1. Operators : Mathematical operators + - * / % =
Comparison operators > ,< , >,= ,<,= , != ,==;
Incrementing and decrementing ++, --
Logical operators & ,| ,! , && , ||;
Conditional ( ternary ) operator a>b?a:b
a. Mathematical operators: the closure of operations ( the closure of complement operations ), the closure of java operations is the closure of complement operations
For example , the final result of a value operation of type int is type int .
b. Operation rules in java
Participating operations of the same type ( may require automatic type conversion )
Returns the same type:
The three types of byte, short, and char are representations in java , and they are operated according to int at the bottom layer .
Summary :
1. In the + - * operation, the main focus is on the type of operation, and the problem of overflow
2. The division operation is an integer division .
3. Surplus :
The remainder of 0 for any number is 0.
modulo a fixed value, the result is a periodic function
(n++%3)
The remainder of a negative number is a negative number
The remainder of a positive number is a positive number
c . Self-increase and self-decrease
++ --
Operator ++ that increments ( decrements ) the current variable by itself
int a ;
Before ++++ a;
post ++a++;
From the ++ expression itself:
There is no difference between before and after ++ ,
are given to the current variable
self increase 1
From the whole operation expression:
a++: first set the value of a as the value of this a++ expression, then increment a by 1
++a: first add 1 to the value of a, and then use it as the value of the ++a expression to participate in the operation
d. Logical expression:
Short circuit with &&
①&&②
When condition ① is false ,
Condition ② is not executed and is short-circuited
short circuit or ||
①||②
When condition ① is true ,
Condition ② is not executed and is short-circuited
e. Ternary operator:
logical expression (true/false) ? expression1 : expression2
Decide to execute expression 1 according to the result of the logical expression
or expression 2
true : expression1 _
false: expression 2
2. Branch flow control statement:
1. Branch flow control statement
a.if
1. if ( condition ) { code body }
通过判断条件的true/false，来决定是否执行{}. 如果为true,则执行，为false,不执行
2. if(条件){//条件为true}else{//条件为false}
通过判断条件的true/false,来决定执行哪个大括号中的代码.
如果为true，执行if后面的{}
如果为false,执行else后面的{}
3. if(条件){
}else if(条件){
}.....{
}else{
}
if必须要出现，else if可有0-n个，else可有0-1个。
通过判断每个分支的条件，如果条件为true. 则执行对应的{}
当有一个条件为true后，其他的分支不再判断.如果所有的条件判断都为false，则执行else
b.switch case
switch(整数表达式){
case 条件1：
//代码
break;
case 条件2：
case 条件3：
...........
case 条件n:
default:
}
switch(表达式)，只能是整数(int,byte,char,short)，不能是long或者其他类型；
case条件只能是整数(int，byte,short, char)常量(字面量)，不能是变量及表达式；
case条件的内容可以为空,如果为空，继续执行下面的代码；
default表示如果没有满足case条件的其他 一切情况；
在每一个case分支中，需要写break关键字。如果不写，会出现穿透现象.
注： 穿透现象:下面所有的case分支不再进行判断直接执行.
2.循环控制
循环：重复去做某一些事情。
java中的循环:
while(条件表达式){ //循环体}
通过判断条件表达式的结果决定是否执行循环体.
true:执行循环体, false:退出循环
int a = 3 ;
while(a>0){
System.out.println(a);
a--;
}
一般来说，使用while循环的时候都是使用它的死循环。
while(true){
//跳出循环的判断
if(条件为true){
break;
}
}
do{//循环体}while(条件表达式),先执行循环体，后判断条件