Java快速入门

标签：Java 入门 System value class println 快速 public out

初学者快速学习Java

1- 介绍

2- 创建Java工程

5.4- switch- case -default语句

1- 介绍

首先，学习Java需要什么，先阅这里，这里列出一些开发工具和环境配置：

http://www.yiibai.com/java/what-is-needed-to-get-started-with-java.html

2- 创建一个工程

首先，我们使用Eclipse（注意就是Eclise，如果没有安装好，选安装好再接着下一个步骤）创建一个新的项目，这将在本教程中使用。

输入项目名称：

BasicJavaTutorial

项目已创建：

注：为了能够在除英语工程其他语言可以使用，应该切换到UTF-8编码。右键单击该项目并选择属性：

3- 原始数据类型

JAVA中有八种基本类型

对于整数有4种类型：byte, short, int, long
实数类型： float, double
字符类型： char
布尔：返回 true 或 false 值 (true 或 false)

类型	描述	bit	最小值	最大值
byte	8位整数	8	-128 (-2^7)	127 (2^7-1)
short	16位整数	16	-32,768 (-2^15)	32,767 (2^15 -1)
int	32位整数	32	- 2,147,483,648 (-2^31)	2,147,483,647 (2^31 -1)
long	64位整数	64	-9,223,372,036,854,775,808 (-2^63)	9,223,372,036,854,775,807 (2^63 -1)
float	32位实数	32	-3.4028235 x 10^38	3.4028235 x 10^38
double	64位实数	64	-1.7976931348623157 x 10^308	1.7976931348623157 x 10^308
boolean	逻辑类型		false	true
char	字符	16	'\u0000' (0)	'\uffff' (65,535).

4- 变量

右键点击 src 并选择 "New/Package":

新建命名包是：

com.yiibai.tutorial.javabasic.variable

选择上面创建的包名 com.yiibai.tutorial.javabasic.variable，在弹出的菜单中选择 New 中选择 Class。输入类的名称：

VariableExample1

创建 VariableExample1 类如下：

VariableExample1.java

package com.yiibai.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);

    }
}

运行类 VariableExample1: 在 VariableExample1 类右键单击选择 "Run As/Java Application"：

运行类，在控制台上看到的结果如下：

您也可以一次声明多个变量，下例说明了这一点：创建一个新的类 VariableExample2

VariableExample2.java

package com.yiibai.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);
    }

}

运行类 VariableExample2 的结果 :

5- 控制流

5.1- if-else语句

if-else 语句的结构是：

if(condition1 true)  {
 // Do something here
}elseif(condition2 true) {
 // Do something here
}elseif(condition3 true) {
 // Do something here
}else  { // Other
 // Do something here
}

创建一个类 ElseIfExample1:

ElseIfExample1.java

package com.yiibai.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!");
        }

    }
}

运行 ElseIfExample1 类的结果：

改变在上面的例子中，变量“score”的值，然后重新运行ElseIfExample1类：

int score = 80;

5.2- 常规操作符

> 大于号
< 小于号
>= 大于或等于
<= 小于或等于
&& 且
|| 或
== 等一个值
!= 不等于一个值
! 非

创建一个类 ElseIfExample2

ElseIfExample2.java

package com.yiibai.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");
            }

        }

    }
}

您可以修改 “age” 的值，然后重新运行 ElseIfExample2 类，并查看结果。

5.3- 布尔值

布尔是一种数据类型，它只有两个值true或false。创建一个类 BooleanExample

BooleanExample.java

package com.yiibai.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 语句

这也是类似上面介绍的 if-else 分支语句：

switch( variable_to_test ) {
  casevalue:
   // code_here;
   break;
  casevalue:
   // code_here;
   break;
  default:
   // values_not_caught_above;
}

SwitchExample1.java

package com.yiibai.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");
        }

    }
}

运行类 SwitchExample1 的结果 :

You are 20 year old

请注意case语句是一个特定的值，不能做下面的操作：

// This is not allowed !!
case(age < 18) :
 
// case only accept a specific value eg:
case18:
  // Do something here
  break;

看下面的一个例子：

SwitchExample2.java

package com.yiibai.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");
        }

    }
}

运行结果：

You are 30

5.5- for循环

语法：

for( start_value; end_value; increment_number ) {
  // Code here
}

考虑如下一个例子：

ForLoopExample1.java

packagecom.yiibai.tutorial.javabasic.loop;
 
publicclass ForLoopExample1 {
 
    publicstaticvoidmain(String[] args) {
 
        // Declare a variable, step in loop
        intstep = 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(intvalue = 0; value < 10; value = value + 3) {
 
            System.out.println("Step ="+ step + "  value = "+ value);
 
            // Increase 1
            step = step + 1;
 
