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
| + | 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
| == | 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：
| & | 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：
| && | 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
| = | 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。
| 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.) |