Java stack (stack) and heap

Java stack vs heap (heap)
1. Concept
Stacks and heaps (heap) 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.
Stacks, heaps of data structures
Stacks are like buckets or chests of data.
It is a data structure with a last-in-first-out nature, that is, after the storage of the first fetch, the first storage after the fetch.
It's like we're going to take out something that's under the box (put in an earlier object), and we'll first remove the object that's pressed on it (the relatively late object put in).
Heap like a tree upside down
And the heap is different, the heap is a sort of tree data structure, each node has a value. Usually what we call a heap of data structures is a two fork heap.
The heap is characterized by the minimum (or maximum) value of the root node, and the two subtrees of the root node are also a heap.
Because of this feature of the heap, commonly used to achieve the priority queue, heap access is arbitrary, it is like we in the library shelves to pick up books, although the book is placed in order, but we do not want to take any of the same as the stack, first remove all the previous books, bookshelf This mechanism is different from the box, we can directly take out the book
Stacks: Some basic types of variables and object reference variables defined in the function are allocated in the stack memory of the function. When a variable is defined in a block of code, Java allocates a memory space for the variable in the stack, and when the scope of the variable is exceeded, Java automatically frees the memory space allocated for that variable, which can be used immediately by another.
Heap: Heap memory is used to store objects and arrays created by new. The memory allocated in the heap is managed by the Java Virtual Machine automatic garbage collector. After creating an array or an object in the heap, you can also define a special variable in the stack that is equal to the array or the first address of the object in the heap memory, and this particular variable in the stack becomes the reference variable of the array or object. You can then use the reference variable in the stack memory in your program to access the array or object in the heap, which is the equivalent of an alias, or codename, of an array or object.
A reference variable is a normal variable that is defined when memory is allocated in the stack, and the reference variable is released in the program run to the extraterritorial scope. The array and object itself is allocated in the heap, and even if the program runs beyond the block of code that uses the new generation of arrays and objects, the heap memory that the array and the object itself occupies will not be freed, and arrays and objects become garbage, no longer used, but still occupy memory when no reference variable points to it. is released by the garbage collector at a later indeterminate time. This is also the main reason for the memory of Java comparison, in fact, the variables in the stack point to the heap memory variables, this is the pointer in Java!
2. Pros and cons
The advantage of the stack is that the access speed is faster than the heap, second only to the registers directly in the CPU. However, the disadvantage is that the size and lifetime of the data in the stack must be deterministic and inflexible. In addition, the stack data can be shared.
The advantage of the heap is that the memory size can be allocated dynamically, and the lifetime does not have to tell the compiler beforehand that the Java garbage collector automatically collects the data that is no longer in use. However, the disadvantage is that the access speed is slower due to the dynamic allocation of memory at run time.
3. Concrete examples
Basic types (primitive types), there are 8 kinds, namely int, short, long, byte, float, double, Boolean, char (note, and no basic type of string). The definition of this type is through such as int a = 3; Long B = 255L; the form to be defined, called an automatic variable. It is worth noting that the automatic variable is a literal value, not an instance of a class, that is not a reference to a class, there is no class here. such as int a = 3; Here A is a reference to the int type, pointing to the literal value of 3. The data of these literals, due to the size of the known, the lifetime of the known (these values are fixed in a program block, the program block exits, the field value disappears), for the sake of speed, it exists in the stack.
Stack has a very important particularity, is that there is data in the stack can be shared:
eg
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 looks for an address with a literal value of 3, finds an address that holds the literal value of 3, and then points A to the address of 3. then the int b = 3 is processed, and after the reference variable of B is created, B is pointed directly to the address of 3 because there are already 3 literals in the stack. In this case, A and B both point to 3.
It is particularly important to note that the reference to this literal is different from the reference to the class object. Assuming that a reference to two class objects points to an object at the same time, if an object reference variable modifies the internal state of the object, then another object reference variable will immediately reflect that change. Conversely, modifying its value by a reference to a literal value does not result in another case where a reference to that literal is changed. As in the example above, we define the value of a and B and then make a=4; then B will not be equal to 4 or equal to 3. Inside the compiler, when it encounters A=4, it will re-search the stack for a literal value of 4, and if not, re-open the value of the address 4, and if so, point a directly at the address. Therefore the change of a value does not affect the value of B.

The other is the wrapper class data, such as Integer, String, double, and so on, the corresponding basic data types are wrapped up class. These classes of data all exist in the heap, and Java uses the new () statement to tell the compiler that it is dynamically created as needed at run time, so it is more flexible, but the disadvantage is that it takes more time. 4. String is a special wrapper class data. That is, it can be created in the form of string str = new String ("abc"), or in the form of string str = "abc" (In contrast, before JDK 5.0, you have never seen an expression of integer i = 3; because class and literal literals are not can be generic except for string. In JDK 5.0, this expression is possible! Because the compiler is converting the integer i = new Integer (3) in the background. The former is the process of creating a canonical class, that is, in Java, everything is an object, and the object is an instance of the class, all created in the form of new (). Some classes in Java, such as the DateFormat class, can return a newly created class through the class's getinstance () method, which seems to violate this principle. actually otherwise The class uses a singleton pattern to return an instance of the class, except that the instance is created inside the class through new (), and getinstance () hides this detail from the outside. So why is the case in string str = "abc", not created by new (), a violation of the above principle? Not really.

