javadataAbout stack and heap in JAVA(2)

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The stack is much faster than the heap. This is because of the way that memory is allocated on the stack. Allocating memory on the stack is as simple as moving the stack pointer up.

Java use garbage collection to automatically delete memory from the heap, without the programmer having to do anything.

If the stack runs out of a memory, then this is called a stack overflow – and could cause the program to crash. The heap could have the problem of fragmentation, which occurs when the available memory on the heap is being stored as noncontiguous (or disconnected) blocks -  because used blocks of memory are in between the unused memory blocks. When excessive fragmentation occurs, allocating new memory may be impossible because of the fact that even though there is enough memory for the desired allocation, there may not be enough memory in one big block for the desired amount of memory.

For people new to programming, it’s probably a good idea to use the stack since it’s easier. Because the stack is small, you would want to use it when you know exactly how much memory you will need for your data, or if you know the size of your data is very small. It’s better to use the heap when you know that you will need a lot of memory for your data, or you just are not sure how much memory you will need (like with a dynamic array).

In particular, local variable are stored on the stack. Local variables are often stored for short amounts for time while a function/method block uses them to compute a task. Once a function/method has completed its cycle, the space on the stack used by all local variables is freed. This chain of suspended function calls is the <b>stack</b>, because elements in the stack (function calls) depend on each other.

The heap is simply the memory used by programs to store global variables.

Element of the heap (variables) have no dependencies with each other and can always be accessed randomly at any time.

When you look at your computer memory, it is organized into three segments:

(1)    Text(code) segment

(2)    Stack segment

(3)    Heap segment

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The text segment(often called code segment) is where the compiled code of the program itself resides. When you open some EXE file in Notepad, you can see that it includes a lot of “Gibberish” language, something that is not readable to human. It is the machine code, the computer representation of the program instructions. This includes all user defined as well as system functions.

The two sections other from the code segment in the memory are used for data. The stack is the section of memory that is allocated for automatic variables within functions.

Data is stored in stack using the Last In First Out(LIFO) method. This means that storage in the memory is allocated and deallocated at only one end of the memory called the top of the stack. Stack is a section of memory and its associated registers that is used for temporary storage of information in which the most recently stored item is the first to be retrieved.

The memory allocated in the heap area is used and reused during program execution.

It should be noted that memory allocated in heap will contain garbage values left over from previous usage.

Memory space for objects is always allocated in heap. Objects are placed on the heap.

Built-in datatypes like int, double, float and parameters to methods are allocated on the stack.

Even though objects are held on heap, references to them are also variables and they are placed on the stack.

The stack segment provides more stable storage of data for a program. The memory allocated in the stack remains in existence for the duration of a program. This is good for global and static variables. Therefore, global variables and static variables are allocated on the stack.

The most important point is that heap and stack are generic terms for ways in which memory can be allocated. They can be implemented in many different ways, and the terms apply to the basic concepts.

In a stack of items, items sit one on top of the other in the order they were placed there, and you can only remove the top one(without toppling the whole thing over).

In a heap, there is no particular order to the way items are placed. You can reach in and remove items in any order because there is no clear ‘top’ item.

文章结束给大家分享下程序员的一些笑话语录：
一个合格的程序员是不会写出 诸如 “摧毁地球” 这样的程序的，他们会写一个函数叫 “摧毁行星”而把地球当一个参数传进去。

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java和data
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