1.API
1.1 API Overview - use of help documents
-
What is an API
API (Application Programming Interface): application programming interface
-
API in Java
It refers to Java classes with various functions provided in the JDK. These classes encapsulate the underlying implementation. We don't need to care about how these classes are implemented. We just need to learn how to use these classes. We can learn how to use these API s through help documents.
How to use API help documentation:
-
Open help document
-
Locate the input box in the index tab
-
Enter Random in the input box
-
See which package the class is in
-
Look at the description of the class
-
Look at the construction method
-
Look at member methods
1.2 keyboard input string
Scanner class:
next(): if a space is encountered, no more data will be entered. End tag: space, tab
nextLine(): the data can be received completely, and the end mark is carriage return line feed
Code implementation:
package com.itheima.api;
import java.util.Scanner;
public class Demo1Scanner {
/*
next() : If you encounter a space, you will no longer enter data
End tag: space, tab
nextLine() : The data can be received completely
End tag: carriage return line feed
*/
public static void main(String[] args) {
// 1. Create Scanner object
Scanner sc = new Scanner(System.in);
System.out.println("Please enter:");
// 2. Call the nextLine method to receive the string
// ctrl + alt + v: return value of quick generation method
String s = sc.nextLine();
System.out.println(s);
}
}
package com.itheima.api;
import java.util.Scanner;
public class Demo2Scanner {
/*
nextInt When used in conjunction with the nextLine method, the nextLine method has no chance of keyboard entry
Suggestion: when entering data on the keyboard in the future, if the string and integer are accepted together, it is recommended to use the next method to accept the string
*/
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.println("please enter an integer:");
int num = sc.nextInt(); // 10 + enter line feed
System.out.println("Please enter a string:");
String s = sc.nextLine();
System.out.println(num);
System.out.println(s);
}
}
2. String class
2.1 String overview
1 the string class is under the java.lang package, so there is no need to import the package when using it
2 the String class represents a String. All String literals (such as "abc") in the Java program are implemented as instances of this class, that is, all double quoted strings in the Java program are objects of the String class
3 strings are immutable, and their values cannot be changed after creation
2.2 construction method of string class
Common construction methods
Sample code
package com.itheima.string;
public class Demo2StringConstructor {
/*
String Common construction methods of class:
public String() : Create a blank string object that contains nothing
public String(char[] chs) : Create a string object based on the contents of the character array
public String(String original) : Creates a string object based on the passed in string content
String s = "abc"; Create a string object by direct assignment. The content is abc
be careful:
String This class is special. When printing its object name, there will be no memory address
It is the real content recorded by the object
Object oriented - inheritance, object class
*/
public static void main(String[] args) {
// public String(): create a blank string object without any content
String s1 = new String();
System.out.println(s1);
// public String(char[] chs): creates a string object according to the contents of the character array
char[] chs = {'a','b','c'};
String s2 = new String(chs);
System.out.println(s2);
// public String(String original): creates a string object based on the incoming string content
String s3 = new String("123");
System.out.println(s3);
}
}
2.4 comparison of differences in creating string objects
-
Create by construction method
For string objects created through new, each new will apply for a memory space. Although the contents are the same, the address values are different
-
Create by direct assignment
As long as the character sequence of the String given in the form of "=" is the same (order and case), no matter how many times it appears in the program code, the JVM will only create a String object and maintain it in the String pool
2.5 string comparison
2.5.1 string comparison
- ==Compare basic data types: specific values are compared
- ==Compare reference data types: object address values are compared
String class: public boolean equals(String s) compares whether the contents of two strings are the same and case sensitive
code:
package com.itheima.stringmethod;
public class Demo1Equals {
public static void main(String[] args) {
String s1 = "abc";
String s2 = "ABC";
String s3 = "abc";
// equals: compares string contents, case sensitive
System.out.println(s1.equals(s2));
System.out.println(s1.equals(s3));
// equalsIgnoreCase: compares string contents, ignoring case
System.out.println(s1.equalsIgnoreCase(s2));
}
}
2.6 user login case [application]
Case requirements:
Known user name and password, please use the program to simulate user login. Give a total of three opportunities. After logging in, give corresponding prompts
Implementation steps:
- If the user name and password are known, define two string representations
- Enter the user name and password to log in with the keyboard, which is implemented with Scanner
- Compare the user name and password entered on the keyboard with the known user name and password, and give the corresponding prompt.
- The content comparison of the string is implemented with the equals() method
- Use the loop to realize multiple opportunities. The number of times here is clear. Use the for loop to realize it. When the login is successful, use the break to end the loop
Code implementation:
package com.itheima.test;
import java.util.Scanner;
public class Test1 {
/*
Requirements: if the user name and password are known, please use the program to simulate user login.
