SoFunction
Updated on 2025-03-08

Detailed explanation of Java String class usage

1. Introduction

Scratch knowledge points

  • extends Object
  • implements serializable，Comparable< String >,charSequence
  • The String class represents a string, and all string literals are objects of this class.
  • The string is unchanged and the value cannot be changed after creation.
  • Once the object is declared, it cannot be changed. All it changes is the address. The original string still exists and generates garbage.
  • Any "" is a string object, and if there is no assignment, it is an anonymous object.
  • Use "+" to splice strings as much as possible, generally use append+toString
  • StringBuilder (thread unsafe) and StringBuffer (thread safe), ignore thread safety, everything else is the same

Since String objects are immutable,shared(i.e. the same as two stringsShare the same memory address)

//sharedString t1 = "123";
String t2 = "123";
(t1 == t2);//true,(== Comparing memory addresses)

String constant pool

Exist inMethod area(Memory area where the code is loaded) is shared by all threads

Logical division

The heap is logically divided into three parts:
New Generation: Storing newly created objects (the maximum number of GC queries is 15 times, and if >15 is entered, it will enter the old age)
Elderly: Garbage recycling times exceeds 15 times (that is, objects that still survive after 15 times)
Permanent generation: everything that is statically modified (classes, methods, constants...)

The creation of each string object will be put into a permanent generation (every time you create it, the permanent generation will be first, the premise is: no new, new opens up new memory)

2. Create an object

2.1 Direct reference to the constant area

String s = " ";

String str = “abc” ;

2.2 Using the construction method

String()

String str = new String(“abc”);

2.3 The difference between the two instantiation methods

the difference

Direct assignment:

  • Only open up a pile of memory space, and it will automatically enter the pool without garbage
  • Anonymous object "" will be placed into the object pool, and the anonymous object that already exists in the pool will be directly used when different objects are assigned directly.

Construction method:

  • Create an object in heap memory
  • Two heap memory spaces will be opened, one of which will turn into garbage and be recycled by the system, and it cannot be automatically entered into the pool. It needs to be manually entered into the pool through the public String intern(); method.

3. Common methods

Modifier and Type Method Description
char charAt​(int index) Returns the char value at the specified index.
int compareTo​(String anotherString) Compare two strings in dictionary order.
boolean endsWith​(String suffix) Tests whether this string ends with the specified suffix.
boolean equals​(Object anObject) Compare this string to the specified object.
boolean equalsIgnoreCase​(String anotherString) Comparing this String with another String, ignoring case.
void getChars​(int srcBegin, int srcEnd, char[] dst, int dstBegin) Copy the characters from this string into the target character array.
int indexOf​(int ch) Returns the index in the string where the specified character first appears.
int indexOf​(int ch, int fromIndex) Returns the index in this string where the specified character first appears, starting with the search at the specified index.
int indexOf​(String str) Returns the index in the string in which the specified substring first appears.
int indexOf​(String str, int fromIndex) Starting from the specified index, return the index in the string in which the specified substring first appears.
boolean isEmpty() Returns true if and only if length() is 0 .
int lastIndexOf​(int ch) Returns the index in the string where the specified character last appears.
int lastIndexOf​(int ch, int fromIndex) Returns the index in the string where the specified character last appears, starting from the specified index and searching backwards.
int lastIndexOf​(String str) Returns the index in the string that last appears in the specified substring.
int lastIndexOf​(String str, int fromIndex) Returns the index in the string that last appears in the specified substring, searching backwards from the specified index.
int length() Returns the length of this string.
String repeat​(int count) Returns a string whose value is repeated counts for the concatenation of this string.
String replace​(CharSequence target, CharSequence replacement) Replace each substring in this string that matches the literal target sequence with the specified literal replacement sequence.
String replaceAll​(String regex, String replacement) Replace each substring of this string that matches the given regular expression of the given replacement.
String replaceFirst​(String regex, String replacement) Replace the first substring of this string that matches the given regular expression of the given replacement.
boolean startsWith​(String prefix) Tests whether this string begins with the specified prefix.
boolean startsWith​(String prefix, int toffset) Tests whether the substring of this string starting with the specified index begins with the specified prefix.
String stripLeading() Returns a string whose value is this string and deletes all leading white space.
String stripTrailing() Returns a string whose value is this string and removes all trailing white space.
CharSequence subSequence​(int beginIndex, int endIndex) Returns a sequence of characters that are subsequences of this sequence.
String substring​(int beginIndex) Returns a string that is a substring of this string.
String substring​(int beginIndex, int endIndex) Returns a string that is a substring of this string.
char[] toCharArray() Convert this string to a new character array.
String toLowerCase() Use the rules of the default locale to convert all characters to lowercase.
String toUpperCase() Use the rules of the default locale to convert all characters to uppercase.
String toUpperCase​(Locale locale) Convert all characters of this String to uppercase using the rules given Locale.
static String valueOf​(T i) Returns the string representation of the T parameter.

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