String in Java

String in Java

Java String

In java, string is basically an object that represents sequence of char values. An array of characters works same as java string. For example :

char[] ch={'j','a','v','a','t','p','o','i','n','t'};  
String s=new String(ch);
is same as:
String s="javatpoint";

Java String class provides a lot of methods to perform operations on string such as compare(), concat(), equals(), split(), length(), replace(), compareTo(), intern(), substring() etc.

The java.lang.String class implements Serializable, Comparable and CharSequence interfaces.

CharSequence Interface

The CharSequence interface is used to represent sequence of characters. It is implemented by String, StringBuffer and StringBuilder classes. It means, we can create string in java by using these 3 classes.

The java String is immutable i.e. it cannot be changed. Whenever we change any string, a new instance is created. For mutable string, you can use StringBuffer and StringBuilder classes.

We will discuss about immutable string later. Let's first understand what is string in java and how to create the string object.

What is String in java

Generally, string is a sequence of characters. But in java, string is an object that represents a sequence of characters. The java.lang.String class is used to create string object.

How to create String object?

There are two ways to create String object:

By string literal
By new keyword

1) String Literal

Java String literal is created by using double quotes. For Example:

String s="welcome";

Each time you create a string literal, the JVM checks the string constant pool first. If the string already exists in the pool, a reference to the pooled instance is returned. If string doesn't exist in the pool, a new string instance is created and placed in the pool. For example:

String s1="Welcome";  
String s2="Welcome";//will not create new instance

In the above example only one object will be created. Firstly JVM will not find any string object with the value "Welcome" in string constant pool, so it will create a new object. After that it will find the string with the value "Welcome" in the pool, it will not create new object but will return the reference to the same instance.

Note : String objects are stored in a special memory area known as string constant pool.

Why java uses concept of string literal?

To make Java more memory efficient (because no new objects are created if it exists already in string constant pool).

2) By new keyword

String s=new String("Welcome");//creates two objects and one reference variable

In such case, JVM will create a new string object in normal(non pool) heap memory and the literal "Welcome" will be placed in the string constant pool. The variable s will refer to the object in heap(non pool).

Java String Example

public class StringExample
     public static void main(String args[])
          String s1="java";//creating string by java string literal  
          char ch[]={'s','t','r','i','n','g','s'};  
          String s2=new String(ch);//converting char array to string  
          String s3=new String("example");//creating java string by new keyword  

Java String class methods

The java.lang.String class provides many useful methods to perform operations on sequence of char values.

1char charAt(int index)returns char value for the particular index
2int length()returns string length
3static String format(String format, Object... args)returns formatted string
4static String format(Locale l, String format, Object... args)returns formatted string with given locale
5String substring(int beginIndex)returns substring for given begin index
6String substring(int beginIndex, int endIndex)returns substring for given begin index and end index
7boolean contains(CharSequence s)returns true or false after matching the sequence of char value
8static String join(CharSequence delimiter, CharSequence... elements)returns a joined string
9static String join(CharSequence delimiter, Iterable elements)returns a joined string
10boolean equals(Object another)checks the equality of string with object
11boolean isEmpty()checks if string is empty
12String concat(String str)concatinates specified string
13String replace(char old, char new)replaces all occurrences of specified char value
14String replace(CharSequence old, CharSequence new)replaces all occurrences of specified CharSequence
15static String equalsIgnoreCase(String another)compares another string. It doesn't check case.
16String[] split(String regex)returns splitted string matching regex
17String[] split(String regex, int limit)returns splitted string matching regex and limit
18String intern()returns interned string
19int indexOf(int ch)returns specified char value index
20int indexOf(int ch, int fromIndex)returns specified char value index starting with given index
21int indexOf(String substring)returns specified substring index
22int indexOf(String substring, int fromIndex)returns specified substring index starting with given index
23String toLowerCase()returns string in lowercase.
24String toLowerCase(Locale l)returns string in lowercase using specified locale.
25String toUpperCase()returns string in uppercase.
26String toUpperCase(Locale l)returns string in uppercase using specified locale.
27String trim()removes beginning and ending spaces of this string.
28static String valueOf(int value)converts given type into string. It is overloaded.