<> one , summary
String: character string , Use a pair of "" Cause to express .
* String Declare as final Of , Not inheritable
* String Realized Serializable Interface ： Indicates that the string is serializable .
Realized Comparable Interface ： express String You can compare the size
* String Internally defined final char[] value Used to store string data
* In a literal way （ Different from new Assign a value to a string , At this point, the string value is declared in the string constant pool ).
* String constant pool does not store the same content ( use String Class equals() compare , return true) Of the string .
<> two ,String The immutability of
* When a string is reassigned , The memory area assignment needs to be re specified , Cannot use original value Assign values .
* When a connection operation is performed on an existing string , You also need to reassign the memory area , Cannot use original value Assign values .
* When calling String Of replace() Method to modify the specified character or string , You also need to reassign the memory area , Cannot use original value Assign values . String s1 =
"abc";// The definition of literal quantity String s2 = "abc"; s1 = "hello"; System.out.println(s1 == s2);
// compare s1 and s2 Address value of System.out.println(s1);//hello System.out.println(s2);//abc System
.out.println("*****************"); String s3 = "abc"; s3 += "def"; System.out.
println(s3);//abcdef System.out.println(s2); System.out.println(
"*****************"); String s4 = "abc"; String s5 = s4.replace('a', 'm');
System.out.println(s4);//abc System.out.println(s5);//mbc
<> three ,String Different ways of instantiation
* By means of literal definition
* adopt new + The way of constructors // By means of literal definition ： At this time s1 and s2 Data for javaEE Declared in the string constant pool in the method area . String s1 =
"javaEE"; String s2 = "javaEE"; // adopt new +
The way of constructors : At this time s3 and s4 Saved address values , It is the corresponding address value of data after opening up space in heap space . String s3 = new String("javaEE");
String s4= new String("javaEE"); System.out.println(s1 == s2);//true System.out.
println(s1 == s3);//false System.out.println(s1 == s4);//false System.out.
println(s3 == s4);//false
Interview questions : String s = new String("abc"); How to create objects , Several objects are created in memory ? Two :
One is in heap space new structure , The other is char[] Data in the corresponding constant pool ："abc"
<> four ,String Memory resolution of splicing
* The splicing result of constant and constant is in the constant pool .
* As long as one of them is a variable , The result is in the pile .
* If the result of splicing calls intern() method , The return value is in the constant pool . String s1 = "javaEE"; String s2 = "hadoop";
String s3= "javaEEhadoop"; String s4 = "javaEE" + "hadoop"; String s5 = s1 +
"hadoop"; String s6 = "javaEE" + s2; String s7 = s1 + s2; System.out.println(s3
== s4);//true System.out.println(s3 == s5);//false System.out.println(s3 == s6);
//false System.out.println(s3 == s7);//false System.out.println(s5 == s6);
//false System.out.println(s5 == s7);//false System.out.println(s6 == s7);
//false String s8 = s6.intern();// Return to the worthy s8 The value of the constant used already exists “javaEEhadoop” System.out.
println(s3 == s8);//true **************************** String s1 = "javaEEhadoop"
; String s2 = "javaEE"; String s3 = s2 + "hadoop"; System.out.println(s1 == s3);
//false final String s4 = "javaEE";//s4: constant String s5 = s4 + "hadoop"; System.out
.println(s1 == s5);//true
<> five ,String common method
int length()： Returns the length of the string ： return value.length char charAt(int index)： Returns the character at an index
return value[index] boolean isEmpty()： Determine whether it is an empty string ：return value.length == 0 String
toLowerCase()： Use default locale , take String Converts the selected characters in to lowercase String toUpperCase()： Use default locale , take
String Converts the selected characters in to uppercase Stringtrim()： Returns a copy of a string , Ignore leading and trailing blanks boolean equals(Object obj)
： Compare whether the contents of the string are the same boolean equalsIgnoreCase(String anotherString)： And equals The method is similar , ignore case
Stringconcat(String str)： Concatenates the specified string to the end of this string . Equivalent to using “+” int compareTo(String
anotherString)： Compare the size of two strings String substring(int beginIndex)
： Returns a new string , It is the slave of this string beginIndex Start to intercept to the last substring . Stringsubstring(int beginIndex, int
endIndex) ： Returns a new string , It is this string from beginIndex Start intercepting to endIndex( Not included ) A substring of . boolean
endsWith(String suffix)： Tests whether the string ends with the specified suffix boolean startsWith(String prefix)
： Tests whether the string starts with the specified prefix boolean startsWith(String prefix, int toffset)
： Tests whether the substring of this string starting at the specified index starts with the specified prefix boolean contains(CharSequence s)： If and only if the string contains the specified char
Value sequence , return true int indexOf(String str)： Returns the index of the first occurrence of the specified substring in the string int indexOf(String
str, int fromIndex)： Returns the index of the first occurrence of the specified substring in the string , Starts with the specified index int lastIndexOf(String str)
： Returns the index of the rightmost occurrence of the specified substring in the string int lastIndexOf(String str, int fromIndex)
： Returns the index of the last occurrence of the specified substring in the string , Starts a reverse search from the specified index notes ：indexOf and lastIndexOf Methods are returned if not found -1 replace ：
Stringreplace(char oldChar, char newChar)： Returns a new string , It's by using newChar Replace all occurrences in this string
oldChar Got it . Stringreplace(CharSequence target, CharSequence replacement)
： Replaces the substring of the literal target sequence matched by this string with the specified literal replacement sequence . StringreplaceAll(String regex, String
replacement)： Use the given replacement Replaces the substring of the given regular expression that this string matches . String replaceFirst(
String regex, String replacement)： Use the given replacement Replace this string to match the first substring of the given regular expression . matching
: boolean matches(String regex)： Tells if the string matches the given regular expression . section ： String[] split(String
regex)： Splits the string according to the match of the given regular expression . String[] split(String regex, int limit)
： Splits the string by matching the given regular expression , No more than limit individual , If it exceeds , The rest is in the last element .
