Differences between heap and Stack in Java

Stacks and heaps are places that Java uses to store data in RAM. Unlike C + +, Java automatically manages stacks and heaps, and programmers cannot directly set up stacks or heaps.
The Java heap is a run-time data area in which the objects of the class allocate space. These objects are established through directives such as new, NewArray, Anewarray, and Multianewarray, and they do not require program code to be explicitly released. Heap is responsible for garbage collection, the advantage of the heap is the ability to dynamically allocate memory size, the lifetime does not have to tell the compiler beforehand, because it is at runtime to allocate memory dynamically, Java garbage collector will automatically take away these no longer use data. However, the disadvantage is that the access speed is slower due to the dynamic allocation of memory at run time.

 栈的优势是，存取速度比堆要快，仅次于寄存器，栈数据可以共享。但缺点是，存在栈中的数据大小与生存期必须是确定的，缺乏灵活性。栈中主要存放一些基本类型的变量（,int, short, long, byte, float, double, boolean, char）和对象句柄。栈有一个很重要的特殊性，就是存在栈中的数据可以共享。

Let's say we define both:
int a = 3;
int b = 3;

The compiler processes int a = 3 First, it creates a reference to a variable in the stack, and then finds out if there is a value of 3 in the stack, and if it does not, it stores the 3 in and then points a to 3. then the int b = 3 is processed, and after the reference variable of B is created, because there are already 3 values in the stack, B points directly to 3. In this case, A and B both point to 3. At this point, if you make a=4 again, then the compiler will re-search the stack for 4 values, if not, then store 4 in, and a point to 4; Therefore the change of a value does not affect the value of B.

It is important to note that this sharing of data with two object references also points to an object where this share is different, because the modification of a does not affect B, which is done by the compiler, which facilitates space saving. An object reference variable modifies the internal state of the object, affecting another object reference variable.
String is a special wrapper class data.

Can be used:
String str = new String ("abc");
String str = "ABC";

Two forms, the first is to create new objects with new (), which is stored in the heap. A new object is created each time the call is made. The second is to create an object reference to the string class in the stack str, and then find whether there is no "ABC" in the stack, if not, put "ABC" into the stack, and make str point to "ABC", if there is already "ABC" directly to the "ABC" Str.

Use the Equals () method when comparing values within a class, and when testing two wrapper classes for reference to the same object, use = =, the following example illustrates the above theory.
String str1 = "abc";
String str2 = "abc";
System.out.println (STR1==STR2); True
You can see that str1 and str2 are pointing to the same object.

String str1 =new string ("abc");
String str2 =new string ("abc");
System.out.println (STR1==STR2); False
The new method is to generate different objects. Each time one is generated.
So create multiple "abc" strings in the second way (string str1 = "abc"), in which only one object exists in memory. This writing is advantageous and saves memory space. At the same time it can improve the speed of the program to some extent, because the JVM will automatically determine whether it is necessary to create new objects based on the actual data in the stack. For the code of string str = new String ("abc"), it is necessary to create new objects in the heap, regardless of whether their string values are equal, to create new objects, thereby aggravating the burden of the program.
On the other hand, it is important to note that when you define a class using a format such as String str = "ABC", you always want to assume, of course, that the object that created the string class is Str. Worry about traps! The object may not have been created! Instead, it might just point to an object that was previously created. Only through the new () method can you guarantee that a new object is created each time.
Because of the immutable nature of the string class, you should consider using the StringBuffer class to improve program efficiency when a string variable needs to change its value frequently.

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Differences between heap and Stack in Java