Java cheatsheet

 

Great question! Java is strict with rules. Here's a quick checklist of the most important things you MUST remember while writing Java code—like semicolon, braces, and more.

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✅ JAVA CODE RULES TO ALWAYS REMEMBER

1. Semicolon (;)

🔹 Use it to end every statement.
📌 Remember: If you forget it, compiler will give an error.

int x = 10;   // ✅ correct
System.out.println(x);  // ✅ correct

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2. Curly Braces { }

🔹 Use for class, method, loops, if-else blocks.
📌 Open { and always close with }.

if (x > 0) {
   System.out.println("Positive");
} // ✅ braces are matched

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3. Main Method Syntax (fixed format)

🔹 This is where your program starts running.
📌 Don’t change the spelling or structure.

public static void main(String[] args) {
    // code
}

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4. Class Name = File Name (Case Sensitive)

🔹 File name must match the class name (if public).

public class Hello {
    public static void main(String[] args) {
        System.out.println("Hi");
    }
}
// Must be saved as Hello.java ✅

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5. Case Sensitive

🔹 Hello ≠ hello ≠ HELLO
📌 Variable and method names are case-sensitive.

int num = 5;
System.out.println(num);  // ✅
System.out.println(Num);  // ❌ Error

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6. Use new to Create Object

🔹 You can't use a class directly—use new.

Student s = new Student();  // ✅

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7. Use System.out.println() for Output

🔹 Java’s standard way to print something.

System.out.println("Hello");

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8. Always Close Quotes and Brackets

🔹 Forgetting a " or ) causes errors.

System.out.println("Hi");  // ✅
System.out.println("Hi);   // ❌ missing "

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🧠 BONUS MEMORY HACK (CLUE WORDS)

💡 What	🧠 Clue	✅ Tip
End statement	Semicolon ;	Like a full stop .
Group code	Curly braces {}	Open door → do stuff → close door
Output	System.out.println()	Say something out loud
Create object	new	"new object born"
Start program	main method	Like main() in C/C++
File name = class	Match exactly	If class Cat, file = Cat.java

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✅ PRACTICE CHECKLIST FOR EVERY JAVA PROGRAM

✔ Starts with class
✔ Has main method
✔ Ends each line with ;
✔ Uses {} correctly
✔ Uses System.out.println() for output
✔ Object created using new
✔ File name = public class name

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If you want, I can give you a template to copy-paste every time. Want that?

Absolutely! Here's more detailed guidance on writing correct and clean Java code—beyond semicolons and syntax. This will help you avoid errors and write code like a pro.

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🧠 PART 1: STRUCTURE TO REMEMBER

✅ Basic Java Program Template

// Class name (must match file name if public)
public class MyProgram {

    // Main method (entry point)
    public static void main(String[] args) {
        // Your code here
        System.out.println("Hello, Java!");
    }
}

💡 Tip: Save file as MyProgram.java

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🔑 PART 2: COMMON MISTAKES & REMINDERS

1. Every statement ends with ;

int a = 5;        // ✅
a = a + 10;       // ✅

❌ Forgetting ; causes "expected ;" error

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2. Curly braces {} must always be paired

if (a > 0) {
   System.out.println("Positive");
} // ✅ closed

❌ Forgetting } gives "reached end of file while parsing" error

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3. Case-sensitive

int Age = 20;
System.out.println(Age); // ✅
System.out.println(age); // ❌ age not defined

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4. Class name = File name (important!)

public class HelloWorld {} 
// Must save as HelloWorld.java ✅

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5. Use correct method names (spelling matters!)

System.out.println("Hello"); // ✅
System.out.print("No newline"); // ✅

❌ system.out.Println() will cause errors (capitalization matters)

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6. Create objects with new

Car myCar = new Car(); // ✅

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7. Avoid using public class more than once in same file

public class A {}   // ✅
class B {}          // ✅

❌ Only one public class per file

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🔧 PART 3: TIPS TO WRITE CLEAN CODE

✅ DO	❌ AVOID
Use indentation (tabs/spaces)	Writing all code in one line
Use meaningful variable names	Names like x1, var2
Use comments // to explain logic	No comments makes code confusing
Group related code with {}	Writing code without structure

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📚 PART 4: Java Keyword Categories (Mini Reference)

🔹 Data Types

int, float, double, char, boolean, String

🔹 Control Flow

if, else, switch, while, for, break, continue

🔹 OOP Basics

class, object, new, this, public, private, protected

🔹 Error Handling

try, catch, throw, throws, finally

🔹 Other

static, void, return, import, package

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🧪 PART 5: MINI PRACTICE CHALLENGE

Try writing this yourself:

✅ Create a simple class Student with:

• Attributes: name, roll
• Method: displayDetails() which prints both

Expected Output:

Name: Ravi
Roll: 101

Need help with the answer? I can give you the full code after you try!

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Would you also like a Java cheat sheet PDF or a beginner Java roadmap next?

References

https://chatgpt.com/share/6882f1b3-e66c-8001-8cca-a4783ad31d32