        }
 
    }
 
}

运行 ForLoopExample1 类结果：

Step =1  value = 0
Step =2  value = 3
Step =3  value = 6
Step =4  value = 9

另一实例中，从1至100的数字求和：

ForLoopExample2.java

package com.yiibai.tutorial.javabasic.loop;

public class ForLoopExample2 {
    public static void main(String[] args) {
        int sum = 0; for (int i = 0; i <= 100; i = i + 1) {
            sum = sum + i;
        }
       System.out.println(sum); }
}

结果：

5.6- while循环

这是 while 循环结构：

// While the condition is true, then do something.
while( 条件为真 ) {
  // Do something here.
}

参见图示

WhileExample1.java

publicclassWhileExampe1 {
 
     
    publicstaticvoidmain(String[] args)  {
         
        intvalue = 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循环

下面是do-while循环的结构：

// 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 );

如下图的示例：

DoWhileExample1.java

package com.yiibai.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);

    }
}

结果：

Value = 3
Value = 6
Value = 9

6- Java数组

6.1- 什么是数组？

数组是元素存储在彼此相邻列表。让我们来看看，一个数组有5个int型的元素。

6.2- 使用数组

如何在Java中声明数组。

// 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 = newint[100];
 
// Declare an array specifies the number of elements
// The element has not been assigned a specific value
double[] array2 = newdouble[10];
 
// Declare an array whose elements are assigned specific values.
// This array with 4 elements
long[] array3= {10L, 23L, 30L, 11L};

让我们来看一个例子：

ArrayExample1.java

package com.yiibai.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]);
        }
    }
}

结果：

Array Length=5
myArray[3]=27
Element 0 = 10
Element 1 = 14
Element 2 = 36
Element 3 = 27
Element 4 = 18

举一个实例来说明使用一个for循环来对元素赋值：

ArrayExample2.java

package com.yiibai.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]);
    }
}

输出结果：

Array Length=5
myArray[3] = 903

7- 类, 继承, 构造器

有三个概念需要进行区分：

类
构造
继承

当我们讨论树，它是抽象的东西，它是一个类。但是，当我们指出了一个特定的树（比如：槟榔树），很明显，那就是实例。或者，当我们谈论的人，这是抽象的，它是一个类。但是，当指向你或我，这是两种不同的情况下，都是同一个 Person 类。

Person.java

package com.yiibai.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类没有任何main函数。 TestPerson类通过构造函数初始化Person对象实例

PersonTest.java

package com.yiibai.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);

       }

    }

运行示例的结果如下：

Person 1: Edison
Person 2: Bill Gates

8- 字段

在本节中，我们将讨论一些概念：字段

一般字段
静态字段
final字段
static final 字段

下面看看字段和静态字段的例子。

FieldSample.java

package com.yiibai.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 com.yiibai.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);
    }
}

运行示例的结果：

obj1.myValue= Value1
obj1.MY_STATIC_FIELD= 100
FieldSample.MY_STATIC_FIELD= 100
obj2.myValue= Value2
obj2.MY_STATIC_FIELD= 100
 ------------- 
obj2.MY_STATIC_FIELD= 200

最后一个字段是不能一个新值分配给它的，它就像一个常数。

FinalFieldExample.java

package com.yiibai.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- 方法

有关方法的种类：

方法
静态方法
final 方法 (将在类的继承中说明)

MethodSample.java

package com.yiibai.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 com.yiibai.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);
    }

}

执行上面的程序输出结果如下：

Text = Some text
Text = New Text
Sum  10,20,30= 60
Sum  20,30,40= 90

10- 在Java中的继承

Java允许从其他类扩展类。类扩展另一个类称为子类。子类必须有继承父类中的字段和方法的能力。

Animal.java

package com.yiibai.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 com.yiibai.tutorial.javabasic.inheritance;

public class Cat extends Animal {

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

}

Dog.java

package com.yiibai.tutorial.javabasic.inheritance;

public class Dog extends Animal {

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

Ant.java

package com.yiibai.tutorial.javabasic.inheritance;

public class Ant extends Animal {

}

AnimalTest.java

package com.yiibai.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();
    }
}

运行示例的结果如下：

cat instanceof Animal?true
I am Cat
I am Cat
<nothing>

//更多请阅读：https://www.yiibai.com/java/java_quick_guide.html#article-start

标签：Java,入门,System,value,class,println,快速,public,out
From： https://www.cnblogs.com/cnetsa/p/17080870.html

1- 介绍

2- 创建一个工程

3- 原始数据类型

4- 变量

5- 控制流

5.1- if-else语句

5.2- 常规操作符

5.3- 布尔值

5.4- switch- case -default 语句

5.5- for循环

5.6- while循环

5.7- do-while循环

6- Java数组

6.1- 什么是数组？

6.2- 使用数组

7- 类, 继承, 构造器

8- 字段

9- 方法

10- 在Java中的继承

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