java String class entry

　　1 Overview

　　As long as the string is called double quotes

　　Java.lang.String String class in the package, as long as the java.lang package, when writing a program does not need to import to come

　　Contents of the string once created, no longer change, is a constant

　　It is precisely because the string can not be changed, so the strings can be shared use. Imagine if you can share things easily modified with a residue once, who wanted to share it, think of all sick, everyone can use a shared bike, you can easily use when representatives spoil it, dismantle it ???

　　The effect is equivalent to a string char [] array of characters, but the underlying principle is the byte [] array

　　2. Create a String class (3 + 1 ways)

　　2.1 Method three configurations

　　public String (); create a blank string does not contain any content

　　public String (char [] array); the contents of the string array to create the corresponding character string

　　public String (byte [] array); according to the content byte array to create the corresponding character string

　　Three configurations way to demonstrate it again in code

　　public class ConstructString {

　　public static void main(String[] args) {

　　// The first constructor: public String (); create a blank string does not contain any content

　　String str1 = new String();

　　System.out.println ( "first string:" + str1); // first string:

　　// The second constructor: public String (char [] array); the contents of the string array to create the corresponding character string

　　char[] charArray = {'A', 'B', 'C'};

　　String str2 = new String(charArray);

　　System.out.println ( "second string:" + str2); // second string: ABC

　　The third // constructor: public String (byte [] array); according to the content byte array to create the corresponding character string

　　byte [] byteArray = {97, 98, 99}; // Note that the byte array element, corresponding to the value of the ASCII character code table

　　String str3 = new String(byteArray);

　　System.out.println ( "third string:" + str3); // third string: abc

　　}

　　2.2 A direct creation

　　String str4 = "HelloWorld!"

　　Whether or not new, as long as the stuff inside double quotes, JVM will be judged to be a string, so you can directly create

　　3. constant pool

　　3.1 string of relatively little knowledge about

　　We want to know, in java, using if (String1 == String2) to compare two strings, the comparison is whether the addresses of the two strings of the same, and if they want to know whether the value of the same, using a String equals methods, such as:

　　public static void main(String[] args) {

　　String str1 = new String("hello");

　　String str2 = new String("hello");

　　// compare str1 address and the address is not the same as str2

　　System.out.println (str1 == str2); // false is not the same

　　// compare the value of str1 and str2 is not the same

　　System.out.println (str1.equals (str2)); // true, it is hello

　　}

　　3.2 heap constant pool (heap) in

　　Str1 and str2 result of the comparison is true, prove that they are the same string, but there are only two names, in memory of all point to the same address, they are in a place called constant pool, so the constant pool of characters hello string can be shared, not be modified.

　　str1 and str3 not the same as proof of their addresses are not the same, string created by the new, not in the constant pool, but also a constant

　　public class ConstantPool {

　　public static void main(String[] args) {

　　String str1 = "hello";

　　String str2 = "hello";

　　String str3 = new String("hello");

　　System.out.println(str1 == str2); //true

　　System.out.println(str1 == str3); //false

　　System.out.println(str2 == str3); //false

　　}

　　A method of comparing strings

　　4.1 String equals method

　　3. Tips from the constant pool, we know the value of comparing two strings if the same, using the equals method, the return value is a Boolean value, the same is true, is not the same as false.

　　public boolean equals(Object obj)

　　NOTE: Any object can be received by Object, String class of course be.

　　equals method has symmetry, i.e. the result str1.equals (str2) and str2.equals (str1) is the same

　　Note: If you compare the two sides is a constant a variable, string constants recommendation EDITORIAL

　　That recommendation "abc" .equals (str) is not recommended: str.equals ( "abc")

　　Because if the string str is a null, the result of "abc" .equals (str) is certainly false, but str.equals ( "abc") written will result in a null pointer exception NullPointerException, do not casually do an exception out .

　　4.1 String method of equalsIgnoreCase

　　The difference between it and the equals method is that it ignores the case, Ignore to ignore, Case case

　　public static void main(String[] args) {

　　String str1 = "hello";

　　String str2 = "HellO";

　　// value comparison of the value of str1 and str2 is not as rigorous case-sensitive

　　System.out.println( str1.equals(str2) ); //false

　　// compare the value of str1 and str2 is not the same, ignoring case

　　System.out.println( str1.equalsIgnoreCase(str2)); //true

　　}

　　5. String associated with the acquisition process

　　Outline

　　public int length (); Get the number of characters contained in the character string which, to get the string length

　　public String concat (String str); and the current parameter string concatenation string becomes the return value as a new string

　　public char charAt (int index); obtaining a single character at the specified index. (0 indexed)

　　public int indexOf (String str); lookup index position of the first occurrence of the parameter string of this character string in which, if there is no value -1

　　5.1 length()和concat(String str)

　　public static void main(String[] args) {

　　// length () method, get the length of the string

　　int length = "abcdefghijk".length();

　　System.out.println ( "length of the string are:" + length);

　　// concat (String str), string concatenation

　　String str1 = "Hello";

　　String str2 = "World";

　　String str3 = str1.concat (str2); // str1 after splicing produces a new string, but itself unchanged str1

　　// print string splicing

　　System.out.println(str1);

　　System.out.println(str2);

　　System.out.println(str3);

　　}

　　/ * Output:

　　Length of the string is: 11

　　Hello

　　World

　　HelloWorld

　　5.2 charAt(int index)和indexOf(String str)

　　public static void main(String[] args) {

　　// charAt (int index), gets the specified index position of a single character

　　char ch = "hello".charAt(1);

