Master Core Java Programming From Scratch

What is a String?

A String represents a sequence of characters. In Java, String is a class provided by the java.lang package.

Strings are objects, even though Java provides special syntax for creating them using string literals.

String name = "CIIT";

System.out.println(name);

Creating Strings

Strings are commonly created using string literals.

String language = "Java";

String message = "Welcome CIIT world 🌍..!";

A String can also be explicitly created using the constructor, although literals are normally preferred.

String text = new String("Java");

String Immutability

Strings in Java are immutable. Once a String object is created, its character contents cannot be changed.

String text = "CIIT";

text = text + " Institute";

System.out.println(text);

The original String object is not modified. A new String object is created for the concatenated result, and the variable is updated to reference it.

String Pool

Java maintains a special pool for string literals. Identical literals can share the same pooled String object.

String first = "CIIT";

String second = "Institute";

System.out.println(first == second);

true

The == comparison here checks whether the two variables refer to the same object.

Comparing Strings with equals()

Use equals() when you want to compare String contents.

String first = "CIIT";

String second = new String("Institute");

System.out.println(first.equals(second));

true

Case-Insensitive Comparison

String first = "ciit";

String second = "INSTITUTE";

System.out.println(
    first.equalsIgnoreCase(second)
);

true

String length()

The length() method returns the number of UTF-16 code units in the String.

String text = "CIIT";

System.out.println(text.length());

4

charAt()

The charAt() method returns the character at a specified zero-based index.

String text = "Java";

System.out.println(text.charAt(0));

System.out.println(text.charAt(2));

J
v

substring()

The substring() method extracts part of a String. The ending index is exclusive.

String text = "Programming";

System.out.println(
    text.substring(0, 4)
);

System.out.println(
    text.substring(4)
);

Prog
ramming

contains()

String text = "Java Programming";

System.out.println(
    text.contains("Java")
);

System.out.println(
    text.contains("Python")
);

true
false

startsWith() and endsWith()

String file = "report.pdf";

System.out.println(
    file.startsWith("report")
);

System.out.println(
    file.endsWith(".pdf")
);

true
true

indexOf() and lastIndexOf()

String text = "Java Programming";

System.out.println(
    text.indexOf("a")
);

System.out.println(
    text.lastIndexOf("a")
);

replace()

Since Strings are immutable, replace() returns a new String instead of modifying the original.

String text = "Java is easy";

String updated =
    text.replace("easy", "powerful");

System.out.println(updated);

Java is powerful

toLowerCase() and toUpperCase()

String text = "Java Programming";

System.out.println(
    text.toLowerCase()
);

System.out.println(
    text.toUpperCase()
);

java programming
JAVA PROGRAMMING

trim() and strip()

trim() removes certain leading and trailing characters with code points at or below U+0020. Modern Java also provides strip(), which uses Unicode-aware whitespace rules.

String text = "   Java   ";

System.out.println(
    text.trim()
);

System.out.println(
    text.strip()
);

String Concatenation

Strings can be combined using the + operator.

String firstName = "Samadhan";

String lastName = "Patole";

String fullName =
    firstName + " " + lastName;

System.out.println(fullName);

concat()

String first = "CIIT";

String second = " Institute";

String result = first.concat(second);

System.out.println(result);

split()

The split() method divides a String according to a regular expression.

String languages =
    "Java,Python,CSharp";

String[] values =
    languages.split(",");

for (String value : values) {

    System.out.println(value);

}

Java
Python
CSharp

String.join()

String.join() can combine multiple strings using a delimiter.

String result = String.join(
    ", ",
    "Java",
    "Python",
    "CSharp"
);

System.out.println(result);

Java, Python, CSharp

String Formatting

String.format() creates formatted text using placeholders.

String name = "Amit";

int age = 25;

String result = String.format(
    "Name: %s, Age: %d",
    name,
    age
);

System.out.println(result);

Name: Amit, Age: 25

Text Blocks

Modern Java supports text blocks for convenient representation of multiline text.

String message = """
        Hello
        Welcome to Java
        Programming
        """;

StringBuilder

When text needs to be modified repeatedly, use StringBuilder instead of creating many temporary String objects.

StringBuilder builder =
    new StringBuilder();

builder.append("Java");
builder.append(" ");
builder.append("Programming");

System.out.println(
    builder.toString()
);

Java Programming

StringBuffer

StringBuffer is another mutable character sequence. Its methods are synchronized, making it different from StringBuilder.

StringBuffer buffer =
    new StringBuffer("Java");

buffer.append(" Programming");

System.out.println(buffer);

String vs StringBuilder vs StringBuffer

Checking Empty and Blank Strings

String text = "";

System.out.println(text.isEmpty());

String value = "   ";

System.out.println(value.isBlank());

true
true

isEmpty() checks whether the String has zero characters. isBlank() also considers strings containing only whitespace to be blank.

String and null

A String variable can contain null, meaning it does not currently reference a String object.

String text = null;

if (text != null) {

    System.out.println(text.length());

}

String and Unicode

Java Strings use UTF-16 internally. Characters outside the Basic Multilingual Plane may require a surrogate pair, so char is not always equivalent to one Unicode code point.

String text = "Hello";

System.out.println(
    text.codePointAt(0)
);

Example: Username Validation

String username = "  Sam2210  ";

username = username.strip();

if (!username.isBlank()
        && username.length() >= 5) {

    System.out.println(
        "Valid username"
    );

} else {

    System.out.println(
        "Invalid username"
    );

}

String Best Practices

• Use equals() for content comparison.
• Avoid using == when your intention is to compare String contents.
• Remember that Strings are immutable.
• Use StringBuilder for repeated local string modifications.
• Handle possible null references safely.
• Use strip() when Unicode-aware whitespace handling is desired.

Interview Questions