1. Description

If the code is not a problem yesterday, I probably would not notice the difference between Double and double use;

Question 1 : If an int data type of cast can turn into a double, but I accidentally wrote double Double (main problem is I do not understand the use of Double and double, confused) in the project;
then error: can not cast 'int' to 'java.lang.Double', examples are as follows:

The error can be seen that a Double java class: java.lang.Double; is the basic data type double

Question 2 : Item inspection found a type of double data can be converted into a Double, and not being given

        double smD = 2.22;
        System.out.println((Double)smD);
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Indeed, in jdk1.5 above, (automatic automatic packing and unpacking) Double double wrapper class is, if converted into double Double type data, then this data can be used in all methods Double ;

Auto-boxing and auto-unboxing:

Double d =new Double(dou)可将double基本数据类型装箱为Double包装类
double dou =d.doubleValue()可将Double的包装类拆箱为基本数据类型
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And double the difference 2.Double

Double is a class object is created, stored on the heap;
basic data type is a double, reference is created is stored in the stack;
Double is a double package type, built many ways to achieve String to double conversion, and acquires the attribute values (MAX_VALUE, SIZE, etc.) type double
Double basic methods:

3.java basic data types

Since the basic data type conversion is not familiar with, so look back, do a little summary.

3.1 automatic type conversion

Defined : small numbers indicate the type of data can be automatically converted into a wide range of data types;

Data overflow : multiplying two int, int result is outside the range expressed.
Solution : a first generally converts data into a wide range of data types and other data types and then calculates;

问题：
int count = 100000000;
int price = 1999;
long totalPrice = count * price;
输出结果为负数；

解决：
int count = 100000000;
int price = 1999;
long totalPrice = (long) count * price;

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3.2 Mandatory conversion

Conversion to a forced display data type to another data type, data overflow careful attention to :

int ii = 300;
byte b = (byte)ii;
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300 has exceeded the scope of the byte type represents, it will be converted into a digital meaningless.

3.3 type promotion

The so-called type promotion refers to data in a variety of different types of expressions, the type is automatically told of a large range of data types to enhance the value of

long count = 100000000;
int price = 1999;
long totalPrice = price * count;
price 为 int 型，count 为 long 型，运算结果为 long 型，运算结果正常，没有出现溢出的情况。
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Use of the difference between Double and double, to avoid the pit