Revise introduction to Java Booleans

concepts/booleans/introduction.md

@@ -1,20 +1,224 @@
-# Introduction
+# Java Booleans – Complete Beginner Guide
 
-Booleans in Java are represented by the `boolean` type, which values can be either `true` or `false`.
+## What Are Booleans?
 
-Java supports three boolean operators:  
+A **boolean** in Java can store only two values:
 
-- `!` (NOT): negates the boolean  
-- `&&` (AND): takes two booleans and results in true if they're both true  
-- `||` (OR): results in true if any of the two booleans is true  
+* `true`
+* `false`
 
-## Examples
+Booleans are widely used in **conditions, loops, decisions, validations, and logical operations**.
+
+---
+
+## Boolean Operators
+
+Java supports three main boolean operators:
+
+### NOT (`!`)
+
+Reverses the value.
+
+```
+!true  → false
+!false → true
+```
+
+### AND (`&&`)
+
+True only if **both** values are true.
+
+```
+true && true   → true
+true && false  → false
+false && true  → false
+false && false → false
+```
+
+### OR (`||`)
+
+True if **any one** value is true.
+
+```
+true || false  → true
+false || true  → true
+false || false → false
+```
+
+---
+
+## Truth Tables
+
+### AND (&&)
+
+| A | B | A && B |
+| - | - | ------ |
+| T | T | T      |
+| T | F | F      |
+| F | T | F      |
+| F | F | F      |
+
+### OR (||)
+
+| A | B | A || B |
+|---|---|---------|
+| T | T | T       |
+| T | F | T       |
+| F | T | T       |
+| F | F | F       |
+
+### NOT (!)
+
+| A | !A |
+| - | -- |
+| T | F  |
+| F | T  |
+
+---
+
+## Operator Precedence
+
+Java evaluates operators in this order:
+
+1. Parentheses `()`
+2. NOT `!`
+3. AND `&&`
+4. OR `||`
+
+Example:
+
+```
+true || true && false
+```
+
+Steps:
+
+* true && false → false
+* true || false → true
+
+Final result → `true`
+
+To avoid confusion:
 
-```java
-!true // => false
-!false // => true
-true && false // => false
-true && true // => true
-false || false // => false
-false || true // => true
 ```
+true || (true && false)
+```
+
+---
+
+## Short-Circuit Evaluation
+
+Java stops evaluation early if the final result is already known.
+
+### AND (&&)
+
+If first operand is false:
+
+```
+false && something → second part not checked
+```
+
+### OR (||)
+
+If first operand is true:
+
+```
+true || something → second part not checked
+```
+
+This prevents errors like null pointer exceptions:
+
+```
+if (str != null && str.length() > 0)
+```
+
+If `str` is null, second part is NOT evaluated.
+
+---
+
+## Practical Examples
+
+### Example 1 – Age Check
+
+```
+int age = 20;
+boolean canVote = age >= 18; // true
+```
+
+### Example 2 – Multiple Conditions
+
+```
+int temperature = 35;
+boolean isHot = temperature > 30 && temperature < 50; // true
+```
+
+### Example 3 – Login Validation
+
+```
+boolean usernameCorrect = true;
+boolean passwordCorrect = false;
+
+if (usernameCorrect && passwordCorrect) {
+    System.out.println("Login successful");
+} else {
+    System.out.println("Invalid credentials");
+}
+```
+
+### Example 4 – Checking Grades
+
+```
+char grade = 'A';
+
+if (grade == 'A' || grade == 'B') {
+    System.out.println("Pass");
+}
+```
+
+---
+
+## Common Mistakes
+
+### ❌ Using `=` instead of `==`
+
+```
+if (isReady = true)  // WRONG: assignment
+```
+
+### ✔ Correct
+
+```
+if (isReady == true)
+```
+
+Even better:
+
+```
+if (isReady)
+```
+
+### ❌ Unnecessary comparison
+
+```
+if (isOpen == true) // redundant
+```
+
+✔ Use:
+
+```
+if (isOpen)
+```
+
+---
+
+## Memory Tricks
+
+* **AND (&&)** → both conditions must be true → "two locks must open"
+* **OR (||)** → at least one must be true → "one key is enough"
+* **NOT (!)** → flip value → "reverse mode"
+
+---
+
+
+
+