Chapter3.Know Your Variables

variables types

• Variables come in two flavors

  • primitive

    • as object state(instance variables)

  • reference

    • as local variables( variables decleard within a method )

primitive types

Type	Bit Depth	Value Range
boolean	JVM-specific	true of false
char	16 bits	0 to 65535
byte	8 bits	-128 to 127
short	16 bits	-32768 to 32767
int	32 bits	-2147483648 to 2147483647
long	64 bits	-huge to huge
float	32 bits	varies
double	64 bits	varies

controlling your object

• There is actually no such thing as an object variable.

• There's only an object reference variable.

• An object reference variable holds bits that represent a way to access an object.

• It doesn't hold the object itself,but it holds something like a pointer.Or an address.

You can't stuff an object into a variable.Objcet live in one place and one place only -- the garbage collectible heap!

When obeject is not assigned,it is abandoned and eligible for Garbage Collection(GC).

变量的类型

• 变量有两种类型

  • 基本类型

    • 作为对象里的声明(实例变量)

  • 引用类型

    • 作为局部变量(变量声明在方法内)

      基本类型

      类型	位深	取值范围
      boolean	JVM-specific	true of false
      char	16 bits	0 to 65535
      byte	8 bits	-128 to 127
      short	16 bits	-32768 to 32767
      int	32 bits	-2147483648 to 2147483647
      long	64 bits	-很多到+很多
      float	32 bits	变化的
      double	64 bits	变化的

      控制你的对象

• 实际上没有对象变量

• 仅仅只是对象引用变量

• 对象引用变量是二进制数，代表进入对象的方法

• 对象引用变量不是对象，但它像指针或者地址。

你不能把对象塞进变量。对象只存在一个地方--垃圾回收堆(GC)!

当对象没有被变量指定，它将会被丢弃，并符合垃圾收集(GC)的条件。