Quick Learning Java for beginners

package org.o7planning.tutorial.javabasic.variable;

public class VariableExample1 {

	public static void main(String[] args) {

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

		// Assigning value to firstNumber
		firstNumber = 10;

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

		// Declare a variable type of float (32-bit real number)
		// This number is assigned a value of 10.2
		// The 'f' character at the end makes java understand that this is a float.
		float secondNumber = 10.2f;

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

		// Declare a number type of double (64-bit real number).
		// 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 'f'.
		double thirdNumber = 10.2d;

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

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

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

	}

}

package org.o7planning.tutorial.javabasic.variable;

public class VariableExample2 {

	public static void main(String[] args) {

		// Declare 3 numbers type of long (64-bit Integer)
		long firstNumber, secondNumber, thirdNumber;

		// Assign value to firstNumber
		// Character '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);
	}

}

if(condition 1)  {
   // Do something here
}else if(condition 2) {
   // Do something here
}else if(condition 3) {
   // Do something here
}
// Else
else  { 
   // Do something here
}

package org.o7planning.tutorial.javabasic.controlflow;

public class ElseIfExample1 {

	public static void main(String[] args) {

		// Declaring a varible of type int (32 bit integer)
		// Represent your test scores.
		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!");
		}

	}
}

int score = 60;

package org.o7planning.tutorial.javabasic.controlflow;

public class ElseIfExample2 {

	public static void main(String[] args) {

		// Declare a variable of type int, represents your age.
		int age = 20;

		// Test if 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 if 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");
			}

		}

	}
}

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");
		}


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

// variable_to_test: A varible to test.
switch ( variable_to_test ) {
  case value1:
   // Do something here ...
   break;
  case value2:
   // Do something here ...
   break;
  default:
   // Do something here ...
}

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");
		}

	}

}

// This is not allowed !!
case (age < 18) :

// case only accept a specific value, eg:
case 18:
  // Do something here ..
  break;

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");
			break;
		// 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");
		}

	}

}

// start_value: Start Value
// end_value: End Value
// increment_number: Added value after each iteration.
for ( start_value; end_value; increment_number ) {
   // Do something here
}

package org.o7planning.tutorial.javabasic.loop;

public class ForLoopExample1 {

	public static void main(String[] args) {

		// Declare a variable 'step', 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;

		}

	}

}

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 iteration 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);

	}

}

// While condition is true then do something.
while ( condition ) {
   // Do something here...
}

package org.o7planning.tutorial.javabasic.loop;

public class WhileExampe1 {

	public static void main(String[] args) {

		int value = 3;

		// While the 'value' is still less than 10, the loop still works.
		while (value < 10) {

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

			// Increase 'value' by adding 2
			value = value + 2;
		}
	}
	
}

// The do-while loop performs at least one iteration.
// And while the condition is true, it continues to work
do {
   // Do something here.
}
while (condition);

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 by 3
			value = value + 3;

		} while (value < 10);

	}
}

// Declare an array, not a specified number of elements.
int[] array1;

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

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

// Declare an array with elements assigned a specific values.
// This array has 4 elements
long[] array3= {10L, 23L, 30L, 11L};

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 value to the second element (index 1)
		myArray[1] = 14;

		myArray[2] = 36;
		myArray[3] = 27;

		// Assigning value to 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]);
		}
	}

}

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]);
	}

}

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
	// The constructor always has the same name as the class name.
	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;
	}

}

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 
		Person edison = new Person("Edison");

		// Class Person has the getName() method.
		// 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 
		Person billGate = new Person("Bill Gates");

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

	}

}

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 to initialize the FieldSample object.
	public FieldSample(String myValue) {
		this.myValue = myValue;
	}

}

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 - MY_STATIC_FIELD
		// Access via instance of class (an object)(Not recommended)
		System.out.println("obj1.MY_STATIC_FIELD= " + obj1.MY_STATIC_FIELD);

		// Print out static value - MY_STATIC_FIELD
		// Access via class (Recommended)
		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);

		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);
	}
	
}

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;

}

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;
	}

	// This is a static method.
	// return type of int, has 3 parameters.
	public static int sum(int a, int b, int c) {
		int d = a + b + c;
		return d;
	}
}

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 setText(String) method
		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 object.
		// This way is not recommended. (**)
		int sum2 = obj.sum(20, 30, 40);

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

}

package org.o7planning.tutorial.javabasic.inheritance;

public class Animal {

	public Animal() {

	}

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

	// The cry of this animal.
	public void say() {
		System.out.println("<nothing>");
	}

}

package org.o7planning.tutorial.javabasic.inheritance;

public class Cat extends Animal {

	// Override method of the Animal class.
	public void say() {
		System.out.println("Meo");
	}

}

package org.o7planning.tutorial.javabasic.inheritance;

public class Dog extends Animal {

	// Override method of the Animal class.
	public void say() {
		System.out.println("Go");
	}

}

package org.o7planning.tutorial.javabasic.inheritance;

// Ant
public class Ant extends Animal {

}

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

		// ==> Meo
		// say() method of Cat is called (Not Animal)
		cat2.say();

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

		// Ant has no say() method.
		// ==> It call to say() method,
		// that inherited from the parent class (Animal)
		ant.say();
	}
}