Java Strings Methods

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Index

1. [[#length|length()]]
2. [[#tolowercase|toLowerCase()]]
3. [[#touppercase|toUpperCase()]]
4. [[#charat|charAt()]]
5. [[#substring|substring()]]
6. [[#concat|concat()]]

Danger

Java String Builder

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length()

Description:
The length() method returns the length of a String object. The length is equal to the number of 16-bit Unicode characters in the string.

Syntax:

int length = string.length();

Parameters:
The length() method does not take any parameters.

Return Value:
This method returns an int value representing the number of characters in the String.

Example:

String str = "Hello, world!";
 
int len = str.length();   // 13
 

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toLowerCase()

Description:
The toLowerCase() method converts all the characters in a String to lower case.

Syntax:

String lowerCaseString = string.toLowerCase();

Parameters:
The toLowerCase() method does not take any parameters.

Return Value:
This method returns a String that represents the original string converted to lowercase.

Example:

String str = "Hello, World!";
String lowerCaseStr = str.toLowerCase();    // "hello, world!"

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toUpperCase()

Description:
The toUpperCase() method converts all the characters in a String to upper case.

Syntax:

String upperCaseString = string.toUpperCase();

Parameters:
The toUpperCase() method does not take any parameters.

Return Value:
This method returns a String that represents the original string converted to uppercase.

Example:

String str = "Hello, World!";
 
String upperCaseStr = str.toUpperCase();    // "HELLO, WORLD!"

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charAt()

Description:
The charAt() method returns the character at the specified index in a String. The index value should lie between 0 and length()-1.

Syntax:

char ch = string.charAt(index);

Parameters: The charAt() method takes one parameter:

• index (required): This is the index of the character that you want to retrieve. Indexing starts from 0.

Return Value:
This method returns the char value at the specified index of this string. The first char value is at index 0.

Example:

String str = "Hello, world!";
char ch = str.charAt(7);    // 'w'

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substring()

Description:
The substring() method returns a new string that is a substring of the original string. The substring begins with the character at the specified index and extends to the end of the string or up to endIndex - 1, if the second argument is given.

Syntax:

String sub = string.substring(beginIndex); // or
String sub = string.substring(beginIndex, endIndex);

Parameters:
The substring() method takes one or two parameters:

• beginIndex (required): This is the beginning index, inclusive.
• endIndex (optional): This is the ending index, exclusive.

Return Value:
This method returns a new String that is a substring of this string.

Example:

String str = "Hello, world!";
String subStr1 = str.substring(7);     // "world!"
String subStr2 = str.substring(0, 5);  // "Hello"
 

In subStr1, we only provide the beginIndex, so the substring includes all characters from index 7 to the end of the string.

In subStr2, we provide both beginIndex and endIndex, so the substring includes characters from index 7 up to but not including index 12.

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concat()

Description:
The concat() method concatenates the specified string to the end of this string. If the length of the argument string is 0, then the original string is returned.

Syntax:

String concatenatedString = string.concat(s);

Parameters:
The concat() method takes one parameter:

• s (required): This is the String that is concatenated to the end of this String.

Return Value:
This method returns a String that represents the concatenation of this String object and the String argument.

Example:

String str1 = "Hello, ";
String str2 = "world!";
String concatenatedStr = str1.concat(str2);    // "Hello, world!"

Differences between concat() and +

The concat() method and the + operator both concatenate strings in Java, but there are some differences:

1. Performance: For concatenating a few strings, there’s not much difference in performance between concat() and +. However, if you’re concatenating many strings in a loop, using StringBuilder or StringBuffer is more efficient.

2. Null Handling: If you use concat() and one of the strings is null, a NullPointerException will be thrown. On the other hand, if you use + and one of the strings is null, it will be treated as "null".

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indexOf()

Description:
The indexOf() method returns the index within this string of the first occurrence of the specified character. If a character with value c occurs in the character sequence represented by this String object, then the index of the first such occurrence is returned.

Syntax:

int index = string.indexOf(c);

Parameters:
The indexOf() method takes one parameter:

• c (required): This is the character for which to find the first occurrence.

Return Value:
This method returns the index of the first occurrence of the character in the character sequence, or -1 if the character does not occur.

Example:

String str = "Hello, world!";
int index = str.indexOf('w');    // 7

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replace()

Description:
The replace() method returns a new string resulting from replacing all occurrences of oldChar in this string with newChar.

Syntax:

String replacedString = string.replace(oldChar, newChar);

Parameters:
The replace() method takes two parameters:

• oldChar (required): This is the old character.
• newChar (required): This is the new character.

Return Value:
This method returns a new String derived from this String by replacing every occurrence of oldChar with newChar.

Example:

String str = "Hello, world!";
String replacedStr = str.replace('o', 'a');   // "Hella, warld!"

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equals()

Description:
The equals() method compares this string to the specified object. The result is true if and only if the argument is not null and is a String object that represents the same sequence of characters as this object.

Syntax:

boolean isEqual = string1.equals(string2);

Parameters:
The equals() method takes one parameter:

• string2 (required): This is the String to compare with string1.

Return Value:
This method returns a boolean - true if the given object represents a String equivalent to this string, false otherwise.

Example:

String str1 = "Hello, world!";
String str2 = "Hello, world!";
boolean isEqual = str1.equals(str2);    // true

Differences between equals() and ==

In Java, equals() and == operator are used for comparison. However, they work differently when it comes to objects like String.

• equals(): This method compares the “content” of the strings. If two strings contain the same characters in the same order, equals() will return true.

• == operator: This compares the “references” of the strings, not their content. It checks if both strings point to the same object in memory. So, even if two strings contain exactly the same characters, == will return false if they are not the same object in memory.

Therefore, when comparing String values in Java, it’s generally recommended to use equals(), not ==.

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