Java基础之标示符，变量，流程控制

java注释

Java的注释分三种：单行、多行以及文档注释。

      单行： //后面的内容就是单行注释；

      多行：/* 中间部分 就是多行注释 */；

Java标识符

     标识符(合法的名字)：类名、方法名等。标识符由字母、下划线_、美元符号$或数字组成。

     标识符应由字母、下划线或美元符号开头，且不能是Java的关键字和保留字 。共53个关键字，

     其中有2个保留字：const、goto，它们在Java中目前没有被 

     使用，因此不具有意义。我们的类名应尽量有意义。

java 运算符

Java提供了一组运算符丰富的操纵变量。我们可以把所有的Java操作符为以下几组：

    1.>算术运算符

        算术运算符用于在数学表达式中，他们是在代数中使用的方法相同。下表列出了算术运算符：

        假设整型变量A=10和变量B=20，则：

        算术运算实例

       运算符 描述 实例

运算符 描述 实例

+	Addition - Adds values on either side of the operator	A + B = 30
-	Subtraction - Subtracts right hand operand from left hand operand	A - B = -10
*	Multiplication - Multiplies values on either side of the operator	A * B = 200
/	Division - Divides left hand operand by right hand operand	B / A = 2
%	Modulus - Divides left hand operand by right hand operand and returns remainder	B % A = 0
++	Increment - Increases the value of operand by 1	B++ =21
--	Decrement - Decreases the value of operand by 1	B-- =19

    2.>关系运算符

       假设变量A=10和变量B=20，则：

       关系运算符实例  

运算符 描述 实例

==	Checks if the values of two operands are equal or not, if yes then condition becomes true.	(A == B) is not true.
!=	Checks if the values of two operands are equal or not, if values are not equal then condition becomes true.	(A != B) is true.
>	Checks if the value of left operand is greater than the value of right operand, if yes then condition becomes true.	(A > B) is not true.
<	Checks if the value of left operand is less than the value of right operand, if yes then condition becomes true.	(A < B) is true.
>=	Checks if the value of left operand is greater than or equal to the value of right operand, if yes then condition becomes true.	(A >= B) is not true.
<=	Checks if the value of left operand is less than or equal to the value of right operand, if yes then condition becomes true.	(A <= B) is true.

    3.>位运算符

         Java定义了几个位运算符，它可以应用到整数类型，长型，整型，短整型，字符和字节。

         位运算符作用于位，并执行逐位操作。假设当a =60和b= 13; 现在以二进制格式，他们将会如下：

          a = 0011 1100

          b = 0000 1101

           -----------------

           a&b = 0000 1100

           a|b = 0011 1101

           a^b = 0011 0001

           ~a  = 1100 0011

            下表列出了按位运算符：

            假设整型变量A=60和变量B=13，则：            

运算符 描述 实例

&	Binary AND Operator copies a bit to the result if it exists in both operands.	(A & B) will give 12 which is 0000 1100
|	Binary OR Operator copies a bit if it exists in either operand.	(A | B) will give 61 which is 0011 1101
^	Binary XOR Operator copies the bit if it is set in one operand but not both.	(A ^ B) will give 49 which is 0011 0001
~	Binary Ones Complement Operator is unary and has the effect of 'flipping' bits.	(~A ) will give -61 which is 1100 0011 in 2's complement form due to a signed binary number.
<<	Binary Left Shift Operator. The left operands value is moved left by the number of bits specified by the right operand.	A << 2 will give 240 which is 1111 0000
>>	Binary Right Shift Operator. The left operands value is moved right by the number of bits specified by the right operand.	A >> 2 will give 15 which is 1111
>>>	Shift right zero fill operator. The left operands value is moved right by the number of bits specified by the right operand and shifted values are filled up with zeros.	A >>>2 will give 15 which is 0000 1111

    4.> 逻辑运算符

          下表列出了逻辑运算符：

           假设布尔变量A=ture，变量B=false，那么：

运算符 描述 实例

&&	Called Logical AND operator. If both the operands are non-zero, then the condition becomes true.	(A && B) is false.
||	Called Logical OR Operator. If any of the two operands are non-zero, then the condition becomes true.	(A || B) is true.
!	Called Logical NOT Operator. Use to reverses the logical state of its operand. If a condition is true then Logical NOT operator will make false.	!(A && B) is true.

