Java Basics Interview Questions (JDK vs JRE vs JVM, Data Types, String & Coding)

1. Difference between JDK, JRE, and JVM

JDK (Java Development Kit)

• Full package for Java development
• Includes JRE + development tools (compiler, debugger)
• Used to write and compile Java programs

JRE (Java Runtime Environment)

• Provides environment to run Java programs
• Includes JVM + libraries
• Used to execute Java programs

JVM (Java Virtual Machine)

• Executes Java bytecode
• Converts bytecode into machine code
• Platform independent

Flow:

Java Code → Compiled by JDK → Bytecode → Executed by JVM (inside JRE)

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2. Data Types in Java (Primitive vs Non-Primitive)

Primitive Data Types

• int, float, char, boolean, byte, short, long, double
• Store actual values
• Fixed size
• Faster performance

Example:

int a = 10;
char c = 'A';
boolean flag = true;

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Non-Primitive Data Types

• String, Array, Class, Object
• Store reference (address)
• Can be null
• More flexible

Example:

String name = "Java";
int[] arr = {1, 2, 3};

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3. String vs StringBuilder vs StringBuffer

String

• Immutable (cannot change)
• Thread-safe
• Slower when modifying

Example:

String s = "Hello";
s = s + " World";

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StringBuilder

• Mutable
• Not thread-safe
• Faster

Example:

StringBuilder sb = new StringBuilder("Hello");
sb.append(" World");

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StringBuffer

• Mutable
• Thread-safe
• Slower than StringBuilder

Example:

StringBuffer sb = new StringBuffer("Hello");
sb.append(" World");

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Key Difference Table:

Feature	String	StringBuilder	StringBuffer
Mutability	Immutable	Mutable	Mutable
Thread Safe	Yes	No	Yes
Performance	Slow	Fast	Medium

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4.Coding Questions

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i. Reverse a String

Logic:

• Convert string to char array
• Swap characters from start and end

Code:

public class ReverseString {
    public static void main(String[] args) {
        String str = "hello";
        String reversed = "";

        for (int i = str.length() - 1; i >= 0; i--) {
            reversed += str.charAt(i);
        }

        System.out.println(reversed);
    }
}

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ii. Check Palindrome String

Logic:

• Reverse the string
• Compare with original

Code:

public class Palindrome {
    public static void main(String[] args) {
        String str = "madam";
        String reversed = "";

        for (int i = str.length() - 1; i >= 0; i--) {
            reversed += str.charAt(i);
        }

        if (str.equals(reversed)) {
            System.out.println("Palindrome");
        } else {
            System.out.println("Not Palindrome");
        }
    }
}

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