Java Learning Notes-5. Common Data Structures

One, array

1.Java is the implementation of an array as an object, rather than a contiguous storage space

2. In Java, an array is a class that provides methods and properties, such as the number of Array.Length

The array name in 3.Java can be seen as an object reference, and the length cannot be defined directly at the time of initialization

Example: int a[3]; Method of declaring errors

4. An element in an array can also be a composite data type, when an array element is actually an object reference

Cases:

New Complex[3]; int i;  for (i = 0; i < arr.length; i++) {    new  Complex ();}

5. Declaration of one-dimensional array: type arrayname[]; or type[] arrayname;

Dynamic initialization: type[] arrayname = new Type[length];

Static initialization: type[] arrayname = {val0, Val1, ..., valn};

6. Declaration of two-dimensional arrays: type[][]arrayname; or type arrayname[][];

Dynamic initialization: type[][] arrayname = new Type[length1][length2]; Rule array

Or: type[][] arrayname = new type[length1][]; Irregular arrays

Array[0] = new Type[length2_1];

ARRAY[1] = new Type[length2_2]; ......

Static initialization: type[][] Arrayname = {{1, 2, ...}, {1, 2, ...}, ...};

Note: Arrayname.length represents the number of rows in a two-dimensional array

Arrayname[i].length represents the number of columns in row I of a two-dimensional array

For statements in 7.foreach form

Form: for (data type iteration variable: Array | collection) {

Loop body

}

Note: The value of an element in the array cannot be modified by iterating through a variable in the loop body of the foreach form

8.Arrays Class Common methods

Serial number	Method	Meaning
1	static int BinarySearch (type[] A, type key)	Binary lookup returns the value of the element in the array a with the value key
2	Static Boolean Equals (Type[] A, type[] b)	Determine if arrays A and B are equal
3	Static Boolean deepequals (Object[] A, object[] b)	Determines whether arrays A and b are equal, for multidimensional arrays
4	static void Fill (type[] A, type Val)	Assigns the elements in array a a value of Val
5	static void sort (type[] a)	Non-descending ordering of elements in array a
6	static void sort (type[] A, int from, int. to)	The non-descending ordering of the elements from the lower-to-to element in array a
7	Static String toString (type[] a)	Returns the string representation of the contents of the array a
8	Static String deeptostring (type[] a)	Returns a string representation of the contents of an array, for use in multidimensional arrays

Second, string

1. String constants that appear in Java are converted to objects of type string at compile time

2. Creating a String Object

How to construct the String class: String () string (string str) string (char[] arr)

To create a method:

(1) String str1; str1 = new String ("Hello World");

(2) String str2 = "Hello World";

(3) char[] arr = {' A ', ' B ', ' C '}; String arr3 = new string (arr);

Common methods for 3.String classes

(1) operator +: Connection operation for two strings

(2) operator charAt (int position): The position character in a string (counting from 0)

(3) operator equals (string str): Compares two strings for the same

Note:

New String ("Hello"new string ("Hello");

Str1. The result of equals (STR2) is true

The result of str1 = = Str2 is false because the "= =" operator is used to compare str1 and str2 values, and the two reference variables hold separate string object addresses

Comparison of ignoring case: Equalsignorecase ()

(4) operator indexOf (): Find string, parameter can be character or string

(5) operator split (): Used for the segmentation of a string, the string is divided according to the given parameters, the argument can be a character or a string

(6) operator tolowcase () touppercase (): Convert case

Parameters of the 4.main method

Cases:

class Argsdemo {    publicstaticvoid  main (string[] args) {        for (int i = 0; i < args.length; i++) {            System.out.println ("args[" + i + "]:" + Args[i] );        }    }}

5.StringBuffer and StringBuilder class

The string class is used to handle string variables that are not changed after creation, so using StringBuffer or StringBuilder is more efficient when dealing with variable-content strings

The StringBuffer class supports multi-threading, so it uses StringBuffer

Construction Method:

New StringBuffer ();    // Create an empty StringBuffer object (initial capacity is.) New StringBuffer (ten);    // Create a StringBuffer object with an initial capacity of 10 New StringBuffer ("Hello");    // Create a StringBuffer object with a content of hello (initial capacity is string length Plus +) // StringBuffer s4 = "Hello";    For the illegal

6.StringBuffer class method

(1) public void append (type T); function to append new content

public void Insert (int i, type T); function to insert new content

Type types can be all basic data types, character arrays, object references, and so on

I means inserting new content in the word I prompt

(2) Conversion of the string class to the StringBuffer class

stringbuffer->string:string s2 = s1.tostring ();

String->stringbuffer:stringbuffer S1 = new StringBuffer (s2);

Third, Vector class

1. Vector classes are used to describe vectors, defined in the Java.util package, as variable-length arrays that allow different types of elements to coexist

3 properties of the vector class: length, capacity, and increment

Length: Number of elements stored in vectors

Capacity: vector size, general capacity greater than or equal to length

Increment: The amount of each expansion when the capacity is insufficient

Vector capacity expansion is automatic, that is, when the new element exceeds its capacity, the capacity of the vector will automatically grow

Construction Method:

Method	Meaning
Public Vector ()	Constructs an empty vector object with a capacity of 10 and an increment of 0
Public Vector (Collection c)	Used to construct a vector containing the elements in the specified collection
Public Vector (int initialcapacity)	Vector initial capacity is initialcapacity, increment is 0
Public Vector (int initialcapacity, int capacityincrement)	Vector initial capacity is initialcapacity, increment is capacityincrement

The smaller the vector increment set, the higher the utilization of the memory space, but the greater the number of memory allocations

The larger the vector increment is set, the less memory allocation will be performed, but it is possible to generate memory waste

Adding elements to a vector: the Add () method or the Insertelementat () method

After inserting an element, the elements after the insertion position are moved backward by one unit

Delete an element in a vector: Removeelement () method

After deleting an element, the element after the delete position moves one unit forward

Clear all elements in the vector: the Removeallelement () method or the clear () method

Accessing the specified subscript element: ElementAt () method

Find the position of an element in a vector: IndexOf () method

Java Learning Notes-5. Common Data Structures