  • Instance
When we talk about the tree, it is something abstract, it is a class. But when we pointed to a specific tree, it was clear, and that is the instance
Or when we talk about the Person, that's abstract, it is a class. But when pointed at you or me, it is two different instances, the same class of People.
  • Person.java
package org.o7planning.tutorial.javabasic.javastructure;

public class Person {

    
    // This is field
    // The name of Person
    public String name;
    
    // This is a Constructor
    // Use it to initialize the object (Create new object)
    // This constructor has one parameter
    // Constructor always have the same name as the class.
    public Person(String persionName) {
        // Assign the value of the parameter into the 'name' field
        this.name = persionName;
    }

    // This method returns a String ..
    public String getName() {
        return this.name;
    }

}
Person class has no main function. TestPerson class is initialized instance of the Person object via the constructor
  • PersonTest.java
package org.o7planning.tutorial.javabasic.javastructure;

public class PersonTest {

   public static void main(String[] args) {

       // Create an object of class Person
       // Initialize this object via constructor of class Person
       // Specifically, Edison
       Person edison = new Person("Edison");

       // Class Person has the method getName()
       // Use the object to call getName():
       String name = edison.getName();
       System.out.println("Person 1: " + name);

       // Create an object of class Person
       // Initialize this object via constructor of class Person
       // Specifically, Bill Gates
       Person billGate = new Person("Bill Gates");

       // Class Person has field name (public)
       // Use objects to refer to it.
       String name2 = billGate.name;
       System.out.println("Person 2: " + name2);

   }

}
Results of running the example:

8- Field

In this section we will discuss some of the concepts:
Field
  • Normal field
  • static Field
  • final Field
  • static final Field
See an example with the field and static fields.
  • FieldSample.java
package org.o7planning.tutorial.javabasic.javastructure;

public class FieldSample {
    
    // This is static field.
    public static int MY_STATIC_FIELD = 100;
    
    // This is normal field.
    public String myValue;
    
    
    // Constructor
    public FieldSample(String myValue)  {
        this.myValue= myValue;
    }

}
  • FieldSampleTest.java
package org.o7planning.tutorial.javabasic.javastructure;

public class FieldSampleTest {

    public static void main(String[] args) {

        // Create the first object.
        FieldSample obj1 = new FieldSample("Value1");

        System.out.println("obj1.myValue= " + obj1.myValue);

        // Print out static value, access via instance of class (an object).
        System.out.println("obj1.MY_STATIC_FIELD= " + obj1.MY_STATIC_FIELD);

        // Print out static value, access via class.
        System.out.println("FieldSample.MY_STATIC_FIELD= "
                + FieldSample.MY_STATIC_FIELD);

        // Create second object:
        FieldSample obj2 = new FieldSample("Value2");

        System.out.println("obj2.myValue= " + obj2.myValue);

        // Print out static value, access via instance of class (an object)
        System.out.println("obj2.MY_STATIC_FIELD= " + obj2.MY_STATIC_FIELD);

        System.out.println(" ------------- ");

        // Set new value for static field.
        // (Or using: FieldSample.MY_STATIC_FIELD = 200)
        obj1.MY_STATIC_FIELD = 200;

        // It will print out the value 200.
        System.out.println("obj2.MY_STATIC_FIELD= " + obj2.MY_STATIC_FIELD);
    }
}
Results of running the example:
The final field is the field that can not assign a new value to it, it's like a constant.
  • FinalFieldExample.java
package org.o7planning.tutorial.javabasic.javastructure;

public class FinalFieldExample {

    // A final field.
    // Final Field does not allow to assign new values.
    public final int myValue = 100;

    // A static final field.
    // Final field does not allow to assign new values.
    public static final long MY_LONG_VALUE = 1234L;
}

9- Method

Method
  • Method.
  • static Method
  • final Method. (Will be mentioned in the inheritance of the class).
  • MethodSample.java
package org.o7planning.tutorial.javabasic.javastructure;

public class MethodSample {

    public String text = "Some text";
    
    // Default Constructor
    public MethodSample()  {
        
    }

    // This method return a String
    // and has no parameter.
    public String getText() {
        return this.text;
    }

    // This is a method with one parameter String.
    // This method returns void (not return anything)    
    public void setText(String text) {
        // this.text reference to the text field.
        // Distinguish the text parameter.        
        this.text = text;
    }

    // Static method
    public static int sum(int a, int b, int c) {
        int d =  a + b + c;
        return d;
    }
}
  • MethodSampleTest.java
package org.o7planning.tutorial.javabasic.javastructure;

public class MethodSampleTest {

    public static void main(String[] args) {

        // Create instance of MethodSample
        MethodSample obj = new MethodSample();

        // Call getText() method
        String text = obj.getText();

        System.out.println("Text = " + text);

        // Call method setText(String)
        obj.setText("New Text");

        System.out.println("Text = " + obj.getText());

        // Static method can be called through the class.
        // This way is recommended. (**)
        int sum = MethodSample.sum(10, 20, 30);

        System.out.println("Sum  10,20,30= " + sum);

        // Or call through objects
        // This way is not recommended. (**)        
        int sum2 = obj.sum(20, 30, 40);

        System.out.println("Sum  20,30,40= " + sum2);
    }

}

10- Inheritance in Java

Java allows classes which extend from other class. Class extends another class called subclasses. Subclasses have the ability to inherit the fields and methods from the parent class.
  • Animal.java
package org.o7planning.tutorial.javabasic.inheritance;

public class Animal {

 public Animal() {

 }

 public void move() {
     System.out.println("Move ...!");
 }

 public void say() {
     System.out.println("<nothing>");
 }

}
  • Cat.java
package org.o7planning.tutorial.javabasic.inheritance;

public class Cat extends Animal {

    // Override method of the Animal class.
    public void say() {
        System.out.println("Meo");
    }
}
  • Dog.java
package org.o7planning.tutorial.javabasic.inheritance;

public class Dog extends Animal {

   // Override method of the Animal class.
   public void say() {
       System.out.println("Go");
   }
}
  • Ant.java
package org.o7planning.tutorial.javabasic.inheritance;

public class Ant extends Animal {

}
  • AnimalTest.java
package org.o7planning.tutorial.javabasic.inheritance;

public class AnimalTest {

    public static void main(String[] args) {

        // Declaring a Cat object.
        Cat cat = new Cat();

        // Check 'cat' instance of Animal.
        // The result is clearly true.
        boolean isAnimal = cat instanceof Animal;
        System.out.println("cat instanceof Animal?"+ isAnimal);

        // ==> Meo
        // Call the method say() of the Cat.
        cat.say();


        // Declare an object Animal
        // Initialize the object through the Constructor of the Cat.
        Animal cat2 = new Cat();

        // ==> Meo
        // Call to say() of Cat (Not Animal)
        cat2.say();

        // Create the object Animal
        // Through the Constructor of the class Ant.        
        Animal ant = new Ant();

        // Ant has no say() method.
        // It call to say() method that inherited from the parent class (Animal)        
        ant.say();
    }
}
Results of running the example:

11- Using Javadoc

Javadoc is the reference document for Java API. You can download Jdk9 Javadoc in CHM format at: