Java Novice Learning Guide [day13]---Commonly used classes

1. Packaging

Packaging class: java provides a packaging class for each basic data type

• byte     Byte
  char     Character
  short    Short
  int      Integer
  float    Float
  double   Double
  boolean  Boolean
  

  Function: The packaging class and the corresponding basic data type all represent the corresponding value, but the packaging class can better reflect the object-oriented thinking, for example, including methods: max, min

  Packing: Convert the basic data type to the corresponding packaging class

  Unboxing: Convert the packaging class to the corresponding basic data type

  The above process is divided into automatic and manual

           //手动装箱
  		Integer i1 = new Integer(99);
  		//手动拆箱
  		int i2 = i1.intValue();
  		//自动装箱
  		Integer i3 = 111;
  		//自动拆箱
  		int i4 = i3;
  

  Flyweight mode: only for integer types, in jvm, [-128,127] will be cached in an area in the heap.

  String constant pool: The string created by direct assignment will enter a specific area constant pool in the jvm heap.

  The difference between Flyweight and Constant Pool: Flyweight is cached in advance with JVM, while constant pool is cached when the string is created.

  2. Math method

  Commonly used method: PI abs (absolute value) pow (power operation)

  BigInteger: a larger number than the long type

  BigDecimal: Represents a decimal with higher precision than double

  3、System/Runtime

  System：arrayCopy currentTimeMills

  Runtime: Enables the application to connect to the environment in which it runs.

  4、 String/StringBuilder/StringBuffer

  Master some commonly used String methods

  StringBuilder: A variable character sequence. Concatenating strings is the most efficient

  StringBuffer: A thread-safe variable character sequence. Concatenating strings is more efficient