<> six ,String Transformation with other structures
6. String Transformation with other structures 6.1 And basic data types , Conversion between packaging classes String --> Basic data type , Packaging ： Calling static methods of wrapper classes ：
parseXxx(str) Basic data type , Packaging --> String: call String Overloaded valueOf(xxx) @Test public void
test1(){ String str1 = "123"; // int num = (int)str1;// FALSE int num = Integer.
parseInt(str1); String str2 = String.valueOf(num);//"123" String str3 = num + ""
; System.out.println(str1 == str3); } 6.2 Conversion between and character array String --> char[]: call String Of
toCharArray() char[] --> String: call String The constructors of @Test public void test2(){ String
str1= "abc123"; // subject ： a21cb3 char[] charArray = str1.toCharArray(); for (int i =
0; i < charArray.length; i++) { System.out.println(charArray[i]); } char[] arr =
new char[]{'h','e','l','l','o'}; String str2 = new String(arr); System.out.
println(str2); } 6.3 Conversion between and byte array code ：String --> byte[]: call String Of getBytes() decode ：byte
[] --> String: call String The constructors of code ： character string --> byte ( I can understand it ---> Incomprehensible binary data ) decode ： The reverse process of coding , byte -->
character string （ Incomprehensible binary data ---> I can understand it explain ： When decoding , The character set used for encoding must be the same as that used for decoding , Otherwise, there will be garbled code . @Test public
void test3() throws UnsupportedEncodingException { String str1 = "abc123 China ";
byte[] bytes = str1.getBytes();// Use the default character set , Code . System.out.println(Arrays.
toString(bytes)); byte[] gbks = str1.getBytes("gbk");// use gbk Character set . System.out.
println(Arrays.toString(gbks)); System.out.println("******************");
String str2= new String(bytes);// Use the default character set , Decode . System.out.println(str2);
String str3= new String(gbks); System.out.println(str3);// Garbled . reason ： The encoding set is inconsistent with the decoding set !
String str4= new String(gbks, "gbk"); System.out.println(str4);
// There's no garbled code . reason ： The coding set is consistent with the decoding set ! } 6.4 And StringBuffer,StringBuilder Conversion between String -->
StringBuffer,StringBuilder: call StringBuffer,StringBuilder constructor
StringBuffer,StringBuilder-->String:① call String constructor ;②StringBuffer,StringBuilder Of
toString()
<> seven , JVM Description of storage location of string constant pool in
* jdk 1.6 (jdk 6.0 ,java 6.0): The string constant pool is stored in the method area （ Permanent zone ）
* jdk 1.7: The string constant pool is stored in heap space
* jdk 1.8: The string constant pool is stored in the method area （ Meta space ）
<> eight ,String,StringBuffer,StringBuilder Comparison of the three
* String: Immutable character sequence ; Underlying use final char[] storage
* StringBuffer: Variable character sequence ; Thread safe , Low efficiency ; Underlying use char[] storage
* StringBuilder: Variable character sequence ;jdk5.0 New , Thread unsafe , efficient ; Underlying use char[] storage 2.
StringBuffer And StringBuilder Memory resolution of with StringBuffer take as an example ： String str= new String();
//char[] value = new char[0]; String str1 = new String("abc");//char[] value =
new char[]{'a','b','c'}; StringBuffer sb1 = new StringBuffer();//char[] value =
new char[16]; The bottom layer creates a length of 16 Array of . System.out.println(sb1.length());// sb1.append('a'
);//value[0] = 'a'; sb1.append('b');//value[1] = 'b'; StringBuffer sb2 = new
StringBuffer("abc");//char[] value = new char["abc".length() + 16]; // problem 1.
System.out.println(sb2.length());//3 // problem 2. Capacity expansion : If you want to add data, the underlying array won't hold , Then you need to expand the underlying array .
By default , Expansion to the original capacity 2 times + 2, At the same time, the elements in the original array are copied to the new array . Guiding significance ： It is suggested to use in the development ：StringBuffer(int
capacity) or StringBuilder(int capacity) 3.
contrast String,StringBuffer,StringBuilder The implementation efficiency of the three From high to low ：StringBuilder> StringBuffer >
String4.StringBuffer,StringBuilder The common methods in the design increase ：append(xxx) Delete ：delete(int start,int
end) change ：setCharAt(int n ,char ch) / replace(int start, int end, String str) check ：
charAt(int n ) insert ：insert(int offset, xxx) length ：length(); * ergodic ：for() + charAt() /
toString()
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