2.0 Table of Contents

2.1 Session 1 Review

2.1.1 Types

  • boolean: truth values (e.g. true, false)
  • int: integer numbers (e.g. 1, 2, 3, 4, 5, …)
  • double: real numbers (e.g. -2.0, -1.0, 0.0, 1.0, 2.0, …)
  • string: text (e.g. “Hello World!”)

2.1.2 Variables

location in memory that stores a value

String a = "letter a";
String b = "letter b";
String c = a + " and " + b;
System.out.println(c);

2.1.3 Operators

  • Arithmetic Operators are used to perform common mathematical operations.
  • Assignment Operators are used to assign values to variables.
  • Comparison Operators are used to compare two values.
  • Logical Operators are used to determine the logic between variables or values.

2.1.4 Solution to Homework 1

View Solution

2.2 More Types

2.2.1 Division

Division works differently for integers and doubles. For integers, division is integer division. For doubles, division is floating point division.

double a = 5.0/2.0; // a = 2.5
int b = 5/2; // b = 2
double c = 5/2; // c = 2.0

2.2.2 Type Mismatch

String a = 5.0; // error
double b = "hello"; // error
int c = 5.0; // error

Error Message:

test.java.2: incompatible types
found: double
required: java.lang.String
String a = 5.0;

2.2.3 Casting

int a  = 2; // a = 2
double a = 2; // a = 2.0 (implicit casting)

int a  =2.0; // error
int a = (int) 2.7; // a = 2

double a = 2/3; // a = 0.0
double a = (double) 2/3; // a = 0.666...

2.3 Methods

2.3.1 Basic Method

  • A method is a block of code which only runs when it is called.
  • You can pass data, known as parameters, into a method.
  • define the code once, and use it many times.
  • method must exist within a class
public class Main {
  static void myMethod() {
    // code to be executed
  }
}
  • To Call the method: myMethod();
  • myMethod() is the name of the method
  • static means that the method belongs to the Main class and not an object of the Main class.
  • void means that this method does not have a return value.

NewLines.java

class NewLines {
  public static void newLine() {
    System.out.println();
  }
  public static void threeLines() {
    newLine();
    newLine();
    newLine();
  }

  public static void main(String[] args) {
    System.out.println("First line.");
    threeLines();
    System.out.println("Second line.");
  }
}

2.3.3 Parameters

public static void myMethod(TYPE NAME) {
  // code to be executed
}
  • to call the method: myMethod(VALUE);

Square.java

class Square{
  public static void printSquare(int x) {
    System.out.println(x * x);
  }

  public static void main(String[] args){
    int value = 2;
    printSquare(value); // 4
    printSquare(3); // 9
    printSquare(2 * value); // 16
  }
}

2.3.4 Common Errors

Square2.java

class Square2{
  public static void printSquare(int x) {
    System.out.println(x * x);
  }

  public static void main(String[] args){
    printSquare("hello");
    printSquare(5.5);
  }
}

Square3.java

class Square3{
  public static void printSquare(double x) {
    System.out.println(x * x);
  }

  public static void main(String[] args){
    printSquare(5);
  }
}

2.3.5 Multiple Parameters

...
public static void myMethod(TYPE NAME1, TYPE NAME2){
  // code to be executed
}
...

Multiply.java

class Multiply{
  public static void times(double x, double y) {
    System.out.println(x * y);
  }

  public static void main(String[] args){
    times(2, 3); // 6
    times(4, 5); // 20
    times(6, 7); // 42
  }
}

2.3.6 Return Values

...
public static TYPE myMethod(){
  // code to be executed
  return VALUE;
}
...
  • return statement is used to return a value from the method
  • void means that this method does not have a return value.

Square4.java

class Square4{
  public static double square(double x) {
    return x * x;
  }

  public static void main(String[] args){
    System.out.println(square(5)); // 25.0
    double result = square(3); // 9.0
    System.out.println(result); // 9.0
  }
}

2.3.7 Variable Scope

  • The scope of a variable is the part of the program where the variable is accessible.
  • variables only exist within the block {} they are defined in

Scope.java

class Scope1{
  public static double printSquare(int x) {
    System.out.println("printSquare x=" + x);
    x *= x;
    System.out.println("printSquare x=" + x);
  }

  public static void main(String[] args){
    int x = 5;
    System.out.println("main x=" + x);
    printSquare(x);
    System.out.println("main x=" + x);
  }
}

2.3.8 Abstraction

  • complex programs are built from simple methods.
  • methods can be individually developed, tested, and reused.
  • user of method does not need to know how it works.
  • this is called abstraction

2.4 Conditionals

2.4.1 If Statements

if (CONDITION) {
  // code to be executed if condition is true
}

public static void test(int x) {
  if (x > 5) {
    System.out.println("x is greater than 5");
  }
}

public static void main(Stringp[] args){
  test(6); // x is greater than 5
  test(5);
  test(4);
}

Class AbsoluteValue{
  public static int abs(int x) {
    if (x < 0) {
      return -x;
    } else {
      return x;
    }
  }

  public static void main(String[] args){
    System.out.println(abs(5)); // 5
    System.out.println(abs(-5)); // 5
  }
}

2.4.2 Comparison Operators

Operator Description Example
== equal to (= is assignment) x == y
!= not equal to x != y
> greater than x > y
< less than x < y
>= greater than or equal to x >= y
<= less than or equal to x <= y

2.4.3 Logical Operators

Operator Description Example
&& and x && y
|| or x || y
! not !x 
if (x>6){
  if (x<10){
    System.out.println("x is between 6 and 10");
  }
}

if (x>6 && x<10){
  System.out.println("x is between 6 and 10");
}

2.4.4 If-Else Statements

if (CONDITION) {
  // code to be executed if condition is true
} else {
  // code to be executed if condition is false
}

public static void test(int x){
  if (x > 5) {
    System.out.println("x is greater than 5");
  } else {
    System.out.println("x is less than or equal to 5");
  }
}

public static void main(String[] args){
  test(6); // x is greater than 5
  test(5); // x is less than or equal to 5
  test(4); // x is less than or equal to 5
}

2.4.5 If-Else-If Statements

if (CONDITION1) {
  // code to be executed if condition1 is true
} else if (CONDITION2) {
  // code to be executed if the condition1 is false and condition2 is true
} else {
  // code to be executed if the condition1 is false and condition2 is false
}

public static void test(int x){
  if (x > 5) {
    System.out.println("x is greater than 5");
  } else if (x < 5) {
    System.out.println("x is less than 5");
  } else {
    System.out.println("x is equal to 5");
  }
}

public static void main(String[] args){
  test(6); // x is greater than 5
  test(5); // x is equal to 5
  test(4); // x is less than 5
}

2.5 Assignment 2 - Wage Calculator

Write a method that calculates the weekly wage of an employee. The method should take as input the number of hours worked and the hourly wage, and print the total wage.

  • An employee gets paid (hours worked) × (hourly rate), for each hour up to 40 hours.
  • For every hour over 40, they get overtime = (hourly rate) × 1.5.
  • The base pay must not be less than the minimum wage ($15.00 an hour). If it is, print an error.
  • If the number of hours is greater than 60, print an error message.

Specifications:

  • Name the method calculateWage
  • input: int hours, double wage
  • output: no return value and print the result

Cases:

  • find the wage for an employee who worked 50 hours at $9.99/hour
  • find the wage for an employee who worked 50 hours at $15.00/hour
  • find the wage for an employee who worked 63 hours at $25.00/hour

View Solution

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