Java Programming thought Fourth Edition--Everything is an object

2.1 Manipulating objects with handles

Everything in Java is "considered" as an object. The way the object is manipulated is through a handle to the object (also known as a reference or pointer). But the handle does not necessarily point to the object, and the action handle gets an error (run-time)

For example: String s;s.length ();

The handle s,s is created here and does not point to the object. If you send a message to s at this point, you will get an error. The compiler prompts "The local variable s may not have been initialized"

string s = new String ("hello,world!") ; S.length ();

The handle is created here S,s points to an object (the object is located in a heap area) and sends a message to s normal.

2.2 All objects must be created

When you create a handle, it is generally also pointed to an object. The object is created in the New keyword.

string s = new String ("hello,world!");

2.2.1 Where data is saved

① registers, located inside the processor. is assigned by the compiler as needed.

The ② stack, which resides in the normal Ram area, is second only to register. The object handle is saved in the stack.

③ Heap, a general-purpose memory pool (in RAM). The flexibility is higher than the stack, but the performance is low (allocating more storage space within the heap) and Java objects are saved in this area.

④ static storage, located in RAM. The static keyword indicates that a particular element of an object is static. However, the Java object itself will never be placed in a static store.

The ⑤ constant is stored in read-only storage (ROM). Save constant value

⑥ non-RAM storage, streaming objects, or fixed objects.

2.2.2 Main types

Also known as the basic type, mainly includes: boolean,char,byte,short,int,long,float,doule,void

For these types, instead of creating a variable from new, you create an "automatic" variable that is not a handle, which holds a specific value. On the stack.

The main type has its own wrapper class

Basic data Types

Main type	Size	Minimum value	Maximum Value	Wrapper type
Boolean	1			Boolean
Char	16	0	2^16-1	Character
Byte	8	-128	127	Byte
Short	16	-2^15	2^15-1	Short
Int	32	-2^31	2^31-1	Integer
Long	64	-2^63	2^63-1	Long
Float	32	IEEE754	IEEE754	Float
Double	64	IEEE754	IEEE754	Double

Java Programming thought Fourth Edition--Everything is an object