Быстрое изучение Java для начинающих

1- Введение

Для начала, что нужно для обучения Java, смотрите по ссылке:

2- Создать новый Project

Сначала мы создаем новый Project, он будет использоваться в данной инструкции.
Введите название project:
  • BasicJavaTutorial
Это изображение созданного Project:
Примечание: Чтобы можно было писать другими языками помимо английского в Project, вам нужно поменять кодирование на UTF8.
Нажмите на правую кнопку мыши в Project выберите Properties:

3- Виды примитивных данных (Primitive Data Types)

Имеется 8 видов примитвных данных (primitive data) в JAVA:
  • Для целого числа есть 4 вида:   byte, short, int, long
  • Вид настоящих данных есть : float, double
  • Вид символа: char
  • Логический вид: возвращает значение true или false (правильно или неправильно)
Type Description bit Min value Max value
byte 8 bit integer number 8  -128 (-2^7) 127 (2^7-1)
short 16 bit integer number 16 -32,768 (-2^15) 32,767 (2^15 -1)
int 32 bit integer number 32 - 2,147,483,648
(-2^31)
2,147,483,647
(2^31 -1)
long 64 bit integer number 64 -9,223,372,036,854,775,808
(-2^63)
9,223,372,036,854,775,807
(2^63 -1)
float 32 bit real number 32 -3.4028235 x 10^38 3.4028235 x 10^38
double 64 bit real number 64 -1.7976931348623157 x 10^308 1.7976931348623157 x 10^308
boolean Logic type   false true
char Character type 16 '\u0000' (0) '\uffff' (65,535).

4- Переменная (Variable)

Нажмите на правую кнопку мыши в src выберите "New/Package":
Назовите package:
  • org.o7planning.tutorial.javabasic.variable
Введите название класса:
  • VariableExample1
Класс  VariableExample1 создан:
Изменить код класса VariableExample1:
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);

    }

}
Запустить класс  VariableExample1:
Нажать на правую кнопку мыши на класс VariableExample1 выбрать "Run As/Java Application":
Результаты запуска класса для просмотра на Console:
Вы так же можете объявить несколько переменные одновременно, следующий пример иллюстрирует это:
Создать новый класс 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);
    }

}
Результаты запуска класса  VariableExample2:

5- Управление потоком программы (Control flow)

5.1- Команда if - else

Структура команды if - else является:
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
}
Создать 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!");
        }

    }
}
Результаты запуска класса ElseIfExample1:
Изменить значение переменной  "score" в примере выше и перезапустать класс ElseIfExample1:
int score = 20;

5.2- Обычные операторы

  • > Больше
  • < Меньше
  • >= Больше или равно
  • <= Меньше или равно
  • && и
  • || или
  • == Сравнить с
  • != Сравнить разницу
  • ! Отрицание
Создать класс 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");
            }

        }

    }
}
Вы можете изменить значение "age" и запустить класс ElseIfExample2 и посмотреть результаты

5.3- Значение boolean

boolean это вид данных, имеющий только 2 значения true или  false (Правильно или неправильно).
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- Команда switch- case -default

Это так же является команда из ветки схожей с if-else представленной выше.
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");
        }

    }

}
Результаты запуска класса  SwitchExample1:
Заметьте, что с командой  case должно быть определенное значение вы не можете сделать следующее:
// This is not allowed !!
case (age < 18) :

// case only accept a specific value eg:
case 18:
  // Do something here
  break;
Смотрите другой пример:
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");
        }

    }

}
Результаты запуска примера:

5.5- Цикл for

Это структура цикла:
for ( start_value; end_value; increment_number ) {
  // Code here
}
Рассмотрим изображенный пример:
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;

        }

    }

}
Результаты запуска класса ForLoopExample1:
Смотрите другой пример, посчитать сумму чисел от 1 до 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);

    }

}
Результаты:

5.6- Цикл while

Это структура цикла  while:
// While the condition is true, then do something.
while ( condition ) {
 // Do something here.
}
Смотрите изображенный пример:
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- Цикл do-while

Это структура цикла  do-while:
// 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 );
Изображенный пример:
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);

   }
}
Результат:

6- Массив в Java (Array)

6.1- Что такое массив?

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- Работа с массивом

Как объявить массив в 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};
Посмотрим пример:
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]);    
        }
    }
}
Результат:
Пример изображает использоватние цикла для прикрепления значений элементам:
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]);
    }
}
Результат:

7- Class, объект и конструктор (Class, Instance, Constructor)

Вам нужно различать 3 понятия
  • Класс
  • Конструктор (Constructor)
  • Объект (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 method. 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: