Object Objects
Let's start by introducing the API
API (Application programming Interface): Application Programming Interface
Java API
- is the class that Java provides to us, these classes encapsulate the underlying implementation.
- We don't need to care how these classes are implemented, just learn how these classes are used.
Ojbect is a super class, any class will inherit with the object class, you do not write, the parent class is object by default
An overview of Object
- object is the root class of a class hierarchy
- All classes inherit directly or indirectly from the object class.
- How to construct the object class: Public object ()
- The constructor of a subclass is accessed by default for the parameterless construction method of the parent class
The Hashcode method of object
- Object has a method public int hashcode ()
- method returns a hash code value, which, by default, is calculated based on the address of the object
- Hashcode () for different objects are generally not the same, but the hashcode value of the same object is definitely the same
- Note: Hashcode is not the actual address value of an object and can be understood as a logical address value
Public classTest { Public Static voidMain (string[] args) {studenttest std1=NewStudenttest ("null", 11); Studenttest STD2=NewStudenttest ("null", 12); Studenttest std3=STD1; System.out.println ("STD1:" +Std1.hashcode ()); System.out.println ("STD2:" +Std2.hashcode ()); System.out.println ("STD3:" +Std3.hashcode ()); } }classstudenttest{String name; intAge ; PublicStudenttest (String name,intAge ) { This. Name =name; This. Age =Age ; }}
The GetClass method of object
- Returns this object run-time class
- You can get the full name of the real class of the object through a method in class
PackageDay15_nullnull; Public classTest { Public Static voidMain (string[] args) {studenttest std1=NewStudenttest ("null", 11); Studenttest STD2=NewStudenttest ("null", 12); Studenttest std3=STD1; System.out.println (Std1.getclass ()); System.out.println (Std2.getclass ()); System.out.println (Std3.getclass ()); } }classstudenttest{String name; intAge ; PublicStudenttest (String name,intAge ) { This. Name =name; This. Age =Age ; }}
The ToString method of the object class
- The ToString method returns the string representation of this object
- General equivalent: package name [email protected]+integer.tohexstring (D.hashcode ())
- But when we print an object, we call the ToString method by default, which is equivalent to the __str__ in Python.
- This method is one that we use to customize the string output
Suggested overriding the ToString method
The Equals method of the object class
- Indicates whether the object is equal to other objects
- By default, the reference (address) of the object is the same
- Because the reference to the comparison object is meaningless, it is generally recommended to override this method
Add: = = differs from the Equals method
- = = is a comparison cloud algorithm, that is, you can compare basic data types, or you can compare reference data types
- The base data type compares the value, and the reference data type compares the address value
- The Equals method is a method that can only compare data types
- The default equals and = = Compare reference data types without any difference
- But by customizing, we can do what we want to compare the results
Some introduction to the scanner class
Scanner is a simple text scanner that can use regular expressions to parse basic types and strings
Scanner Method of Construction:
Scanner (InputStream Source)
system.in Introduction
- There is a static field under the System class
- public static final InputStream in standard input stream, corresponding to keyboard entry
Member Methods for scanner:
More IMPORTANT:
Hasnext. Determine if there is a next entry, where ... can be int,double and so on, if you need to determine whether to include a string, you can omit ...
Next .... Get the next entry
Commonly used for:
- public int nextint (); Gets a value of type int
- Public String nextline (); Gets a string of type value, which is the string
- Note: It is best to use the same
The string class describes how to construct a string:
- Public String ()
- public string (byte[] bytes) Converts a byte array to a string
- public string (byte[] bytes,int index,int length) converts part of a byte array to a string
- public string (char[] Value,int index,int count) converts part of a character array to a string
- The public string (string original) Initializes a newly created string object that represents a sequence of characters that is the same as the argument; that is, the newly created string is a copy of the parameter string (there is no thought of a deep copy)
Packageday15_nullnull_01; Public classnull_string { Public Static voidMain (string[] args) {String str1=NewString (); System.out.println (STR1); byte[] bytes1 = {97,98,99}; String str2=NewString (bytes1); System.out.println (STR2); byte[] Bytes2 = {97,98,99,100,101,102,103}; String STR3=NewString (bytes2,1,4); System.out.println (STR3); Char[] Chr = {' A ', ' B ', ' C '}; String STR4=NewString (CHR); System.out.println (STR4); } }
Let's take a look at some questions.
// 1. Determine if S1 and S2 that are defined as string types are equal String S1 = "abc"= "abc"= = = s2); System.out.println (s1.equals (S2));
true true // A constant has only one copy in memory
Answer
// 2. The following sentence creates several objects in memory? New String ("abc");
// a total of two objects were created // a constant pool and a heap area each have one
Answer
// 3. Determine if S1 and S2 that are defined as string types are equal New String ("abc"= "abc"= = s2); System.out.println (s1.equals (S2));
Not the same, one in the heap address area, one in the constant area
Answer
// 4. Determine if S1 and S2 that are defined as string types are equal String S1 = "a" + "B" + "C"= "abc"= = = s2); System.out.println (s1.equals (S2));
Java's constant optimization mechanism in fact, when editing the S1 is already "ABC"
Answer
// 5. Determine if S1 and S2 that are defined as string types are equal String S1 = "AB"= "abc"= s1 + "C"= = s2); System.out.println (s3.equals (S2));
= = falsetrue
AnswerThe judging function of the string class
- public boolean equals (Object anobject) to determine if the string is the same
- public boolean equalsignorecase (string anotherstring) determines if the string is the same, ignoring the case
- Public Boolean contains (Charsequence s) determines whether the string contains those characters
- public boolean startsWith (string prefix) determines whether a string begins with what
- public boolean endsWith (string suffix) determines whether the string ends with
- public Boolean isEmpty () determines whether the string is an empty string
Get function of String class
- int length (): Gets the length of the string
- char charAt (int index): Gets the character at the specified index position
- int indexOf (int ch): Gets the index of the specified character at the first occurrence of this string
- int indexOf (String str): Returns the index at the first occurrence of the specified string in this string
- int indexOf (int ch,int fromIndex): Returns the index at the first occurrence of the specified character after the specified position in this string
- int indexOf (String str,int fromIndex): Returns the index at the first occurrence of the specified string after the specified position in this string
- LastIndexOf () The last occurrence of the position
- string substring (int start): Intercepts the string starting at the specified position, default to the end
- String substring (int start,int end): Ends The Intercept word from the specified position to the specified position metalized
Conversion capabilities of the String class
- Byte[] GetBytes () converts a string into a byte array
- Char[] ToCharArray () converts a string into a character array
- static string ValueOf (char[] CHS) converts a character array to a string
- static string valueOf (int i) converts data of type int to string
- Add: The valueof method of string can convert any type of data into a string
- String toLowerCase () turns the string into lowercase
- String toUpperCase () turns the string into uppercase
- String concat (String str) stitching strings and only stitching strings.
Some other features of the String class
- Public String replace (char Oldchar,char newchar) substitution character
- public string replace (charsequence target,charsequence replacement) Replacement string
- Stringtrim (); Space before and after removal
- public int CompareTo (String anotherstring) comparison
- public int comparetoignorecase (String str) comparison
Overview of the StringBuffer class
thread -safe variable-character sequence, a string-like buffer similar to that of string
StringBuffer internal implementation is a string array
The difference between string and StringBuffer:
- String is an immutable sequence of characters
- StringBuffer is a variable sequence of characters
- StringBuffer thread safety, can be locked
The construction method of StringBuffer class
- Public StringBuffer () constructs a string buffer with no characters, with an initial capacity of 16 characters
- public stringbuffer (int capacity) constructs a string buffer with no characters, but with a specified initial capacity
- Public StringBuffer (charsequence seq) constructs a string buffer that contains the same characters as the specified charsequence
- Public StringBuffer (String str) constructs a string buffer and initializes its contents to the specified string content
Methods of StringBuffer
- public int Capacity () returns the current capacity. (theoretical value?) ）
- The public int length () returns the length (number of characters) (actual value)
StringBuffer Add-on features
- Public stringbuffer Append (String str) can add any type of data to the character buffer and return a character buffer itself
- Public stringbuffer Insert (int offset,string str) inserts any type of data into the character buffer at the specified location and returns the string buffer itself
StringBuffer removal function
- Public stringbuffer deletecharat (int index) deletes the character at the specified position and returns itself
- Public StringBuffer Delete (int start,int end) Deletes the content starting at the specified position to the end of the specified position and returns itself
StringBuffer Replacement and rollover functions
- Public stringbuffer replace (int start,int end,string str) is replaced with Str from start to end
- Public StringBuffer reverse () Flip of string
Interception of StringBuffer
- Public String substring (int start) intercepts from the specified position to the end
- Public String substring (int start,int end) intercepts from the specified position to the end position, including the start position, not including the end position
- Note: The previous return value is StringBuffer, this time returning a string
Conversion between string and StringBuffer
String===>stringbuffer
1. Through the construction method, when the StringBuffer is created, the string is passed as a parameter
2. Through the apped () method
Stringbuffer===>string
1. The same over-construction method
2. Via the ToString () method of the StringBuffer
3. Using substring (start,end) Slicing method
Add:
The difference between 1.StringBuffer and StringBuilder
- StringBuffer is a jdk1.0 version, thread-safe, and inefficient.
- StringBuilder is a jdk1.5 version and is thread insecure, but also highly efficient
The difference between 2.String and StringBuffer and StringBuilder
- String is an immutable sequence of characters
- StringBuffer and StringBuilder are a variable sequence of characters
Java Learning Path (v): Common object manipulation
