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Tuesday, May 2, 2017

String Class and its methods

1 - Java  strings class

A string is any sequence of contiguous characters, for example, "hello" is a string of length 5 (length of string = Number of characters in the string). A string can be considered as a variable in Java language and is declared to Using the String statement.
String  myString = "Hello" ;
// declaration of a String variable named myString and having the value = "Hello"
More generally, a string is an object associated with the String class

Example

String myString = new String("Hello");
//create the object myString of type String

Example (String concatenation)

String auto="auto";
String bus="bus";
System.out.println(auto+bus);  // Which display :  "autobus "

2 – The methods associated with the String class

A string is an object of type String, and since a java object has a number of methods and properties, a String object is itself equipped with a lot of methods allowing to do many operations: calculate the length of a string, test whether a string contains a character or a sub-string, extract a sub-string ... Here is a collection of these methods:

char charAt(int index)

Returns the character that is at index. (The first character is at index 0.)
This method returns an exception of type IndexOutOfBoundsException.

int compareTo(String chaine_a_comparer)

Compares two String alphabetically.

int compareToIgnoreCase(String chaine)

Compares two String alphabetically, ignoring case.

boolean contains(char s)

Returns true only if the String contains the same sequence of characters.
This method returns an exception of type NullPointerException.

boolean startsWith(String prefix)

Returns true if the String starts with prefix.

boolean endsWith(String suffixe)

Returns true if the String ends with a suffix.

boolean equals(Object other_obj)

Compare the String with other_obj.

int indexOf(int ch [, int index])

Returns the index of the first occurrence of the character ch.
If index is specified, the search starts from this index.

int indexOf(String s [, int index])

Returns the index of the first occurrence of the String s.
If index is specified, the search starts from this index.

int lastIndexOf(int ch [, int fromIndex])

Returns the index of the last occurrence of the character ch.
If index is specified, the search starts from this index.

int lastIndexOf(String str [, int fromIndex])

Returns the index of the last occurrence of the String s.
If fromIndex is specified, the search starts from this index.

int length()

Returns the size of the String.

boolean isEmpty()

Returns true if the size of the String is 0.

boolean matches(String regex)

Returns true if the String matches the regular regex expression.
This method returns an exception of type PatternSyntaxException.

String concat(String s)

Concatenates the string s at the end of the String that calls the method.

String replace(char old, char new)

Replaces all occurrences of old with new, and then returns the modified string.

String replaceAll(String regex, String new)

Replaces all occurrences corresponding to the regular regex expression with new, and then returns the modified string.
This method returns an exception of type PatternSyntaxException.

String replaceFirst(String regex, String new)

Replaces the first occurrence corresponding to the regular regex expression with new, and then returns the modified string.
This method returns an exception of type PatternSyntaxException.

String[] split(String regex)

Trim the String using the regular regex expression.
This method returns an exception of type PatternSyntaxException.

String substring(int start)

Returns a new String that contains the characters between the start index and the end of the String.
This method returns an exception of type IndexOutOfBoundsException.

String substring(int start, int fin)

Returns a new String that contains the characters between the start index and the end index.
This method returns an exception of type IndexOutOfBoundsException.

String toLowerCase()

Converts all characters to lowercase.

String toUpperCase()

Converts all characters to uppercase.

String trim()

Returns the String without spaces at the beginning and end.

Example

String b="Hello";
String b1=b.Substring(0,3);
//This example extracts from the string b a sub-string b1 = "Hel"

Example

String s1 = "Antihypertenseur";
String s2 =s1.substring(4,9);
System.out.println(s2);
// Returns the sub-string "hyper"

Example

String s="Hello";
if (s.startsWith("Hel")){
System.out.println("The String s start with  \"Hel\" ");
     }
else {
System.out.println("The  string s does not start with \"Hel\" ");   
    }
//Displays on screen the message: 'The string s starts with "Hel"

Example

String s1 = "start";
String s2 = "arrival";
if (s1.endsWith("rt")){
        System.out.println("The string s1 ends with \ "rt \" ");
    }
    else {
        System.out.println("The string s1 does not end with \"rt\" ");   
    }
// Displays the message: 'The string s1 ends with "rt" '

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