Unit 1 Knowledge Handout: Using Objects and Methods

Java Source (.java) → [ Compiler ] → Bytecode (.class)
 (Human Readable)     (Strict Check)   (Machine Code)

// Syntax Error — missing semicolon
System.out.println("Hello")   // ❌ compile error

// Logic Error — runs but gives wrong answer
int average = 10 / 4;         // result is 2, not 2.5
System.out.println(average);  // prints 2 (you probably wanted 2.5)

// Runtime Error — crashes during execution
int x = 5 / 0;                // ❌ ArithmeticException

public class MyFirstProgram {

    public static void main(String[] args) {
        // All execution logic goes here
    }
}

System.out.print("Apple ");    // print without newline
System.out.println("Banana");  // print with newline

System.out.println("Red\n\"Blue\"");
// Output:
// Red
// "Blue"

System.out.println("Type \"Y\" to continue");
// Output: Type "Y" to continue

// Single-line comment

/*
   Multi-line comment:
   This spans multiple lines
*/

/**
 * Javadoc comment: used to generate API documentation
 * @param name the student's name
 */

Type + Name = Value

int score = 100;          // integer (whole number)
double price = 19.99;     // decimal (floating-point)
boolean isReady = true;   // logical (only true or false)

import java.util.Scanner;

Scanner input = new Scanner(System.in);
int age = input.nextInt();
double price = input.nextDouble();
String word = input.next();

int score = 10;
score = score + 5;   // compute score + 5 = 15, then store 15 back into score

int a = 10;
int b = a;    // copy a's value into b
a = 20;       // changing a does NOT affect b
System.out.println(b);  // still 10

System.out.println(10 / 4);      // 2 (not 2.5!)
System.out.println(5 / 2);       // 2
System.out.println(1 / 3);       // 0
System.out.println(10 / 4.0);    // 2.5 (double present → double result)

1. ( ) parentheses
2. *, /, % (left to right)
3. +, - (left to right)

System.out.println((2 + 3) * 4);  // 20
System.out.println(10 - 3 * 2);   // 4 (multiplication first)
System.out.println(7 % 3 + 1);    // 2

double result = 10 / 4.0;  // 2.5
double result2 = 10.0 / 4; // 2.5

int x = 5;
double y = x;    // ✅ int → double auto-promotion, y = 5.0

double d = 3.5;
int i = d;       // ❌ compile error! double → int cannot auto-convert

final double PI = 3.14159;
PI = 3.14;  // ❌ compile error!

(targetType) expression

System.out.println((int) 3.9);    // 3 (not 4!)
System.out.println((int) -3.8);   // -3 (toward zero)
System.out.println((double) 5);   // 5.0

System.out.println((int) 9.9 * 2);     // 18: (int)9.9 = 9, then 9 * 2
System.out.println((int)(9.9 * 2));    // 19: 9.9*2 = 19.8, truncated to 19
System.out.println((double) 10 / 4);   // 2.5: 10 → 10.0, then 10.0 / 4
System.out.println((double)(10 / 4));  // 2.0: 10/4 = 2, then 2 → 2.0

int x = (int) 7.9;          // 7
int y = (int) -3.8;         // -3
double z = (double) 5;      // 5.0
int a = (int)(8.6 + 0.5);   // 9 (can be used for rounding)

int x = 10;
x += 5;    // x = 15
x *= 2;    // x = 30

int y = 10;
y %= 3;    // y = 1

x++;    // equivalent to x = x + 1
x--;    // equivalent to x = x - 1

int count = 5;
count++;     // count = 6
count--;     // count = 5
count += 1;  // count = 6 (all three are equivalent)
count = count + 1;  // count = 7

System.out.println(Integer.MAX_VALUE);          // 2147483647
System.out.println(Integer.MAX_VALUE + 1);      // -2147483648 (overflow!)
System.out.println(Integer.MIN_VALUE - 1);      // 2147483647 (wraps back)

System.out.println(0.1 + 0.2);  // 0.30000000000000004 (not 0.3!)

double calcSpeed(int miles)

// Method that returns a value
int result = Math.abs(-5);      // result = 5
double root = Math.sqrt(81);    // root = 9.0

// void method (returns nothing)
System.out.println("Hello");    // no return value

int adjust(int value)
double adjust(double value)
int adjust(int value, int amount)

Math.methodName(arguments)

double result = Math.sqrt(Math.pow(3, 2));  // 3.0
double value = Math.abs(-4.5) + 1.5;        // 6.0
double power = Math.pow(5, 0);              // 1.0
double power = Math.pow(-3, 2);             // 9.0

int dice = (int) (Math.random() * 6) + 1;   // 1 ~ 6

Math.random()                   → [0.0, 1.0)
* 6                             → [0.0, 6.0)
(int) ...                       → {0, 1, 2, 3, 4, 5}
+ 1                             → {1, 2, 3, 4, 5, 6}

int result = (int) (Math.random() * rangeSize) + offset;

class  -> blueprint
object -> one concrete instance

int score = 100;                    // primitive: stores value directly
double average = 92.5;              // primitive: stores value directly
boolean isReady = true;             // primitive: stores value directly
String name = new String("Ava");    // reference: stores a reference to an object

String message = new String("Hello");

String message = "Hello";                // literal (more common)
String message2 = new String("Hello");   // explicit constructor (same result)

String text = "A\"B\\C\nD";
System.out.println(text.length()); // 7

String word = "coding";
// index: c=0, o=1, d=2, i=3, n=4, g=5

String first = "Hello";
String name = "Ava";

String greeting = first + name;            // "HelloAva"
String message = first + ", " + name;      // "Hello, Ava"
String label = "Score: " + 42;             // "Score: 42"

System.out.println("Score: " + 4 + 2);    // "Score: 42"
System.out.println(4 + 2 + " points");    // "6 points"

String word = "coding";
String letter = word.substring(2, 3);    // "d"

"ant".compareTo("apple");   // negative: ant < apple (lexicographically earlier)
"dog".compareTo("cat");     // positive: dog > cat
"dog".compareTo("dog");     // 0: equal

String word = "coding";

word.substring(0, 3);              // returns "cod", but it's not captured!
System.out.println(word);          // still "coding"

word = word.substring(0, 3);       // captured the return value
System.out.println(word);          // "cod"

String word = "coding";
word.substring(0, 3);              // ❌ return value not captured, lost
System.out.println(word);          // "coding" (not "cod"!)

word = word.toUpperCase();         // ✅ reassign to capture
String result = word.substring(0, 3);  // ✅ store in new variable

String a = "";       // empty string: a real String object with no content
String b = null;     // null: the variable does not point to any object

String empty = "";
String nickname = null;

System.out.println(empty.length());     // 0 ✅
System.out.println(nickname.length());  // NullPointerException ❌

int result = 10 / 4;    // 2 (not 2.5!)

(int) 9.9 * 2     // 18 — cast only applies to 9.9
(int) (9.9 * 2)   // 19 — multiply first, then truncate

String s = "hello";
s.substring(0, 2);      // not captured, result lost
System.out.println(s);  // "hello" (unchanged!)

"Score: " + 4 + 2    // "Score: 42" (left to right, String met first)
4 + 2 + " points"    // "6 points" (numeric addition first)

double d = 5;     // ✅ int auto-promotes to double
int i = 3.5;      // ❌ compile error
int i = (int)3.5; // ✅ OK with explicit cast

Integer.MAX_VALUE + 1  // becomes Integer.MIN_VALUE (negative!)

"".length()      // 0 ✅
null.length()    // NullPointerException ❌

int a = 5;
int b = a;
a = 10;
// b is still 5, not 10!