Learning Java from the basics is a great choice! Java is a versatile, object-oriented programming language widely used for Android development, web applications, enterprise systems, and more. Below is a structured roadmap to learn Java fundamentals:
---
### **1. Introduction to Java**
- **What is Java?**
- Platform-independent (Write Once, Run Anywhere - WORA)
- Object-oriented, robust, secure, and multithreaded
- **Java Editions**: SE (Standard Edition), EE (Enterprise Edition), ME (Micro Edition)
- **JDK vs. JRE vs. JVM**
- **JDK (Java Development Kit)**: Tools to develop Java programs (compiler, debugger, etc.).
- **JRE (Java Runtime Environment)**: Runs compiled Java programs.
- **JVM (Java Virtual Machine)**: Executes bytecode.
---
### **2. Setting Up the Environment**
1. **Install JDK** (from [Oracle](https://www.oracle.com/java/technologies/javase-downloads.html) or OpenJDK).
2. **Set `JAVA_HOME`** environment variable.
3. **Verify Installation**:
```bash
java -version
javac -version
```
4. **Use an IDE** (e.g., IntelliJ IDEA, Eclipse, or VS Code) or a simple text editor.
---
### **3. Basic Syntax & Structure**
```java
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, World!");
}
}
```
- **Class**: `HelloWorld` is a class (file name must match the class name).
- **`main` method**: Entry point of the program.
- **`System.out.println()`**: Prints output to the console.
---
### **4. Data Types & Variables**
- **Primitive Types**:
```java
int num = 10; // Integer
double pi = 3.14; // Floating-point
char letter = 'A'; // Character
boolean flag = true; // Boolean
```
- **Non-Primitive Types**: `String`, arrays, classes.
- **Variable Naming Rules**:
- Start with a letter, `_`, or `$`.
- Case-sensitive (`age` ≠ `Age`).
---
### **5. Operators**
- **Arithmetic**: `+`, `-`, `*`, `/`, `%` (modulus).
- **Comparison**: `==`, `!=`, `>`, `<`, `>=`, `<=`.
- **Logical**: `&&` (AND), `||` (OR), `!` (NOT).
- **Assignment**: `=`, `+=`, `-=`.
---
### **6. Control Flow**
#### **Conditionals**
```java
if (age >= 18) {
System.out.println("Adult");
} else {
System.out.println("Minor");
}
```
- **`switch` Statement**:
```java
switch (day) {
case 1: System.out.println("Monday"); break;
default: System.out.println("Invalid day");
}
```
#### **Loops**
- **`for` Loop**:
```java
for (int i = 0; i < 5; i++) {
System.out.println(i);
}
```
- **`while` Loop**:
```java
while (i < 5) {
System.out.println(i);
i++;
}
```
- **`do-while` Loop**:
```java
do {
System.out.println(i);
i++;
} while (i < 5);
```
---
### **7. Arrays**
```java
int[] numbers = {1, 2, 3};
String[] names = new String[3]; // Fixed size
```
- **Access Elements**: `numbers[0]` → `1`.
- **Loop Through Array**:
```java
for (int num : numbers) {
System.out.println(num);
}
```
---
### **8. Methods (Functions)**
```java
public static int add(int a, int b) {
return a + b;
}
```
- **Calling a Method**:
```java
int sum = add(5, 3); // Returns 8
```
- **Method Overloading**: Same name, different parameters.
---
### **9. Object-Oriented Programming (OOP)**
#### **Classes & Objects**
```java
class Dog {
String name; // Field
void bark() { // Method
System.out.println(name + " says Woof!");
}
}
// Create an object
Dog myDog = new Dog();
myDog.name = "Buddy";
myDog.bark();
```
#### **4 OOP Pillars**:
1. **Encapsulation**: Hide data (use `private` fields + `getter/setter` methods).
2. **Inheritance**:
```java
class Animal { void eat() { ... } }
class Dog extends Animal { ... }
```
3. **Polymorphism**:
- Method overriding (same method, different implementation in child class).
- Method overloading (same name, different parameters).
4. **Abstraction**:
- Abstract classes (`abstract class Animal { ... }`).
- Interfaces (`interface Drawable { void draw(); }`).
---
### **10. Exception Handling**
```java
try {
int result = 10 / 0;
} catch (ArithmeticException e) {
System.out.println("Cannot divide by zero!");
} finally {
System.out.println("Cleanup code here.");
}
```
---
### **11. Collections Framework**
- **Lists**: Ordered, allows duplicates (`ArrayList`, `LinkedList`).
```java
List<String> fruits = new ArrayList<>();
fruits.add("Apple");
```
- **Sets**: No duplicates (`HashSet`, `TreeSet`).
- **Maps**: Key-value pairs (`HashMap`, `TreeMap`).
```java
Map<String, Integer> ages = new HashMap<>();
ages.put("Alice", 25);
```
---
### **12. File I/O**
```java
import java.io.File;
import java.io.FileWriter;
File file = new File("test.txt");
try (FileWriter writer = new FileWriter(file)) {
writer.write("Hello, File!");
}
```
---
### **13. Multithreading**
```java
class MyThread extends Thread {
public void run() {
System.out.println("Thread is running.");
}
}
MyThread t1 = new MyThread();
t1.start();
```
---
### **Next Steps**
1. **Practice**: Solve problems on platforms like LeetCode, HackerRank.
2. **Build Projects**: Calculator, To-Do List, Simple Game.
3. **Explore Advanced Topics**:
- JavaFX (GUI).
- JDBC (Database connectivity).
- Spring Framework (Enterprise Java).
---
### **Recommended Resources**
- **Books**:
- *Head First Java* (Beginner-friendly).
- *Effective Java* by Joshua Bloch (Best practices).
- **Online Courses**:
- MOOC by University of Helsinki ([Java Programming](https://java-programming.mooc.fi/)).
- Coursera/Udemy courses.
- **Documentation**: [Oracle Java Docs](https://docs.oracle.com/en/java/).
---
Would you like a deeper dive into any specific topic or hands-on examples? Let me know how I can assist further!
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