Documentation

Unit 1

Algoritm: a step-by-step process to follow when completing a task or solving a problem.
The sequencing of the tasks and clear instructions are essential.
Many algorithms canbe used to achieve the same goal- through different processes.

Syntax Error

a mistake in the program where the rules of the language are not followed

Logic Error

mistakes in the program causes it to behave inccorrectly or unexpectedly

Run-Time Error

mistakes in the program that occurs during the execution of the program

Data Types

The type of information that available can store and what you can do with that information. There are five types:

1- Integer: Whole numbers.
2- Double: Numbers with decimals.
3- Boolean: Absolute. True or false.
4- String: Number of words, letters etc.
5- Char: One singular character.

public class Example {
public static void main(String[] args) {

public: acessible from anywhere in your program.
class: blueprint for objects in java, all code must be in classes.
static: belongs to the class itself, not to an object.
void: method does not return anything.
main: entry point of java.

All of the following are the same:
a = a + 1;
a += 1;
a++;

Input

In order to have input from user you have to "Scanner in=new Scanner(System.in);"

Literals and Escape Sequences

A literal is a fixed value, a constant.
Using "\" in order to add punctuation marks to the written output
Example: System.out.println("She said \"Hey!\"");
She said "Hey!"
\n = new line
\t = tab

Errors

When you give no value to a variable, the error of variable might NOT be INITIALIZED occurs.
If a variable is assigned to the wrong data tye, ERROR: INCOMPATIBLE TYPES occurs.

Objectss and Calling Methods

Use scanner to get input. Here is an example of using scanner:

Java Code


import java.util.Scanner;

public class BMI {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter weight (kg): ");
        double weight = scanner.nextDouble();

        System.out.print("Enter height (m): ");
        double height = scanner.nextDouble();

        double bmi = weight / (height * height);
        System.out.println("BMI: ", bmi);
    }
}

Widening

Converting an integer to a double.

Narrowing

Converting a double to an integer. Note: even is the double is 9.9, norrowing converts it into 9 not 10. Rounding can be made by (+0.5) while converting.

java.lang = already uploaded libraries ( String, math, system, runtime )
java.util = the libraries you upload ( Scanner, tandom, ArrayList, HashMap)
Java package = a collection of related classes grouped together.

API

Application Programming ınterface. Provides documentation that explains what hose libraries do and how to call the methods or actions. It is like an instruction manual.

CSAwesome Notes — Unit 1

Contents

Minimal program

Your smallest Java file uses a public class named like the file and a main method as the entry point. 

public class Main {
    public static void main(String[] args) {
        System.out.println("hi");
    }
}

Primitive types & Strings

Java primitives: int (32-bit), double (64-bit), boolean (true/false), char (16-bit). String is a class (sequence of characters). 

int count = 0;
double price = 19.99;
boolean ok = true;
char grade = 'A';
String name = "Ada";
System.out.println("Hello " + name);

Expressions & precedence

Precedence: () > * / % > + -. Integer division truncates; one double operand makes the whole expression double. 

int a = 7, b = 3;
System.out.println(a / b);    // 2  (int division)
System.out.println(a / 3.0);  // 2.3333333333333335
System.out.println("Total: " + 5 + 3);   // "Total: 53"
System.out.println("Total: " + (5 + 3)); // "Total: 8"

Assignment, compound ops, ++/--

These are equivalent: a = a + 1; • a += 1; • a++;
Prefix vs postfix matters only when used inside an expression or print. 

int x = 5;
x += 3;      // 8
System.out.println(x++); // prints 8, then x becomes 9
System.out.println(++x); // x becomes 10, then prints 10

Type conversion (casting)

Widening (e.g., int→double) is automatic. Narrowing (double→int) needs an explicit cast and drops the fraction. Cast the whole expression when you convert. 

double y = 2.9;
int k = (int) y;           // 2  (truncation)

int total = 284;
// WRONG: loses fraction first, then casts → 94.0
double avgWrong = (double)(total / 3);
// RIGHT: cast before division → 94.666...
double avgRight = (double) total / 3;

// "lossy conversion" fix: cast the result or use double destination
int i = (int)(2 * 2.2);    // 4
double dx = 2 * 2.2;       // 4.4

Input with Scanner

Import, construct with System.in, then call typed methods like nextInt(), nextDouble(). 

import java.util.Scanner;

public class BMI {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter weight (kg): ");
        double weight = scanner.nextDouble();

        System.out.print("Enter height (m): ");
        double height = scanner.nextDouble();

        double bmi = weight / (height * height);
        System.out.println("BMI: " + bmi);
    }
}

Common errors

Quick charts

Visual cheatsheets (inline SVG; no extra CSS needed).

Primitive sizes (relative) double (64-bit) int (32-bit) char (16-bit) boolean (1-bit*) * conceptual
Relative storage sizes of common primitives.
Frequent pitfalls (sample) Integer division 33 Lossy conversion 26 Using == for doubles 21 Uninitialized local 17
Illustrative counts to remind what to watch for.

Mini-drills

Copy, run, and predict outputs before you execute.

// 1) Integer vs double division
int p = 5, q = 2;
System.out.println(p / q);         // ?
System.out.println(p / (double)q); // ?

// 2) Precedence sanity
System.out.println(2 + 3 * 4);     // ?
System.out.println((2 + 3) * 4);   // ?

// 3) Safe average
int s1 = 87, s2 = 92, s3 = 70;
double avg = (s1 + s2 + s3) / 3.0;
System.out.printf("avg=%.2f%n", avg);

// 4) Overflow demo
System.out.println(Integer.MAX_VALUE + 1); // ?

Next: control flow (if/else, boolean logic, and input validation).