5. About the internal work of string str = "abc". Inside Java, this statement is translated into the following steps:

(1) First define an object reference variable named str to the String class: String str;

(2) in the stack to find whether there is a value of "ABC" address, if not, then open a store with a literal "ABC" address, then create a new String Class object O, and the string value of O point to the address, and in the stack next to this address note the referenced object o. If you already have an address with a value of "ABC", look for the object o and return the address of O.

(3) Point Str to the address of the object o.

It is important to note that the string values in the generic string class are directly stored values. But like string str = "abc"; In this case, the string value is a reference to the data in the existing stack!

To better illustrate this problem, we can verify it by following several code.
Copy content to Clipboard code:
String str1 = "abc";
String str2 = "abc";
System.out.println (STR1==STR2); True
Note that we do not use Str1.equals (STR2) in this way, as this will compare the values of two strings for equality. = = number, as described in the JDK, returns true only if two references point to the same object. And what we're looking at here is whether str1 and str2 all point to the same object.
The result shows that the JVM created two references str1 and str2, but only one object was created, and two references pointed to the object.

Let's go further and change the above code to:
Copy content to Clipboard code:
String str1 = "abc";
String str2 = "abc";
str1 = "BCD";
System.out.println (str1 + "," + str2); BCD, ABC
System.out.println (STR1==STR2); False
This means that the change in the assignment has led to a change in the class object reference, and str1 points to another new object! And str2 still points to the original object. In the example above, when we change the value of str1 to "BCD", the JVM discovers that there is no address for that value in the stack, opens up this address and creates a new object whose string value points to the address.

In fact, the string class is designed to be immutable (immutable) classes. If you want to change its value, yes, but the JVM silently creates a new object at run time based on the new value, and then returns the address of the object to the reference of the original class. This creation process is entirely automatic, but it takes up more time. In the environment that is more sensitive to time requirements, it will have some adverse effects.

Then modify the original code:
Copy content to Clipboard code:
String str1 = "abc";
String str2 = "abc";

str1 = "BCD";

String STR3 = str1;
System.out.println (STR3); Bcd

String STR4 = "BCD";
System.out.println (str1 = = STR4); True
STR3 a reference to this object points directly to the object that str1 points to (note that STR3 does not create a new object). When str1 changes its value, it creates a reference str4 of string and points to the new object created by str1 modifying the value. It can be found that this time STR4 also did not create a new object, thereby re-sharing the data in the stack.

Let's look at the following code again.
Copy content to Clipboard code:
String str1 = new String ("abc");
String str2 = "abc";
System.out.println (STR1==STR2); False creates two references. Two objects were created. Two references point to a different two objects, respectively.

String str1 = "abc";
String str2 = new String ("abc");
System.out.println (STR1==STR2); False
Two references were created. Two objects were created. Two references point to a different two objects, respectively.

The above two code shows that as long as new () is used to create the object, it is created in the heap, and its string is stored separately, even if the data in the stack is the same, it is not shared with the data in the stack.

6. The value of the data type wrapper class cannot be modified. Not only the value of the string class cannot be modified, but all data type wrapper classes cannot change their internal values. 7. CONCLUSIONS AND RECOMMENDATIONS:

(1) When we use a format definition class such as String str = "ABC", we always want to think of course that we created the object str of the String class. Worry about traps! The object may not have been created! The only certainty is that a reference to the string class was created. As to whether the reference is pointing to a new object, it must be considered in terms of context, unless you create a new object with a prominent way through the new () method. Therefore, it is more accurate to say that we have created a reference variable to the object of the String class str, which refers to a variable that points to a string class with the value "ABC". Being aware of this is helpful in troubleshooting bugs that are difficult to find in a program.

(2) The use of string str = "abc", in a way that can improve the speed of the program to a certain extent, because the JVM will automatically based on the actual data in the stack to determine whether it is necessary to create a new object. In the case of string str = new String ("abc"), the code creates a new object in the heap, regardless of whether the string value is equal or not, and it is necessary to create a new object, thereby aggravating the burden of the program. This idea should be the idea of the meta-mode, but it is not known whether the internal JDK implements this pattern.

(3) Use the Equals () method when comparing the values in the wrapper class, and use the = = when testing whether the references to the two wrapper classes point to the same object.

(4) Because of the immutable nature of the string class, you should consider using the StringBuffer class when the string variable needs to change its value frequently to improve program efficiency.

Java stack (stack) and heap