Variables, Scope, Type Conversion, Arrays & Strings in Java

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1️⃣ Variables in Java (Storage Units)

A variable is a named storage location in memory.

In Java, a variable is defined by five things:

Type + Name + Value + Scope + Lifetime

int x = 10;

• int → Data type

• x → Variable name

• 10 → Value

• Scope → Where it can be accessed

• Lifetime → How long it exists in memory

Because Java is strongly typed, every variable must have a declared type.

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2️⃣ Scope & Lifetime (VERY IMPORTANT FOR INTERVIEWS)

Scope Rule

👉 A variable exists only inside its block {}

{
    int x = 10;
}
x = 20; // ❌ Compile-time error

Once the block ends, the variable is destroyed.

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Nested Scope Rule

• Inner scope CAN access outer variables

• Outer scope CANNOT access inner variables

int a = 10;
{
    int b = 20;
    System.out.println(a); // ✔
}
System.out.println(b); // ❌

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Variable Lifetime

• Created → when block starts

• Destroyed → when block ends

for (int i = 0; i < 3; i++) {
    int y = -1;
}

👉 y is recreated and destroyed on every iteration.

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3️⃣ Type Conversion & Casting

Java supports two types of conversions.

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Widening Conversion (Automatic) ✅

Smaller type → Larger type

int → long → float → double

int a = 10;
double d = a; // ✔

✔ Safe
✔ No data loss

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Narrowing Conversion (Manual Cast) ❌

Larger type → Smaller type

int x = (int) 10.5;

⚠️ Fractional part is lost (truncation)

Result:

10

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4️⃣ Type Promotion (CONFUSING BUT CRITICAL)

Rule:

👉 byte, short and char are automatically promoted to int in expressions.

byte a = 50;
byte b = 2;
byte c = (byte)(a * b);

Why casting is required?

• a * b → becomes int

• Java does not assign int to byte automatically

💡 This is a classic Java interview trap.

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5️⃣ Arrays in Java (MOST USED)

What Is an Array?

👉 A collection of same-type values
👉 Stored contiguously in memory

int[] arr = new int[5];

• Index starts from 0

• Size is fixed

arr[0] = 10;

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Array Initialization

int[] days = {31, 28, 31, 30};

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Runtime Safety 🔥

Java checks array bounds at runtime:

arr[10]; // ❌ ArrayIndexOutOfBoundsException

👉 Prevents memory corruption
👉 Makes Java safer than C/C++

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6️⃣ Multidimensional Arrays

Java uses arrays of arrays, not true matrices.

int[][] matrix = new int[3][4];

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Jagged (Irregular) Arrays

int[][] arr = new int[3][];

Each row can have a different size.

📌 Used for:

• Sparse data

• Memory optimization

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7️⃣ var – Type Inference (Java 10+)

var x = 10;    // int
var d = 10.5; // double

Type is inferred at compile time.

Rules:

✔ Only for local variables
✔ Must be initialized

❌ Not allowed for fields
❌ Not allowed for method parameters
❌ Cannot assign null

var x = null; // ❌

💡 var improves readability, not dynamic typing.

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8️⃣ Strings (Introduction Only)

String s = "Java";

Important facts:

• String is an object

• Immutable

• Stored in String Constant Pool

• Rich API with many methods

📌 Covered later in depth:

• String pool

• Immutability

• == vs equals()

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🎓 Final Mentor Summary

• Variables live only within their scope

• Java automatically manages variable lifetime

• Widening conversions are safe, narrowing needs casting

• Type promotion can cause hidden bugs

• Arrays are safe but fixed-size

• var is compile-time inference

• Strings are objects, not primitives