String Handling in Java — ICSE Class 10 Computer Applications

class S1
{
    public static void main(String args[])
    {
        String s = "  Principal Saab  ";
        System.out.println("[" + s + "]");
        System.out.println("[" + s.trim() + "]");
        System.out.println("Length with spaces = " + s.length());
        System.out.println("Length trimmed     = " + s.trim().length());
        System.out.println(s.trim().toUpperCase());
        System.out.println(s.trim().toLowerCase());
    }
}

Output

[  Principal Saab  ]
[Principal Saab]
Length with spaces = 18
Length trimmed     = 14
PRINCIPAL SAAB
principal saab

Square brackets isliye lagaye taaki khaali jagah dikh sake. trim() ne sirf aage-peeche ki jagah hatai — beech wali jagah waisi hi rahi, isliye trimmed length 14 hai (13 akshar + 1 beech ki jagah). Aur dhyaan dijiye s.trim().length() — ek method ka jawaab par doosra method lagana bilkul jaayaz hai, ise chaining kehte hain.

class S2
{
    public static void main(String args[])
    {
        String s = "COMPUTER";
        System.out.println("charAt(0)     = " + s.charAt(0));
        System.out.println("charAt(7)     = " + s.charAt(7));
        System.out.println("indexOf('P')  = " + s.indexOf('P'));
        System.out.println("indexOf('Z')  = " + s.indexOf('Z'));
        System.out.println("lastIndexOf('U') = " + s.lastIndexOf('U'));
        System.out.println("substring(3)  = " + s.substring(3));
        System.out.println("substring(0,4)= " + s.substring(0, 4));
        System.out.println("replace       = " + s.replace('O', '0'));
        System.out.println("concat        = " + s.concat(" SCIENCE"));
    }
}

Output

charAt(0)     = C
charAt(7)     = R
indexOf('P')  = 3
indexOf('Z')  = -1
lastIndexOf('U') = 4
substring(3)  = PUTER
substring(0,4)= COMP
replace       = C0MPUTER
concat        = COMPUTER SCIENCE

Teen baatein yaad rakhne layak hain. indexOf jab kuch na mile toh -1 lautata hai — isiliye “mila ya nahi” wale sawaalon mein == -1 se jaanch ki jaati hai. substring(3) index 3 se aakhir tak deta hai. Aur substring(0, 4) index 0 se shuru karke 4 se pehle tak deta hai — yaani 4 shaamil nahi hai.

class S7
{
    public static void main(String args[])
    {
        int n = 250;
        double d = 3.75;
        char c = 'A';
        boolean b = true;

        String s1 = String.valueOf(n);
        String s2 = String.valueOf(d);
        String s3 = String.valueOf(c);
        String s4 = String.valueOf(b);

        System.out.println("From int     : " + s1 + " , length = " + s1.length());
        System.out.println("From double  : " + s2 + " , length = " + s2.length());
        System.out.println("From char    : " + s3 + " , length = " + s3.length());
        System.out.println("From boolean : " + s4 + " , length = " + s4.length());
        System.out.println("Now a String : " + (s1 + 10));
        System.out.println("Back to int  : " + (Integer.parseInt(s1) + 10));
    }
}

Output

From int     : 250 , length = 3
From double  : 3.75 , length = 4
From char    : A , length = 1
From boolean : true , length = 4
Now a String : 25010
Back to int  : 260

Saboot aakhri do lines mein hai. s1 ab ek String hai, isliye s1 + 10 ne jod nahi kiya, chipka diya → 25010. Aur Integer.parseInt(s1) + 10 ne asli jod kiya → 260. Dhyaan dijiye ki valueOf par length() lagaya ja sakta hai — yani jawaab sach mein String hai. Yeh tareeka tab kaam aata hai jab kisi number ke ank ginne hon.

class S3
{
    public static void main(String args[])
    {
        String a = "Java";
        String b = "java";
        String c = "Java";