Give a total of three opportunities. After logging in, give corresponding prompts
Idea:
1. If the user name and password are known, define two string representations
2. Enter the user name and password to log in with the keyboard, which is implemented with Scanner
3. Compare the user name and password entered on the keyboard with the known user name and password, and give the corresponding prompt.
The content comparison of the string is implemented with the equals() method
4. Use the loop to realize multiple opportunities. The number of times here is clear. Use the for loop to realize it. When the login is successful, use break to end the loop
*/
public static void main(String[] args) {
// 1. If the user name and password are known, define two string representations
String username = "admin";
String password = "123456";
// 2. Enter the user name and password to log in with the keyboard, which is implemented with Scanner
Scanner sc = new Scanner(System.in);
// 4. Use the loop to realize multiple opportunities. The number of times here is clear, and use the for loop to realize
for(int i = 1; i <= 3; i++){
System.out.println("enter one user name:");
String scUsername = sc.nextLine();
System.out.println("Please input a password:");
String scPassword = sc.nextLine();
// 3. Compare the user name and password entered on the keyboard with the known user name and password, and give the corresponding prompt.
if(username.equals(scUsername) && password.equals(scPassword)){
System.out.println("Login succeeded");
break;
}else{
if(i == 3){
System.out.println("Your login times have reached today's limit, Please come back tomorrow");
}else{
System.out.println("Login failed,You still have" + (3-i) +"Second chance");
}
}
}
}
}
2.7 traversal string case [ application ]
Case requirements:
Enter a string on the keyboard and use the program to traverse the string on the console
Implementation steps:
- Enter a string on the keyboard and implement it with Scanner
- To traverse a string, you must first be able to obtain each character in the string. public char charAt(int index): returns the char value at the specified index. The index of the string also starts from 0
- Traverse the string, and then get the length of the string. public int length(): returns the length of the string
- Traversal printing
Code implementation:
package com.itheima.test;
import java.util.Scanner;
public class Test2 {
/*
Requirements: enter a string on the keyboard and use the program to traverse the string on the console
Idea:
1. Enter a string on the keyboard and implement it with Scanner
2. To traverse a string, you must first be able to get each character in the string
public char charAt(int index): Returns the char value at the specified index, and the index of the string starts from 0
3. Traverse the string, and then get the length of the string
public int length(): Returns the length of this string
4. Traversal printing
9
*/
public static void main(String[] args) {
// 1. Enter a string on the keyboard and implement it with Scanner
Scanner sc = new Scanner(System.in);
System.out.println("Please enter:");
String s = sc.nextLine();
// 2. To traverse a string, you must first be able to get each character in the string
for(int i = 0; i < s.length(); i++){
// i: each index of the string
char c = s.charAt(i);
System.out.println(c);
}
}
}
2.8 case of counting the number of characters [ application ]
Case requirements:
Enter a string on the keyboard and use the program to traverse the string on the console
Implementation steps:
- Enter a string on the keyboard and implement it with Scanner
- Split the string into character arrays, public char[] toCharArray(): split the current string into character arrays and return
- Number of traversal characters
Code implementation:
package com.itheima.test;
import java.util.Scanner;
public class Test3 {
/*
Requirements: enter a string on the keyboard and use the program to traverse the string on the console
Idea:
1. Enter a string on the keyboard and implement it with Scanner
2. Splits a string into an array of characters
public char[] toCharArray( ): Splits the current string into an array of characters and returns
3. Traversal character array
*/
public static void main(String[] args) {
// 1. Enter a string on the keyboard and implement it with Scanner
Scanner sc = new Scanner(System.in);
System.out.println("Please enter:");
String s = sc.nextLine();
// 2. Split the string into character arrays
char[] chars = s.toCharArray();
// 3. Traverse character array
for (int i = 0; i < chars.length; i++) {
System.out.println(chars[i]);
}
}
}
2.9 mobile phone number shielding - string interception
Case requirements:
Accept a mobile phone number from the keyboard in the form of string, and shield the middle four numbers
The final effect is: 156 * * * * 1234
Implementation steps:
- Enter a string on the keyboard and implement it with Scanner
- Intercept the first three digits of the string
- Intercept the last four digits of the string
- Splice the two intercepted strings with * * * * in the middle to output the result
Code implementation:
package com.itheima.test;
import java.util.Scanner;
public class Test5 {
/*
Requirement: accept a mobile phone number from the keyboard in the form of string, and shield the middle four numbers
The final effect is: 156 * * * * 1234
Idea:
1. Enter a string on the keyboard and implement it with Scanner
2. Intercept the first three digits of the string
3. Intercept the last four digits of the string
4. Splice the two intercepted strings with * * * * in the middle to output the result
*/
public static void main(String[] args) {
// 1. Enter a string on the keyboard and implement it with Scanner
Scanner sc = new Scanner(System.in);
System.out.println("Please enter your mobile phone number:");
String telString = sc.nextLine();
// 2. Intercept the first three digits of the string
String start = telString.substring(0,3);
// 3. Intercept the last four digits of the string
String end = telString.substring(7);
// 4. Splice the two intercepted strings with * * * * in the middle to output the result
System.out.println(start + "****" + end);
}
}
2.10 sensitive word substitution - string substitution
Case requirements:
Enter a string on the keyboard. If the string contains (TMD), use * * * instead
Implementation steps:
- Enter a string on the keyboard and implement it with Scanner
- Replace sensitive words
String replace(CharSequence target, CharSequence replacement)
Replace the target content in the current string with replacement to return a new string - Output results
Code implementation:
package com.itheima.test;
import java.util.Scanner;
public class Test6 {
/*
Requirement: enter a string on the keyboard. If the string contains (TMD), use * * * instead
Idea:
1. Enter a string on the keyboard and implement it with Scanner
2. Replace sensitive words
String replace(CharSequence target, CharSequence replacement)
Replace the target content in the current string with replacement to return a new string
3. Output results
*/
public static void main(String[] args) {
// 1. Enter a string on the keyboard and implement it with Scanner
Scanner sc = new Scanner(System.in);
System.out.println("Please enter:");
String s = sc.nextLine();
// 2. Replace sensitive words
String result = s.replace("TMD","***");
// 3. Output results
System.out.println(result);
}
}
2.11 cutting strings
Case requirements:
Enter student information from the keyboard in the form of string, for example: "Zhang San, 23"
Cut the valid data from the string and encapsulate it as a Student object
Implementation steps:
-
Write the Student class to encapsulate the data
-
Enter a string on the keyboard and implement it with Scanner
-
Cut the string according to the comma to get (Zhang San) (23)
String[] split(String regex): cut according to the incoming string as a rule
Store the cut content into the string array and return the string array -
Take the element content from the obtained string array and encapsulate it as an object through the parametric construction method of Student class
-
Call the getXxx method of the object, take out the data and print it.
Code implementation:
package com.itheima.test;
import com.itheima.domain.Student;
import java.util.Scanner;
public class Test7 {
/*
Requirement: input student information from the keyboard in the form of string, for example: "Zhang San, 23"
Cut the valid data from the string and encapsulate it as a Student object
Idea:
1. Write the Student class to encapsulate the data
2. Enter a string on the keyboard and implement it with Scanner
3. Cut the string according to the comma to get (Zhang San) (23)
String[] split(String regex) : Cut according to the incoming string as a rule
Store the cut content into the string array and return the string array
4. Take the element content from the obtained string array and encapsulate it as an object through the parametric construction method of Student class
5. Call the getXxx method of the object, take out the data and print it.
*/
public static void main(String[] args) {
// 2. Enter a string on the keyboard and implement it with Scanner
Scanner sc = new Scanner(System.in);
System.out.println("Please enter student information:");
String stuInfo = sc.nextLine();
// stuInfo = "Zhang San, 23";
// 3. Cut the string according to the comma to get (Zhang San) (23)
String[] sArr = stuInfo.split(",");
// System.out.println(sArr[0]);
// System.out.println(sArr[1]);
// 4. Take the element content from the obtained string array and encapsulate it as an object through the parametric construction method of Student class
Student stu = new Student(sArr[0],sArr[1]);
// 5. Call the getXxx method of the object, take out the data and print it.