　　System.out.println ( "character index position is No. 1:" + ch);

　　// indexOf (String str), find the first index parameter string that appears in the original string position among

　　// If not, it returns a value of -1

　　String original = "helloworld!";

　　int index = original.indexOf("ll0");

　　System.out.println ( "first index value are:" + index); // 2

　　}

　　6. The method of interception of the string

　　Outline

　　public String substring (int index); taken from a position aligned to the end of the string parameter and returns the new string. It is [begin, end), containing the left end to the right until

　　public String substring (int begin, int end); taken chest begin start until the end of End, intermediate string. It is [begin, end), containing the left and the right is not included

　　6.1 code shows

　　public static void main(String[] args) {

　　String str1 = "HelloWorld!";

　　// substring (int index) method

　　String str2 = str1.substring (5); // [5, end)

　　System.out.println (str2); // World !, This is a new string

　　//substring(int begin, int end) 方法

　　String str3 = str1.substring(5, 8); // [ 5, 8 )

　　System.out.println(str3); //Wor

　　}

　　7. The method of converting a string

　　Outline

　　public char [] toCharArray (); splitting the current string to a string array as a return value

　　public byte [] getBytes ();! array to obtain the current character string underlying note, not toBytes ()

　　public String replace (CharSequence oldString, CharSequence newString); replace all old and new occurrence of the string into a string, then returns the result as a new replacement string

　　7.1 code shows

　　public static void main(String[] args) {

　　// toCharArray (); splitting the current string to a string array as a return value

　　char[] chars = "hello".toCharArray();

　　System.out.println(chars[0]); //h

　　System.out.println (chars.length); // 5, noted that the methods generally array no parenthesis

　　// byte [] getBytes (); obtain the current array of character strings bottom

　　// Note! No toByte () method

　　byte[] bytes = "abc".getBytes();

　　for(int i = 0; i < bytes.length; i++) {

　　System.out.println (bytes [i]); // 97, 98, 99, respectively, the characters of the ASCII code table values abc

　　}

　　// replace methods, you can do some sensitive words filtered

　　String str1 = "Fuck!";

　　String str2 = str1.replace ( "uc", "**"); // replace ** uc into

　　System.out.println(str2); //F**k!;

　　}

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　　8. The method of dividing a character string

　　Outline

　　public String split (String regex); parameters in accordance with the rules, the string is divided into sections Note that when dividing point English period, the parameter can not be regex, to write! '. "" \\. "

　　8.1 code shows

　　public static void main(String[] args) {

　　// split (String regex) method delimited string

　　String str1 = "AAA,BBB,CCC";

　　String [] array1 = str1.split ( ","); // separated by a comma

　　for(int i = 0; i < array1.length; i++) {

　　System.out.println(array1[i]); // AAA BBB CCC

　　}

　　// Note! Argument split method is actually a regular expression

　　// In other words, according to the English period. "" Be segmented, you must write "\\.", Because the period has a special meaning

　　String str2 = "XXX.YYY.ZZZ";

　　String[] array2 = str2.split("\\.");

　　for(int i = 0; i < array2.length; i++) {

　　System.out.println(array2[i]); //XXX YYY ZZZ

　　}

　　9. Small exercises

　　9.1 Topic

　　Keyboard input a string, and count the number of which various characters appear

　　Types: uppercase letters, lowercase letters, numbers, other

　　9.2 ideas

　　Since it uses keyboard input, certainly Scanner

　　Keyboard input is a string, then: String str = sc.next ();

　　Define four variables, representing the number of sub-emergence of four character classes

　　It requires a word string, a check word, we can use the method charAt String, and the string may be first converted to char [] array traverse

　　Traversal string, the determination of the current type, and performs operations with four variables ++

　　Printout of four variables, representing the number of four character classes appear

　　9.3 code shows

　　public static void main(String[] args) {

　　// 1. Since the keyboard is used, certainly Scanner

　　Scanner sc = new Scanner(System.in);

　　// 2 keyboard input string, then: String str = sc.next ();

　　String str = sc.next();

　　// 3. Define four variables, representing the number of sub-emergence of four character classes

　　int countUpper = 0;

　　int countLower = 0;

　　int countNumber = 0;

　　int countOther = 0;

　　// 4. Traversal string, the determination of the current type, and performs operations with four variables ++

　　for(int i = 0; i < str.length(); i++) {

　　char ch = str.charAt(i);

　　if(ch >= 'A' && ch <= 'Z') {

　　countUpper++;

　　}else if(ch >= 'a' && ch <= 'z') {

　　countLower++;

　　}else if(ch >= '0' && ch <= '9') {

　　countNumber++;

　　}else {

　　countOther++;

　　}

　　// 5. Printout four variables, representing the number of four character classes appear

　　System.out.println ( "number of capital letters:" + countUpper);

　　System.out.println ( "the number of the lowercase letters:" + countLower);

　　System.out.println ( "Digital number:" + countNumber);

　　System.out.println ( "with other numbers:" + countOther);

　　}

　　Haha, because the space on behalf of the end of the input, so in this code, you can not count the number of spaces

　　————————————————

　　Disclaimer: This article is CSDN blogger "Fu 97" original article, follow the CC 4.0 by-sa copyright agreement, reproduced, please attach the original source link and this statement.

　　Original link: https: //blog.csdn.net/weixin_43836046/article/details/96764238

java String class entry

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