Java 简明教程

Java - String Class

Strings 在广泛用于 Java programming 中，是字符序列。在 Java 编程语言中，字符串被视为 objects。

Strings, which are widely used in Java programming, are a sequence of characters. In Java programming language, strings are treated as objects.

Java 平台提供 String class 来创建和操作字符串。

The Java platform provides the String class to create and manipulate strings.

Creating Strings

创建字符串的最直接的方法是编写 −

The most direct way to create a string is to write −

String greeting = "Hello world!";

每当编译器在代码中遇到字符串文字时，都会使用其值创建 String 对象，在本例中为“ Hello world!”。

Whenever it encounters a string literal in your code, the compiler creates a String object with its value in this case, "Hello world!".

与任何其他对象一样，您可以使用*new*关键字和 constructor创建 String 对象。String 类有 11 个构造方法，您可以使用它们通过不同的来源提供字符串的初始值，例如字符数组。

As with any other object, you can create String objects by using the new keyword and a constructor. The String class has 11 constructors that allow you to provide the initial value of the string using different sources, such as an array of characters.

Example to Create Strings in Java

public class StringDemo {
   public static void main(String args[]) {
      char[] helloArray = { 'h', 'e', 'l', 'l', 'o', '.' };
      String helloString = new String(helloArray);
      System.out.println( helloString );
   }
}

hello.

Note - String 类是不可变的，所以一旦创建了 String 对象就无法对其进行更改。如果有必要对字符 Strings 进行大量修改，那么您应该使用 String Buffer & String Builder 类。

Note − The String class is immutable, so that once it is created a String object cannot be changed. If there is a necessity to make a lot of modifications to Strings of characters, then you should use String Buffer & String Builder Classes.

String Length

用于获取有关对象的信息的方法称为*accessor methods*。您可以与字符串一起使用的一种访问器方法是 length() 方法，该方法返回字符串对象中包含的字符数。

Methods used to obtain information about an object are known as accessor methods. One accessor method that you can use with strings is the length() method, which returns the number of characters contained in the string object.

以下程序是 length() 方法 String 类的一个示例。

The following program is an example of length(), method String class.

Example to Get String Length in Java

public class StringDemo {
   public static void main(String args[]) {
      String palindrome = "Dot saw I was Tod";
      int len = palindrome.length();
      System.out.println( "String Length is : " + len );
   }
}

这会产生以下结果 −

This will produce the following result −

String Length is : 17

Concatenating Strings

String 类包括用于连接两个字符串的方法 −

The String class includes a method for concatenating two strings −

string1.concat(string2);

这会返回一个新的字符串，该字符串是 string1 和 string2 添加到其末尾的结果。您还可以在字符串文字中使用 concat() 方法，例如 −

This returns a new string that is string1 with string2 added to it at the end. You can also use the concat() method with string literals, as in −

"My name is ".concat("Zara");

字符串更常与 + 运算符连接，如 −

Strings are more commonly concatenated with the + operator, as in −

"Hello," + " world" + "!"

结果 −

which results in −

"Hello, world!"

让我们看以下示例 −

Let us look at the following example −

Example to Concatenate Strings in Java

public class StringDemo {
   public static void main(String args[]) {
      String string1 = "saw I was ";
      System.out.println("Dot " + string1 + "Tod");
   }
}

这会产生以下结果 −

This will produce the following result −

Output

Dot saw I was Tod

Creating Format Strings

可以使用 printf() 和 format() 方法打印带格式数字的输出。String 类有一个等效类方法 format()，它返回一个 String 对象，而不是一个 PrintStream 对象。

You have printf() and format() methods to print output with formatted numbers. The String class has an equivalent class method, format(), that returns a String object rather than a PrintStream object.

使用 String 的 static format() 方法允许您创建一个可重用的格式化字符串，而不是一个一次性打印语句。例如，而不是 −

Using String’s static format() method allows you to create a formatted string that you can reuse, as opposed to a one-time print statement. For example, instead of −

Example to Create Formatted Strings in Java

System.out.printf("The value of the float variable is " +
   "%f, while the value of the integer " +
   "variable is %d, and the string " +
   "is %s", floatVar, intVar, stringVar);

您可以编写 −

You can write −

String fs;
fs = String.format("The value of the float variable is " +
   "%f, while the value of the integer " +
   "variable is %d, and the string " +
   "is %s", floatVar, intVar, stringVar);
System.out.println(fs);

Java String Class Methods

以下是 String class in Java 的内建方法及其语法和用法 −

The following are the built-in methods of the String class in Java with their syntaxes and usages −