System.out.println(stu.getName() + "..." + stu.getAge());
}
}
2.12 summary of string method
Common methods of String class:
public boolean equals(Object anObject) compares the contents of strings and is strictly case sensitive
Public Boolean equalsignorecase (string otherstring) compares the contents of strings, ignoring case
public int length() returns the length of this string
public char charAt(int index) returns the char value at the specified index
public char[] toCharArray() splits the string into character arrays and returns
public String substring(int beginIndex, int endIndex) intercepts according to the start and end indexes to get a new string (including header and not tail)
public String substring(int beginIndex) is intercepted from the incoming index to the end to get a new string
public String replace(CharSequence target, CharSequence replacement) uses the new value to replace the old value in the string to obtain a new string
public String[] split(String regex) cuts the string according to the incoming rules to get the string array
3 StringBuilder class
3.1 overview of StringBuilder class
Overview: StringBuilder is a variable string class. We can regard it as a container. The variable here means that the content in the StringBuilder object is variable
3.2 differences between StringBuilder class and String class
- String class: the content is immutable
- StringBuilder class: the content is mutable
3.3 construction method of StringBuilder class
Common construction methods
| Method name | explain |
|---|---|
| public StringBuilder() | Create a blank variable string object that contains nothing |
| public StringBuilder(String str) | Creates a variable string object based on the contents of the string |
Sample code
public class StringBuilderDemo01 {
public static void main(String[] args) {
//public StringBuilder(): creates a blank variable string object without any content
StringBuilder sb = new StringBuilder();
System.out.println("sb:" + sb);
System.out.println("sb.length():" + sb.length());
//public StringBuilder(String str): creates a variable string object according to the contents of the string
StringBuilder sb2 = new StringBuilder("hello");
System.out.println("sb2:" + sb2);
System.out.println("sb2.length():" + sb2.length());
}
}
3.4 common member methods of StringBuilder
-
Add and reverse methods
Method name explain Public StringBuilder append (any type) Add data and return the object itself public StringBuilder reverse() Returns the opposite sequence of characters -
Sample code
public class StringBuilderDemo01 {
public static void main(String[] args) {
//create object
StringBuilder sb = new StringBuilder();
//Public StringBuilder append (any type): adds data and returns the object itself
// StringBuilder sb2 = sb.append("hello");
//
// System.out.println("sb:" + sb);
// System.out.println("sb2:" + sb2);
// System.out.println(sb == sb2);
// sb.append("hello");
// sb.append("world");
// sb.append("java");
// sb.append(100);
//Chain programming
sb.append("hello").append("world").append("java").append(100);
System.out.println("sb:" + sb);
//public StringBuilder reverse(): returns the opposite character sequence
sb.reverse();
System.out.println("sb:" + sb);
}
}
3.5 conversion between StringBuilder and String [ application ]
-
Convert StringBuilder to String
public String toString(): you can convert StringBuilder to String through toString()
-
Convert String to StringBuilder
public StringBuilder(String s): you can convert a String into a StringBuilder by constructing a method
-
Sample code
public class StringBuilderDemo02 {
public static void main(String[] args) {
/*
//StringBuilder Convert to String
StringBuilder sb = new StringBuilder();
sb.append("hello");
//String s = sb; //This is wrong
//public String toString(): You can convert StringBuilder to String by toString()
String s = sb.toString();
System.out.println(s);
*/
//Convert String to StringBuilder
String s = "hello";
//StringBuilder sb = s; // This is wrong
//public StringBuilder(String s): you can convert a String into a StringBuilder by constructing a method
StringBuilder sb = new StringBuilder(s);
System.out.println(sb);
}
}
3.6 StringBuilder splicing string cases
Case requirements:
Define a method to splice the data in the int array into a string according to the specified format, return, and call this method,
And output the results on the console. For example, the array is int[] arr = {1,2,3}, The output result after executing the method is: [1, 2, 3]
Implementation steps:
- Define an array of type int, and use static initialization to complete the initialization of array elements
- Defines a method for splicing the data in the int array into a string according to the specified format.
Return value type String, parameter list int[] arr - In the method, use StringBuilder to splice as required, and convert the result into String to return
- Call the method to receive the result with a variable
- Output results
Code implementation:
/*
Idea:
1:Define an array of type int, and use static initialization to complete the initialization of array elements
2:Defines a method for splicing the data in the int array into a string according to the specified format.
Return value type String, parameter list int[] arr
3:In the method, use StringBuilder to splice as required, and convert the result into String to return
4:Call the method to receive the result with a variable
5:Output results
*/
public class StringBuilderTest01 {
public static void main(String[] args) {
//Define an array of type int, and use static initialization to complete the initialization of array elements
int[] arr = {1, 2, 3};
//Call the method to receive the result with a variable
String s = arrayToString(arr);
//Output results
System.out.println("s:" + s);
}
//Defines a method for splicing the data in the int array into a string according to the specified format
/*
Two clear:
Return value type: String
Parameter: int[] arr
*/
public static String arrayToString(int[] arr) {
//In the method, use StringBuilder to splice as required, and convert the result into String to return
StringBuilder sb = new StringBuilder();
sb.append("[");
for(int i=0; i<arr.length; i++) {
if(i == arr.length-1) {
sb.append(arr[i]);
} else {
sb.append(arr[i]).append(", ");
}
}
sb.append("]");
String s = sb.toString();
return s;
}
}