Java Stack and heap

----to these two concepts of unknown long, and finally found a good article, bring to share

1. 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.

2. 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 , see 3rd. 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 theJava 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. There are two kinds of data types in Java.

one is the basic type (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 define, called automatic variable . It should be noted that is literal , Not an instance of the class , , There is no class existence . such as int a = 3; Here A is a reference to the int type, pointing to the literal value of 3. the literal data, Because of the size of the known, the lifetime is 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.

In addition, stack has a very important particularity, . Suppose we define at the same time:  
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int a = 3; 
int b = 3; 
compiler processes int a = 3 First, it creates a reference to a variable in the stack, Then find the address that has a literal value of 3, not found, open a 3 of the value of the address, and then point A to 3 address. 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.

In particular, this literal references and is different. Assume that references to two class objects point to an object , If an object reference variable modifies the internal state of this object , then another object reference variable will immediately reflect that change. opposite , Modify its value by reference to literal values , does not cause another reference to this literal value to change as well. In the above example, after we have defined the values of a and B, then the a=4 is not 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 Takes more time .

4. String is aSpecialWrapper class data. That can be used String str = new String ("ABC") (created in heap);form to create, or you can use theString str = "ABC" (created in stack)The form to create (in contrast, before JDK 5.0, you have never seen an integer i = 3;because classes and literals are not generic, except 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 literal "ABC" address, and then in the heap Create a new object o for the string classand point the string value of O to this address in the stack, and note the referenced object o next to this address in the stack . 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.
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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:
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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, thestring class is designed to be immutable (immutable) classes. This means: If you want to change the value, you can, 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 original class reference . 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:
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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.
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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 pair of images with the newly () 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) using String str = "abc" , the way the data is in the stack, can improve the speed of the program to some extent, because the JVM will automatically determine if it is necessary to create a new object based on the actual data in the stack. 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) the Equals () method is used when comparing the values in the wrapper class with equal value ; when testing whether a reference to two wrapper classes points to the same object, use = =.

(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 and heap