        System.out.println("a.equals(b)           = " + a.equals(b));
        System.out.println("a.equals(c)           = " + a.equals(c));
        System.out.println("a.equalsIgnoreCase(b) = " + a.equalsIgnoreCase(b));
        System.out.println("a.compareTo(b)        = " + a.compareTo(b));
        System.out.println("a.compareTo(c)        = " + a.compareTo(c));
        System.out.println("a.compareToIgnoreCase(b) = " + a.compareToIgnoreCase(b));
        System.out.println("a.startsWith(\"Ja\")    = " + a.startsWith("Ja"));
        System.out.println("a.endsWith(\"va\")      = " + a.endsWith("va"));
    }
}

Output

a.equals(b)           = false
a.equals(c)           = true
a.equalsIgnoreCase(b) = true
a.compareTo(b)        = -32
a.compareTo(c)        = 0
a.compareToIgnoreCase(b) = 0
a.startsWith("Ja")    = true
a.endsWith("va")      = true

-32 kahan se aaya? compareTo pehle alag padne wale character ki Unicode values ka antar deta hai. Yahan pehla hi character alag hai: 'J' ki value 74 aur 'j' ki 106 — toh 74 − 106 = −32. Isliye jawaab rinaatmak hai, jo batata hai ki "Java" varnamala mein "java" se pehle aata hai (capital letters ki value chhoti hoti hai).

class S4
{
    public static void main(String args[])
    {
        String s = "Principal Saab 2026";
        int v = 0, cons = 0, dig = 0, sp = 0;

        for (int i = 0; i < s.length(); i++)
        {
            char c = Character.toLowerCase(s.charAt(i));
            if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u')
                v++;
            else if (Character.isLetter(c))
                cons++;
            else if (Character.isDigit(c))
                dig++;
            else if (c == ' ')
                sp++;
        }
        System.out.println("Vowels     = " + v);
        System.out.println("Consonants = " + cons);
        System.out.println("Digits     = " + dig);
        System.out.println("Spaces     = " + sp);
    }
}

Output

Vowels     = 5
Consonants = 8
Digits     = 4
Spaces     = 2

Do chaalaakiyaan pakadiye. Pehli, Character.toLowerCase() shuru mein hi laga diya, isliye capital aur small dono ek saath sambhal gaye — das conditions ki jagah paanch. Doosri, else if ka kram — vowel ki jaanch pehle hui, isliye jo bacha aur akshar tha wahi consonant gina gaya. Agar isLetter pehle likh dete toh vowels bhi consonants mein gine jaate.

class S5
{
    public static void main(String args[])
    {
        String s = "MALAYALAM";
        String rev = "";

        for (int i = s.length() - 1; i >= 0; i--)
            rev = rev + s.charAt(i);

        System.out.println("Original = " + s);
        System.out.println("Reversed = " + rev);
        if (s.equals(rev))
            System.out.println(s + " is a palindrome");
        else
            System.out.println(s + " is not a palindrome");
    }
}

Output

Original = MALAYALAM
Reversed = MALAYALAM
MALAYALAM is a palindrome

Loop s.length() - 1 se shuru hokar 0 tak ulta chala. rev ko "" (khaali String) se shuru karna zaroori hai — null se shuru karenge toh output mein null chhap jaayega. Aur milane ke liye equals() hi lagaya, == nahi.

class S6
{
    public static void main(String args[])
    {
        String names[] = {"Meera", "Aarav", "Zoya", "Kabir"};

        for (int i = 0; i < names.length - 1; i++)
        {
            for (int j = 0; j < names.length - 1 - i; j++)
            {
                if (names[j].compareTo(names[j + 1]) > 0)
                {
                    String t = names[j];
                    names[j] = names[j + 1];
                    names[j + 1] = t;
                }
            }
        }
        for (int i = 0; i < names.length; i++)
            System.out.println(names[i]);
    }
}

Output

Aarav
Kabir
Meera
Zoya

Poora dhaancha bubble sort ka hi hai. Sirf ek line badli: if (a[j] > a[j+1]) ki jagah if (names[j].compareTo(names[j+1]) > 0). Yaad rakhiye — compareTo ka jawaab 0 se bada hone ka matlab hai ki pehla naam baad mein aata hai, isliye unhein badalna hai.