Java Advanced-Packaging
The packaging class is designed for generics. The collections are still used here. If the collection generics are not written by default, is it the Object type? Every time you have to force it, use generics to solve this problem. But generics need packaging classes instead of ordinary basic data types.
Basic data types: byte, short, int, long, float, double, char, boolean
Corresponding packaging types: Byte, Short, Integer, Long, Float, Double, Character, Boolean
Examples:
Integer a = 9; //自动装箱
int i = a; //自动拆箱
Integer b = new Integer(23);//标准构造
Integer c = new Integer("123");
Integer d = Integer.valueOf(1);//valueOf方法
Integer e = Integer.valueOf("65");
//特殊的例子:byte在-128~127，会缓存在JVM中，
//在Integer创建对象时，如果有相同的值在缓存中，
//则返回原对象的地址，不会再创建对象
//改成-128~127就可以
Integer j = 128;
Integer k = 128;
Integer m = new Integer(128);
Integer n = Integer.valueOf(128);
System.out.println(j==k); //false
System.out.println(j==m); //false
System.out.println(j==n); //false
Character ch = new Character('1');//只能是字符
System.out.println(ch);
Double dg = new Double("34.5f");
System.out.println(dg);
//Number
Number number =new Integer("12");
System.out.println(number);
For example: Character ch = new Character('1');//It can only be a character, it
can only be a single quotation mark, and it can only be a character.
