Compare, find, replace, split and intercept Java strings

1, String comparison
1. equals(): case sensitive, and the return value type is boolean;

//Case sensitive comparison
String str1 = "hello" ; 
String str2 = "Hello" ; 
System.out.println(str1.equals(str2)); // Return false 

2. Case insensitive: equalsIgnoreCase(), the return value type is boolean;

//Case insensitive comparison
String str1 = "hello" ; 
String str2 = "Hello" ; 
System.out.println(str1.equalsIgnoreCase(str2)); // Return true

3. Compare the size relationship between two strings: compareTo(), the return value type is int;
This method will return three contents according to the size relationship:
Equal: return 0;
Less than: the return value is less than 0;
Greater than: the return value is greater than 0;

//compareTo comparison;
System.out.println("A".compareTo("a")); // -32 
System.out.println("a".compareTo("A")); // 32 
System.out.println("A".compareTo("A")); // 0 
System.out.println("AB".compareTo("AC")); // -1 
System.out.println("Liu".compareTo("Yang"));

2, String lookup
1. Contains (charsequences): judge whether a substring exists, and the return value type is boolean;

String str = "helloworld" ; 
System.out.println(str.contains("world")); // true

2. indexOf(String str): finds the position of the specified string from the beginning. If the start index of the return position is found, it returns - 1. The return value type is int;

// Use the indexOf() method to find the location
String str = "helloworld" ; 
System.out.println(str.indexOf("world")); // Returns the index from 5,w
System.out.println(str.indexOf("bit")); // Return - 1, not found
if (str.indexOf("hello") != -1) { 
 System.out.println("You can find the specified string!"); 
}
//Now it is basically done with the contains() method.
//When using indexOf(), it should be noted that if the content is repeated, it can only return to the first location of the search.

// Considerations for using indexOf()
String str = "helloworld" ; 
System.out.println(str.indexOf("l")); // 2 
//When searching, we often judge the beginning or end.

3. indexOf(String str,int fromIndex): find the string position from the specified position, and the return value type is int;

String str = "helloworld" ;
System.out.println(str.indexOf("l",5)); // 8 

4. lastIndexOf(String str): find the position of the substring from the back to the front, and the return value type is int;

String str = "helloworld" ;
System.out.println(str.lastIndexOf("l")); // 8

5. lastIndexOf(String str,int fromIndex): find the position of the substring from back to front from the specified position, and the return value type is int;

String str = "helloworld" ;
System.out.println(str.lastIndexOf("l",6)); // 3

6. startsWith(String prefix): judge whether to start with the specified string, and the return value type is boolean;

String str = "**@@helloworld!!" ; 
System.out.println(str.startsWith("**")); // true 

7. startsWith(String prefix,int toffset): judge whether it starts with the specified string from the specified position, and the return value type is boolean;

String str = "**@@helloworld!!" ; 
System.out.println(str.startsWith("@@",2)); // ture 

8. endsWith(String suffix): judge whether to end with the specified string, and the return value type is boolean;

String str = "**@@helloworld!!" ; 
System.out.println(str.endsWith("!!")); // true

3, String replacement
Replace the existing string data with a specified new string.
1. replaceAll(String regex, String replacement): replace all specified contents and return the replaced string;

String str = "helloworld" ; 
System.out.println(str.replaceAll("l", "_")); //Output - > he__ owor_ d

2. replaceFirst(String regex, String replacement): replace the first content and return the replaced string;

String str = "helloworld" ; 
System.out.println(str.replaceFirst("l", "_"));//Output - > he_ loworld

Note: since the string is an immutable object, the replacement does not modify the current string, but generates a new string;

4, Splitting of strings;
A complete string can be divided into several substrings according to the specified separator.
1. split(String regex): splits all strings, and the return value type is String [];

//Split the string
String str = "hello world hello bit" ; 
String[] result = str.split(" ") ; // Split by space
for(String s: result) { 
 System.out.println(s); 
}

output

2. split(String regex, int limit): split the String part. The length of the split array is the limit of the limit, and the return value type is String [];

//Partial splitting of strings
String str = "hello world hello bit" ; 
String[] result = str.split(" ",2) ; 
for(String s: result) { 
 System.out.println(s); 
}

output

Splitting is a very common operation. You must focus on it. In addition, some special characters may not be segmented correctly as separators, and escape needs to be added

//Split IP address
String str = "192.168.1.1" ; 
String[] result = str.split("\\.") ; 
for(String s: result) { 
 System.out.println(s); 
}

output

matters needing attention
1. The characters "|", "*", "+" must be preceded by the escape character ";
2. If it is "", it must be written as "";
3. If there are multiple separators in a string, you can use "|" as a hyphen;

// Multiple split
String str = "name=zhangsan&age=18" ; 
String[] result = str.split("&") ; 
for (int i = 0; i < result.length; i++) { 
 String[] temp = result[i].split("=") ; 
 System.out.println(temp[0]+" = "+temp[1]); 
}

output

5, String interception
Extract part of the content from a complete string.
1. substring(int beginIndex): intercept from the specified index to the end;

String str = "helloworld" ; 
System.out.println(str.substring(5)); 

2. substring(int beginIndex, int endIndex): intercepts some contents from the specified index;

String str = "helloworld" ; 
System.out.println(str.substring(0, 5));

matters needing attention:
1. Index starts at 0
2. Pay attention to the writing method of the front closed and back open interval. substring(0, 5) indicates a character containing subscript 0, but not subscript 5