Java Strings

Agenda

  • String class & objects

  • String methods

  • String using new

  • String pool

  • Extracting Characters

  • String Builder

  • char arrays vs Strings

The String class represents character strings. All string literals in Java programs, such as "abc", are implemented as instances of this class.

Strings in Java are immutable.

String class

class Main {
    public static void main(String[] args) {
        String str = "abc";
    }
}

The String pool is the area in the heap memory where string literals are stored

String pool

String pool

String s1 = "abc";

String pool

String pool

String s1 = "abc";

String pool

"abc"

String pool

String s1 = "abc";

String pool

"abc"

s1

String pool

String s1 = "abc";
String s2 = "abc";

String pool

"abc"

s1

String pool

String pool

"abc"

s1

s2

String s1 = "abc";
String s2 = "abc";

String pool

String pool

"abc"

s1

s2

String s1 = "abc";
String s2 = "abc";

String pool

String s1 = "abc";
String s2 = "abc";
String s3 = "def";

String pool

"abc"

s1

s2

String pool

String pool

"abc"

s1

s2

"def"

s3

String s1 = "abc";
String s2 = "abc";
String s3 = "def";

String pool

String s1 = "abc";
String s2 = "abc";
String s3 = "def";
s2 = "hello world";

String pool

"abc"

s1

s2

"def"

s3

String pool

String pool

"abc"

s1

s2

"def"

s3

"hello world"

String s1 = "abc";
String s2 = "abc";
String s3 = "def";
s2 = "hello world";

String pool

String pool

"abc"

s1

s2

"def"

s3

"hello world"

String s1 = "abc";
String s2 = "abc";
String s3 = "def";
s2 = "hello world";

String pool

String pool

"abc"

s1

s2

"def"

s3

"hello world"

Heap memory

String s1 = "abc";
String s2 = "abc";
String s3 = "def";
s2 = "hello world";

We can also create new strings using the new keyword. These strings do not reside in the String pool, instead they are created and stored in the heap memory.

Strings using new

class Main {
	public static void main(String args[]) {
		String s = "abc";
		String s2 = new String("abc");


		if (s == s2) {
			System.out.println("This will not work");
		}

		if (s.equals(s2)) {
			System.out.println("This will work");
		}

	}
}

String pool

String s1 = "abc";
String s2 = "abc";
String s3 = "def";
s2 = "hello world";

String pool

"abc"

s1

s2

"def"

s3

"hello world"

Heap memory

String pool

String s1 = "abc";
String s2 = "abc";
String s3 = "def";
s2 = "hello world";
String s4 = new String("abc");

String pool

"abc"

s1

s2

"def"

s3

"hello world"

Heap memory

String pool

String s1 = "abc";
String s2 = "abc";
String s3 = "def";
s2 = "hello world";
String s4 = new String("abc");

String pool

"abc"

s1

s2

"def"

s3

"hello world"

Heap memory

s4

String pool

String s1 = "abc";
String s2 = "abc";
String s3 = "def";
s2 = "hello world";
String s4 = new String("abc");

String pool

"abc"

s1

s2

"def"

s3

"hello world"

Heap memory

s4

"abc"

String

String class methods

String class offers several methods out of the box. We will be looking at some of them in this section.

charAt method allows us to access the character at the specified index.

charAt

String str = "Hello World";
System.out.println(str.charAt(3));  // l
System.out.println(str.charAt(8));  // r

The length() method returns the length of the string.

length

class Main {
    public static void main(String[] args) {
        String str1 = "abc";
        System.out.println(str1.length());  // 3

        String str2 = "Hello World";
        System.out.println(str2.length());  // 11
    }
}

indexOf methods returns the index of the first occurrence of the specified char or string in the given string.

If it is present, then it returns -1.

indexOf


class Main {
    public static void main(String[] args) {
        String str = "Hello World";
        System.out.println(str.indexOf('W'));   // 6
        System.out.println(str.indexOf("ll"));  // 2
        System.out.println(str.indexOf("abc")); // -1
    }
}

Equals method is used to compare whether the two strings contain the same sequence of characters.

equals

class Main {
    public static void main(String[] args) {
        String str = "Hello World";
        String str2 = new String("Hello World");
        System.out.println(str.equals(str2));   // true
        System.out.println(str == str2);        // false
    }
}

Returns true if and only if this string contains the specified sequence of char values

contains


class Main {
    public static void main(String[] args) {
        String str = "Hello World";
        System.out.println(str.contains("ll"));     // true
        System.out.println(str.contains("abc"));    // false
    }
}

Returns new strings after changing the case.

