Detailed explanation of object accommodation in Java programming language

This article mainly provides an overall introduction to the accommodation of objects in Java programming ideas. It has certain reference value and friends in need can learn about it.

If a program only contains a fixed number of objects, and their existence time is known, then the program is quite simple.

Array

There are many ways to accommodate objects. Array is the most common one. It mainly has the following two characteristics: efficiency and type. For Java, the most efficient way to save and access a series of objects (actually handles of objects) is an array. The array is actually a simple linear sequence, so the access speed is very fast, but it also has some limitations. For example, the size of the array is fixed and cannot be changed within the "existence time".
For arrays composed of basic data types, their operation type is similar to that of object arrays. The difference is that the former does not store the handle of the object, but the actual value.

Collection

When programming, I usually don’t know how many objects need to be saved. Sometimes I even want to use a more complicated way to save objects. To solve such problems, Java provides several "collection classes": Vector (vector), BitSet (bit set), Stack (stack) and HashTable (hash table).

Disadvantages of collections: Type information is lost. What it holds is actually a handle to an object of type Object.

Enumerator (iterator)

# Use a collection to save the object before accessing it. You need to know the exact type of the object in the collection in advance, otherwise An exception may occur during use. And iterators can solve this problem. An iterator is an object whose purpose is to iterate over a sequence of objects and select each object in that sequence without letting the client programmer know or care about the underlying structure of that sequence.

Enumeration in Java is a typical iterator, mainly used to do the following things:

(1) Use a method called Elements to ask the collection to be for us Provide an Enumeration that returns the first element in the sequence when its NextElements is first called.

(2) Use NextElements to get the next object.

(3) Use HasMoreElements to check if there are more objects.

Types of collections:

1. Vector

The Vector class can implement a growable array of objects. Like an array, it contains components that can be accessed using integer indexes. However, the size of the Vector can grow or shrink as needed to accommodate items being added or removed after the Vector is created.

2. BitSet

BitSet is actually a Vector composed of "binary bits". If you want to save a large amount of "on-off" information efficiently, you should use BitSet. Each component of the bit set has a boolean value. Index the BitSet's bits as nonnegative integers. Each indexed bit can be tested, set, or cleared. You can use one BitSet to modify the contents of another BitSet using logical AND, logical OR, and logical XOR operations. By default, the initial value of all bits in set is false.

3. Stack

Stack is also called a "last-in-first-out" collection. The Stack class in Java inherits from the Vector class, which extends the Vector class with five operations that allow vectors to be treated as stacks. It provides the usual push and pop operations, as well as the peek method to take the stack vertex, the empty method to test whether the stack is empty, and the search method to find an item in the stack and determine the distance to the top of the stack.

4. HashTable

Hash table is an important storage method and a common retrieval method. The basic idea is to use the value of the relational code as an independent variable, calculate the corresponding function value through a certain functional relationship, interpret this value as the storage address of the node, and store the node in the storage unit corresponding to the calculated storage address. . The search key method is used when searching. Hash tables now have a complete set of algorithms for insertion, deletion, and conflict resolution. In Java, hash tables are used to store objects for fast retrieval.

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