Java – String
Class and its methods
String is a class
in java library
String is
nothing but a sequence of characters, for e.g. “Hello” is a string of 5
characters. In java, string is an immutable object which means it is constant
and can cannot be changed once it has been created. In this tutorial we will
learn about String class and String methods in detail along with many other
Java String tutorials.
Creating a String
There are two
ways to create a String in Java
1. String
literal
2. Using
new keyword
String literal
In java, Strings
can be created like this: Assigning a String literal to a String instance:
String str1 = "Welcome";
String str2 = "Welcome";
The problem with this approach: As I stated in the beginning that
String is an object in Java. However we have not created any string object
using new keyword above. The compiler does that task for us it creates a string
object having the string literal (that we have provided , in this case it is
“Welcome”) and assigns it to the provided string instances.
But if the object already exist in the
memory it does not create a new Object rather it assigns the same old object to
the new instance, that means even though we have two string instances
above(str1 and str2) compiler only created on string object (having the value
“Welcome”) and assigned the same to both the instances. For example there are
10 string instances that have same value, it means that in memory there is only
one object having the value and all the 10 string instances would be pointing
to the same object.
What if we want
to have two different object with the same string? For that we would need to
create strings using new keyword.
Using New Keyword
As we saw above
that when we tried to assign the same string object to two different literals,
compiler only created one object and made both of the literals to point the
same object. To overcome that approach we can create strings like this:
String str1 = new String("Welcome");
String str2 = new String("Welcome");
In this case
compiler would create two different object in memory having the same text.
String Methods
1. char charAt(int index): It returns the
character at the specified index. Specified index value should be between 0 to
length() -1 both inclusive. It throws IndexOutOfBoundsException if
index<0||>= length of String.
2. int
codePointAt(int index):It is similar to the charAt method however it returns
the Unicode code point value of specified index rather than the character
itself.
3. void getChars(int srcBegin, int srcEnd, char[] dest, int
destBegin): It copies the characters of src array to the dest array. Only the specified range is
being copied(srcBegin to srcEnd) to the dest subarray(starting fromdestBegin).
4. boolean equals(Object obj): Compares the
string with the specified string and returns true if both matches else false.
5. boolean contentEquals(StringBuffer sb): It
compares the string to the specified string buffer.
6. boolean equalsIgnoreCase(String string):
It works same as equals method but it doesn’t consider the case while comparing
strings. It does a case insensitive comparison.
7. int compareTo(String string): This method
compares the two strings based on the Unicode value of each character in the
strings.
8. int compareToIgnoreCase(String string):
Same as CompareTo method however it ignores the case during comparison.
9. boolean regionMatches(int srcoffset, String dest, int
destoffset, int len): It compares the substring of input to the
substring of specified string.
10. boolean regionMatches(boolean ignoreCase,
int srcoffset, String dest, int destoffset, int len):
Another variation of regionMatches method with the extra boolean argument to
specify whether the comparison is case sensitive or case insensitive.
11. boolean startsWith(String prefix, int offset):
It checks whether the substring (starting from the specified offset index) is
having the specified prefix or not.
12. boolean startsWith(String prefix): It
tests whether the string is having specified prefix, if yes then it returns
true else false.
13. boolean endsWith(String suffix): Checks
whether the string ends with the specified suffix.
14. int hashCode(): It returns the hash code
of the string.
15. int indexOf(int ch): Returns the index of
first occurrence of the specified character ch in the string.
16. int indexOf(int ch, int fromIndex): Same
as indexOf method however it starts searching in the string from the specified
fromIndex.
17. int lastIndexOf(int ch): It returns the
last occurrence of the character ch in the string.
18. int lastIndexOf(int ch, int fromIndex):
Same as lastIndexOf(int ch) method, it starts search from fromIndex.
19. int indexOf(String str): This method
returns the index of first occurrence of specified substring str.
20. int lastindexOf(String str): Returns the
index of last occurrence of string str.
21. String substring(int beginIndex): It
returns the substring of the string. The substring starts with the character at
the specified index.
22. String substring(int beginIndex, int endIndex):
Returns the substring. The substring starts with character at beginIndex and
ends with the character at endIndex.
23. String concat(String str): Concatenates
the specified string “str” at the end of the string.
24. String replace(char oldChar, char newChar):
It returns the new updated string after changing all the occurrences of oldChar
with the newChar.
25. boolean contains(CharSequence s): It checks whether the string
contains the specified sequence of char values. If yes then it returns true
else false. It throws NullPointerException of ‘s’ is null.
26. String replaceFirst(String regex, String replacement):
It replaces the first occurrence of substring that fits the given regular
expression “regex” with the specified replacement string.
27. String replaceAll(String regex, String replacement):
It replaces all the occurrences of substrings that fits the regular expression regex with the replacement string.
28. String[] split(String regex, int limit):
It splits the string and returns the array of substrings that matches the given
regular expression. limit is a result threshold here.
29. String[] split(String regex): Same as
split(String regex, int limit) method however it does not have any threshold
limit.
30. String toLowerCase(Locale locale): It
converts the string to lower case string using the rules defined by given
locale.
31.String toLowerCase(): Equivalent to
toLowerCase(Locale. getDefault()).
32. String toUpperCase(Locale locale):
Converts the string to upper case string using the rules defined by specified
locale.
33. String toUpperCase(): Equivalent to
toUpperCase(Locale.getDefault()).
34. String trim(): Returns the substring after
omitting leading and trailing white spaces from the original string.
35. char[] toCharArray(): Converts the string
to a character array.
36. static String copyValueOf(char[] data): It
returns a string that contains the characters of the specified character array.
37. static String copyValueOf(char[] data, int offset, int
count): Same as above method with two extra arguments – initial
offset of subarray and length of subarray.
38.static String valueOf(data type): This method returns a string
representation of specified data type.
39. byte[] getBytes(String charsetName): It
converts the String into sequence of bytes using the specified charset encoding
and returns the array of resulted bytes.
40. byte[] getBytes(): This method is similar
to the above method it just uses the default charset encoding for converting
the string into sequence of bytes.
41.int length(): It returns the length of a
String.
42. boolean matches(String regex):
It checks whether the String is matching with the specified regular expression regex.
I). Reference material for Java String Concepts: click here
II). Important String Methods and Operations : click here
