Common tool operations

< p align = "center" > common tool jobs

< p class = "1" > I. fill in the blanks

2、 Multiple choice questions

3、 Judgment question

　　（1） Integer it = 200;

Actually: integer it3 = integer valueOf(100);

Automatic packing function: automatically load int type into integer, box first, and then assign value

　　（2） Integer it2 = it+it1;

Actually: integer it2 = integer valueOf(it2.intValue() + it3. intValue());

First unpack it2 and it3, then add them to get a value of int type, then do the boxing operation on the value of int type, and finally assign a value

(1): generate a new object

String will generate new objects, which is inefficient and wastes memory space

StringBuffer and StringBuilder do not generate new objects, but operate in the original buffer

(2): variable

String is immutable

StringBuffer, StringBuilder variable

(3): is it safe

StringBuffer is secure

StringBuilder is not secure

(4): multithreading

StringBuffer is suitable for multi-threaded operation

StringBuilder is suitable for single thread operation

(5): efficiency

　　　　 StringBuilder>StringBuffer>String

Tip: use the relevant methods of the string class to complete

　　2. Receive the age, score and enrollment time in string format input from the keyboard, convert them to integer, floating point number and date type, and output them on the console.

Tip: use wrapper class integer, double and date conversion class dateformat to implement

6、 Topic selection

Tip: use math Nextdouble() or nextint() implementations of random() and random classes

　　2. Print a visual calendar for a month