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【1】 understand if Conditional statements

if It's a word in English , I believe that as long as you can lose English, you will know its meaning , you 're right , It is " If " It means .

From the literal meaning of understanding , The word is usually associated with an uncertain event . such as ： If tomorrow is anything , I'm just what I am . That is to say, what I have to do is decided according to the result of tomorrow , Such a statement is a conditional statement . such as ： If it doesn't rain tomorrow , I'll go out and play , Or I'll stay at home . So this statement is programmed if To translate is ：

if( It won't rain tomorrow ) {
I'll go out and play
} else {
I'll just stay at home
}

In this code else In English it means “ Other ” Meaning , That is, if the weather is anything else （ It's raining ） situation , I'll just stay at home .

【2】if ... else ...

The next dice , Everyone should have played , Divided into 1,2,3,4,5,6 spot , If only size , that 1、2、3 For small ,4、5、6 For big .

Then it can also be expressed as ：

if( points <=3 ) {
Small
} else {
Big
}

Or for ：

if( points > 3 ) {
Big
} else {
Small
}

I believe many friends have seen this if Using the method of , you 're right , When if If the condition in the parentheses after is true , Then execute the following statement , Otherwise, it will be executed else The words in it .

Everyone should begin to understand the theory , Now let's run the code , The new class Test09, And add the following code ：

package com.java;
/**
* if Conditional statements
*/
public class Test10 {
public static void main(String[] args) {
int x = 2;
if (x <= 3) {
System.out.println(" Small ");
} else {
System.out.println(" Big ");
}
}
}

Run the program ： You can see that when the program x be equal to 2 When , The output is zero " Small ".

We will x The initial value is changed to 6, Then try running the program again ：

You can see , This time it's printed else Inside the braces  “ Big ”.

Some friends have thought of the binocular operator in the last lesson after reading here .

Yes , stay if ... else ... In this structure , He and the ternary operator can be converted to each other .

For example, trinomial operations ：

int x = 10;
int y = (x < 10) ? 0 : 1;

It can be transformed into equivalent if ... else ...

int x = 10;
int y;
if (x < 10) {
y = 0;
} else {
y = 1;
}

Let's verify through the program that ：

The result of the binocular operation ：

if Calculation results ：

It can be seen that the two results are consistent .

As for when to use the ternary operator , When to use if What about conditional statements ？ This will do both , Look at your hobbies .

Generally speaking , The code of ternary operator is less , Code reduction , But compared with if Compare sentences , The code is less readable , When the logic of the code is not high , You can use it according to your hobby .

But in teamwork , If you encounter huge and complex operational logic , Suggest using if , The code written in this way can be easily understood by others , To achieve the goal of joint cooperation .

【3】if ... else if ... else...

In the game of size , Our dice are big and small . But if you play big and small ,1 and 2 For small ,3 and 4 For in the ,5 and 6 For big , Looks like if ... else ... This structure does not support , Because he has only two situations . But we can go through another structure  if ... else if ... else... To show it , As follows ：

if( Points for 1 or 2) {
Small
} else if( Points for 3 or 4) {
in
} else {
Big
}

We change the code and execute it ： You can see , When x by 3 When , Only the middle bracket condition holds , So output the following statement .

Let's give you another example , Here is   New age segment ：

Suppose we use variables age It means age , So we can express it as ：

This is a combination of the first 8 The comparison operator knowledge of the lesson , If you have mastered , Here should be the second to understand .

if(age <6) {
Young children
} else if(age >=7 && age <= 12) {
a juvenile
} else if(age >=13 && age <= 17) {
teenagers
} else if(age >=18 && age <= 45) {
youth
} else if(age >=46 && age <= 69) {
middle-aged
} else {
Big
}

【 Related links 】

Last one ： Xiaobaixue Java The first 9 course ： Operator （ Two ）

Next ： Xiaobaixue Java The first 11 course ：switch Multi branch statement

general programme ： Returns a list of ​​​​​​​