Sr.No.	Method & Description
1	char charAt(int index)This method returns the char value at the specified index.
2	int codePointAt(int index)This method returns the character (Unicode code point) at the specified index.
3	int codePointBefore(int index)This method returns the character (Unicode code point) before the specified index.
4	int codePointCount(int beginIndex, int endIndex)This method returns the number of Unicode code points in the specified text range of this String.
5	int compareTo(String anotherString)This method compares two strings lexicographically.
6	int compareToIgnoreCase(String str)This method compares two strings lexicographically, ignoring case differences.
7	String concat(String str)This method concatenates the specified string to the end of this string.
8	boolean contains(CharSequence s)This method ceturns true if and only if this string contains the specified sequence of char values.
9	boolean contentEquals(CharSequence cs) This method compares this string to the specified CharSequence.
10	boolean contentEquals(StringBuffer sb)This method compares this string to the specified StringBuffer.
11	static String copyValueOf(char[] data) This method returns a String that represents the character sequence in the array specified.
12	static String copyValueOf(char[] data, int offset, int count)This method returns a String that represents the character sequence in the array specified.
13	boolean endsWith(String suffix)This method tests if this string ends with the specified suffix.
14	boolean equals(Object anObject)This method compares this string to the specified object.
15	boolean equalsIgnoreCase(String anotherString)This method compares this String to another String, ignoring case considerations.
16	static String format(Locale l, String format, Object… args)This method returns a formatted string using the specified locale, format string, and arguments.
17	static String format(String format, Object… args)This method returns a formatted string using the specified format string and arguments.
18	byte[] getBytes()This method encodes this String into a sequence of bytes using the platform’s default charset, storing the result into a new byte array.
19	byte[] getBytes(Charset charset)This method encodes this String into a sequence of bytes using the given charset, storing the result into a new byte array.
20	byte[] getBytes(String charsetName)This method encodes this String into a sequence of bytes using the named charset, storing the result into a new byte array.
21	void getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin)This method copies characters from this string into the destination character array.
22	int hashCode()This method returns a hash code for this string.
23	int indexOf(int ch)This method returns the index within this string of the first occurrence of the specified character.
24	int indexOf(int ch, int fromIndex) This method returns the index within this string of the first occurrence of the specified character, starting the search at the specified index.
25	int indexOf(String str)This method returns the index within this string of the first occurrence of the specified substring.
26	int indexOf(String str, int fromIndex)This method returns the index within this string of the first occurrence of the specified substring, starting at the specified index.
27	String intern()This method returns a canonical representation for the string object.
28	boolean isEmpty()This method returns true if, and only if, length() is 0.
29	int lastIndexOf(int ch)This method returns the index within this string of the last occurrence of the specified character.
30	int lastIndexOf(int ch, int fromIndex)This method returns the index within this string of the last occurrence of the specified character, searching backward starting at the specified index.
31	int lastIndexOf(String str)This method returns the index within this string of the rightmost occurrence of the specified substring.
32	int lastIndexOf(String str, int fromIndex)This method returns the index within this string of the last occurrence of the specified substring, searching backward starting at the specified index.
33	int length()This method returns the length of this string.
34	boolean matches(String regex)This method tells whether or not this string matches the given regular expression.
35	int offsetByCodePoints(int index, int codePointOffset)This method returns the index within this String that is offset from the given index by codePointOffset code points.
36	boolean regionMatches(boolean ignoreCase, int toffset, String other, int ooffset, int len)This method tests if two string regions are equal with case ignored.
37	boolean regionMatches(int toffset, String other, int ooffset, int len)This method tests if two string regions are equal.
38	String replace(char oldChar, char newChar)This method returns a new string resulting from replacing all occurrences of oldChar in this string with newChar.
39	String replace(CharSequence target, CharSequence replacement)This method replaces each substring of this string that matches the literal target sequence with the specified literal replacement sequence.
40	String replaceAll(String regex, String replacement)This method replaces each substring of this string that matches the given regular expression with the given replacement.
41	String replaceFirst(String regex, String replacement)This method replaces the first substring of this string that matches the given regular expression with the given replacement.
42	String[] split(String regex)This method splits this string around matches of the given regular expression.
43	String[] split(String regex, int limit)This method splits this string around matches of the given regular expression.
44	boolean startsWith(String prefix)This method tests if this string starts with the specified prefix.
45	boolean startsWith(String prefix, int toffset)This method tests if the substring of this string beginning at the specified index starts with the specified prefix.
46	CharSequence subSequence(int beginIndex, int endIndex)This method returns a new character sequence that is a subsequence of this sequence.
47	String substring(int beginIndex)This method returns a new string that is a substring of this string.
48	String substring(int beginIndex, int endIndex)This method returns a new string that is a substring of this string.
49	char[] toCharArray()This method converts this string to a new character array.
50	String toLowerCase()This method converts all of the characters in this String to lower case using the rules of the default locale.
51	String toLowerCase(Locale locale)This method converts all of the characters in this String to lower case using the rules of the given Locale.
52	String toString()This method returns the string itself.
53	String toUpperCase()This method converts all of the characters in this String to upper case using the rules of the default locale.
54	String toUpperCase(Locale locale)This method converts all of the characters in this String to upper case using the rules of the given Locale.
55	String trim()This method returns a copy of the string, with leading and trailing whitespace omitted.
56	static String valueOf(boolean b)This method returns the string representation of the boolean argument.
57	static String valueOf(char c)This method returns the string representation of the char argument.
58	static String valueOf(char[] data)This method returns the string representation of the char array argument.
59	static String valueOf(char[] data, int offset, int count)This method Returns the string representation of a specific subarray of the char array argument.
60	static String valueOf(double d)This method returns the string representation of the double argument.
61	static String valueOf(float f)This method returns the string representation of the float argument.
62	static String valueOf(int i)This method returns the string representation of the int argument.
63	static String valueOf(long l)This method returns the string representation of the long argument.
64	static String valueOf(Object obj)This method returns the string representation of the Object argument.