Original string remains intact.

toLowerCase, toUpperCase

class Main {
    public static void main(String[] args) {
        String str = "Hello World";
        System.out.println(str.toLowerCase());  // hello world
        System.out.println(str.toUpperCase());  // HELLO WORLD
        System.out.println(str);                // Hello World
    }
}

Replaces the target string (first) with the given replacement string (second) and returns a new string.

replace

class Main {
    public static void main(String[] args) {
        String str = "I love Programming";
        String str2 = str.replace("Programming", "Java");
        
        System.out.println(str);        // I love Programming
        System.out.println(str2);       // I love Java
    }
}

Returns a string that is a substring of this string. The substring begins at the specified beginIndex and extends to the character at index endIndex - 1. Thus the length of the substring is endIndex-beginIndex.

 

If endIndex is not given, it is taken as string length.

substring

class Main {
    public static void main(String[] args) {
        String str = "I love Programming";
        
        // Start at index=7 and extract till the end
        String sub = str.substring(7);      
        System.out.println(sub);            // "Programming"
        
        // Start at index=2 and extract chars till index<6
        String sub2 = str.substring(2, 6);  
        System.out.println(sub2);           // "love"
    }
}
  • Concatenation is the process of appending one string to the end of another string.
  • Java strings can be added by using the '+' operator.
  • When a string is concatenated with primitive types, they are automatically typecasted to strings.
  • When a string is concatenated with non-primitive types, the toString() on that object is called automatically.

String concatenation

class Main {
    public static void main(String[] args) {
        String str1 = "Hello";
        String str2 = "World";

        String str3 = str1 + str2;
        System.out.println(str3);
    }
}

Reverse a string

Given a string as input, print its reverse

 

Sample Input

hello

 

Sample Output

olleh

Java StringBuilder class is used to create mutable (modifiable) String.

StringBuilder

class Main {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("Hello");
        sb.append(" World");
        System.out.println(sb);   // Hello World

        sb.insert(5, '_');
        System.out.println(sb);   // Hello_ World
        
        sb.replace(5, sb.length(), " Java");
        System.out.println(sb);   // Hello Java
    }
}

Is it Palindrome?

Given a string, check whether it is a palindrome.

 

Sample Input

tenet

 

Sample Output

Yes

Is it Palindrome?

Given a string, check whether it is a palindrome.

 

Sample Input

tenet

 

Sample Output

Yes

class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str = sc.next();
    }
}

Is it Palindrome?

Given a string, check whether it is a palindrome.

 

Sample Input

tenet

 

Sample Output

Yes

class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str = sc.next();

        StringBuilder temp = new StringBuilder(str);
        temp.reverse();

    }
}

Is it Palindrome?

Given a string, check whether it is a palindrome.

 

Sample Input

tenet

 

Sample Output

Yes

class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str = sc.next();

        StringBuilder temp = new StringBuilder(str);
        temp.reverse();
        
        String reverse = temp.toString();

    }
}

Is it Palindrome?

Given a string, check whether it is a palindrome.

 

Sample Input

tenet

 

Sample Output

Yes

class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str = sc.next();

        StringBuilder temp = new StringBuilder(str);
        temp.reverse();

        String reverse = temp.toString();

        if (str.equals(reverse)) {
            System.out.println("Yes");
        } else {
            System.out.println("No");
        }
    }
}

Both Strings and character arrays allows us to store and handle sequences of characters. But they are different in some properties.

Strings and char arrays

class Main {
    public static void main(String[] args) {
        String str = "hello";
        char[] arr = {'h', 'e', 'l', 'l', 'o'};

        System.out.println(str);
        System.out.println(arr);

        String str2 = new String(arr);
        System.out.println(str2);

        char[] arr2 = str.toCharArray();
        System.out.println(arr2);
    }
}

Strings vs Char arrays

Strings Character Arrays
Sequence of chars represented as a single data type. Sequential collection of char data type.
Immutable Mutable
Offers several builtin methods. No builtin methods
Stored in the String pool Stored in the Heap
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