Continued

• Modify the program so it also displays the time 55 minutes from now (hint: you may need to use the remainder operator %). For example:

The current time is 12:30.
In 55 minutes, it will be 13:25.

• Test your program out by changing the number of minutes to add.
  • If you coded it right, you should be able to change that number in just one part of your code!
  • And it should continue to work fine!
  • (Or we discover a bug, and we can fix it!)

If Statement

• Issue: 30 + 35 = 65. 1 hour and 5 mintues
• 10:30 + 35 minutes = 11:05, not 11:5!
• Fix? Either:
  • if statement
  • printf format string (see chapter 3 for details)

Strings

The String data type can be used to represent longer text. String literals are written with double quotes around them.

String moby_dick = "Call me Ishmael";

String Concatenation

Strings can be concatenated with the + operator:

System.out.println("Hello " + " world!");

Strings can be concatenated with data of other types:

int x = 5;
String s = "Hi " + x;
System.out.println(s);

Confusion

This can cause some confusion. Java reads the operation left-to-right. Can you guess what is outputted in the following two lines?

System.out.println(5 + 5 + "Hello!");
System.out.println("Hello!" + 5 + 5);

Navigating in IntelliJ

• We can navigate Java libraries using IntelliJ.
• Try holding down the COMMAND key ⌘, and clicking on System.out.
• Useful tool when we are working with libraries that we don’t know fully well.

We can also view information about Java standard libraries online: search for “java PrintStream” for example.

Questions

1. Purpose of import statement?
2. Purpose of new keyword?

Exercise

Write a program that asks a user to input their height in inches, and then displays their height in standard form (feet and inches).

For example, if the user types in 70, we should convert that to 5’10".

Scanner API

• nextInt, nextDouble, nextLong
• next: reads the next “token” (until whitespace, by default), store as a String
• nextLine: reads the entire next line (until a newline), store as a String
• hasNextInt, hasNextDouble, hasNextLine: check if there is more input

hasNextInt may be useful for us in combination with if and while statements

Scanner Pitfall

Scanner sc = new Scanner(System.in);
System.out.println("Enter your age: ");
int age = sc.nextInt();
System.out.println("Enter your name: ");
String name = sc.nextLine();
System.out.println("Hi " + name + ", you are " + age + " years old!");

What happens?

Explanation

• Scanner object parse input as tokens, not as entire lines.
• nextInt: parses all of the input up until the end of the integer.
• “enter” key (newline) not processed by nextInt
• newline processed by nextLine

Fix? Call nextLine after every group of nextInts.

Fix

Scanner sc = new Scanner(System.in);
System.out.println("Enter your age: ");
int age = sc.nextInt();
sc.nextLine();
System.out.println("Enter your name: ");
String name = sc.nextLine();
System.out.println("Hi " + name + ", you are " + age + " years old!");

Structure

• Packages (java.util) group related classes
• classes (Scanner) contain methods (and more…)
• methods: named sequences of statements

All code goes inside a method!

Upcoming

• Read Chapter 3, if you haven’t already.
• Read Chapter 5 on conditionals and logical symbols.
• Read Chapter 6 on loops (for and while)
• Monday: asynchronous / small groups (10-12 students, not everyone).
• Quiz 1: online
  • will be posted tomorrow on BrightSpace
  • 20 minutes
  • includes material from next lesson

Preview

What does the following code snippet do?

Scanner sc = new Scanner(System.in);
System.out.println("n?");
int n = sc.nextInt();
int x = 1;
for (int i = 0; i < n; i++) {
  x *= 2;
}
System.out.println(x);

Make a memory diagram to figure it out.