The difference between heap and Stack in Java

Stacks and heaps are places where Java is used to store data in RAM. Unlike C + +, Java automatically manages stacks and heaps, and programmers cannot directly set stacks or heaps.

Java's heap is a run-time data area in which 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. The heap is responsible for garbage collection, the advantage of the heap is the dynamic allocation of memory size, the lifetime does not have to tell the compiler in advance, because it is dynamically allocating memory at runtime, the Java garbage collector will automatically take away these no longer used data. The disadvantage is that the access rate is slow due to the dynamic allocation of memory at run time.

The advantage of the stack is that the access speed is faster than the heap, after the register, the stack data can be shared. The disadvantage is that the data size and lifetime in the stack must be fixed and inflexible. There are some basic types of variables in the stack (, int, short, long, byte, float, double, Boolean, char), and object handles.

Stack has a very important particularity, is the existence of the stack of data can be shared. Let's say we both define:

int a = 3;

int b = 3;

The compiler deals with int a = 3 First, it creates a reference to a in the stack, finds out if there is a 3 value in the stack, and if not, it stores 3 in, then points a to 3. then process int b = 3, after you create the reference variable for B, because you already have 3 in the stack, point B directly to 3. In this way, there is a case where A and B both point to 3.

At this point, if you make a=4 again, the compiler will search for a 4 value in the stack, and if not, put 4 in and point A to 4, and if so, direct a to this address. Therefore the change of a value does not affect the value of B.

Note that this sharing of data is different from the two-object reference to an object, because the modification of a does not affect B, it is done by the compiler, and it helps save space. While an object reference variable modifies the internal state of the object, it affects another object reference variable.

A string is a special wrapper class data. Can be used:

String str = new String ("abc");

String str = "ABC";

In two forms, the first is to create a new object with new (), which is stored in the heap. Each time a call is made, a new object is created.

And the second is to create an object reference variable str in the stack on a string class, then find out if there is any "ABC" in the stack, and if not, store "ABC" in the Stack and make str point to "ABC", and If "ABC" is already there, direct STR to "ABC".

When comparing the values in a class with the Equals () method, use = = when testing whether references to two wrapper classes are pointing to the same object, use the following example to illustrate 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