    5.>赋值运算符

          

运算符 描述 实例

=	Simple assignment operator, Assigns values from right side operands to left side operand	C = A + B will assign value of A + B into C
+=	Add AND assignment operator, It adds right operand to the left operand and assign the result to left operand	C += A is equivalent to C = C + A
-=	Subtract AND assignment operator, It subtracts right operand from the left operand and assign the result to left operand	C -= A is equivalent to C = C - A
*=	Multiply AND assignment operator, It multiplies right operand with the left operand and assign the result to left operand	C *= A is equivalent to C = C * A
/=	Divide AND assignment operator, It divides left operand with the right operand and assign the result to left operand	C /= A is equivalent to C = C / A
%=	Modulus AND assignment operator, It takes modulus using two operands and assign the result to left operand	C %= A is equivalent to C = C % A
<<=	Left shift AND assignment operator	C <<= 2 is same as C = C << 2
>>=	Right shift AND assignment operator	C >>= 2 is same as C = C >> 2
&=	Bitwise AND assignment operator	C &= 2 is same as C = C & 2
^=	bitwise exclusive OR and assignment operator	C ^= 2 is same as C = C ^ 2
|=	bitwise inclusive OR and assignment operator	C |= 2 is same as C = C | 2

    6.> 其它运算符

              条件运算符也被称为三元运算符。该运算符包括三个操作数，用于评估计算布尔表达式。

             此运算符的目标是确定哪些值应分配给该变量。可写为：

                   variable x = (expression) ? value if true : value if false

             

              instanceof运算符：

                这个操作符只用于对象引用变量。操作检查对象是否为特定类型（类类型或接口类型）。   

                instanceof 运算符被写为：

                      ( Object reference variable ) instanceof  (class/interface type)

  

 流程控制语句

     Java主要的控制语句有三种，选择语句、循环语句、跳转语句。

      选择结构： 在Java中有两种选择语句可以使用：if和switch

       if(条件)  { 

             //语句块1

       } else{ 

             //后续语句

       }

      switch语句：表达式必须是byte、short、int或者是char类型。

                           在case后边的value值必须是跟表达式类型一致类型或 者是可以兼容的类型，不能出现重复的value值。

       switch ( 整型或字符型变量 )

        {

　           case 变量可能值1 ：   分支一; 

　                   break;

               case 变量可能值2 ： 分支二; 

　                  break;

               case 变量可能值3 ：  分支三; 

　                  break;

                       ...

               default : 最后分支; 

      ｝

      

       循环结构：Java中的常用循环形式有三种，for、while、do-while循环

           1> do{

                 //do something...

             }while(返回boolean值的表达式)

 

          2> for(初始化语句;返回boolean值得表达式;每次循环体执行完后执行的代码)

            {

                  //do something...

             }

           3>while(返回boolean值的表达式){

                   do something...

                }

数组

   数组的初始化分为两种：

       1>静态初始化：int[] a = new int[]{1,2,3}//只指定数组的元素，让系统 

               来决定数组的长度,推荐使用这种。还有一种写法：int[] a={1,2,3}; //直接指定数组元素。

       2>动态初始化：int[] a=new int[3];//只指定数组的长度，让系统来决定数组的元素值

Arrays 类:

java.util.Arrays中的类包含各种静态方法用于排序和搜索数组，数组的比较和填充数组元素。这些方法被重载的所有基本类型。

SN 方法和描述

1	public static int binarySearch(Object[] a, Object key) 搜索对象的指定数组（字节，整数，双精度等）使用二进制搜索算法来指定值。该数组必须在进行此调用之前对分类。这将返回索引搜索关键字，如果它被包含在列表 (-(insertion point + 1).
2	public static boolean equals(long[] a, long[] a2) 如果多头的两个指定数组彼此相等返回true。两个数组认为是相等判定方法：如果两个数组包含相同的元素数目，并在两个数组元素的所有相应对相等。如果两 个数组相等，返回true。同样的方法可以用于所有其它的原始数据类型 (Byte, short, Int, etc.)
3	public static void fill(int[] a, int val) 将指定的int值到指定的int型数组中的每个元素。同样的方法可以用于所有其它的原始数据类型(Byte, short, Int etc.)
4	public static void sort(Object[] a) 排序对象指定的数组升序排列，根据其元素的自然顺序。同样的方法可以用于所有其它的原始数据类型( Byte, short, Int, etc.)