Building Java Programs

Lab: Expressions and Variables

Except where otherwise noted, the contents of this document are Copyright 2019 Stuart Reges and Marty Stepp.

Lab goals

Goals for this problem set:

• write and evaluate expressions to compute numeric values
• use variables to store results of a computation into memory

Expressions

Recall that Java has expressions to represent math and other computations. Expressions may use operators, which are evaluated according to rules of precedence. Every expression produces a value of a given type.

Exercise : Expressions (2.1)

Write the results of each of the following expressions. If you're stuck, ask a TA or neighbor.

4

19

-16

3

9.0

1.75

Exercise : More expressions (2.1)

Write the results of each of the following expressions.

"8cs"

"cs 12326"

9.0

2.0

10.5

jShell / Interactions Pane

• Java now has a built-in tool called jShell that allows you to type in Java expressions or statements one at a time and instantly see their results.
• Many Java code editors also have a built-in tool equivalent to jShell. Some call it an Interactions Pane.
• 

continued on the next slide...

Exercise : Using jShell / Interactions Pane

In this exercise, you'll use jShell (or your editor's Interactions Pane) to quickly discover the result of some expressions that would be difficult to evaluate by hand. Copy/paste each expression below to evaluate it, then input the answer into this slide.

55299

7.3535

1024

"4xyz33"

Variables

Recall that you can use a variable to store the results of an expression in memory and use them later in the program.

type name;                       // declare
name = value or expression;        // assign a value
...
type name = value or expression;   // declare-and-initialize together

Examples:

double iphonePrice;
iPhonePrice = 299.95;

int siblings = 3;
System.out.println("I have " + siblings + " brothers/sisters.");

Exercise : Variable declaration syntax

Which of the following choices is the correct syntax for declaring a real number variable named grade and initializing its value to 4.0?

1. grade : 4.0;
2. int grade = 4.0;
3. 4.0 = grade;
4. grade = 4.0;
5. double grade = 4.0;
6. grade = double 4.0;

Exercise : Variable assignment syntax

Suppose you have a variable named grade, set to 1.6:

double grade = 1.6;   // uh-oh

Suppose later in the program's code, we want to change the value of grade to 4.0. Which is the correct syntax to do this?

1. grade : 4.0;
2. double grade = 4.0;
3. grade = 4.0;
4. 4.0 = grade;
5. set grade = 4.0;

Exercise : Variable mutation

Suppose you have a variable named balance, set to 463.23:

double balance = 463.23;

Suppose later in the program's code, we want to add 5 to the account balance. Which is a correct statement to do this?

1. balance + 5;
2. balance = 5;
3. balance = balance + 5;
4. 5 + balance = balance;
5. balance <-- 5;

Exercise : a, b, and c

What are the values of a, b, and c after the following statements? Write your answers in the boxes on the right.

int a = 5;
int b = 10;
int c = b;

a = a + 1;            // a? 6
b = b - 1;            // b? 9
c = c + a;            // c? 16

Exercise : Syntax errors

• The following program contains 9 mistakes! What are they? Copy and paste the following code into your editor and correct the various mistakes. The answer is on the next 2 slides if you need some help.

  1 2 3 4 5 6 7 8 9 10 11 12 13

  public class Oops { public static void main(String[] args) { int x; System.out.println("x is" x); int x = 15.2; // set x to 15.2 System.out.println("x is now + x"); int y; // set y to 1 more than x y = int x + 1; System.out.println("x and y are " + x + and + y); } }

answer on next slide...

Exercise - answer

1. line 4: missing + between "x is" and x
2. line 4: cannot print the value of x before assigning it a value
3. line 6: cannot assign 15.2 into a variable of type int
4. line 6: should not redeclare the variable's type
5. line 7: " mark should be between now and +
6. line 10: should not write the word int here
7. line 10: variable y should be same type as x
8. line 10: does not properly set y to be 1 more than x (should not write the word int here)
9. line 11: and should be in quotes with surrounding spaces

Exercise - corrected version

• Here is a corrected version of the program:

  public class Oops {
      public static void main(String[] args) {
          double x = 0.0;
          System.out.println("x is" + x);
  
          x = 15.2;       // set x to 15.2
          System.out.println("x is now " + x);
  
          double y;       // set y to 1 more than x
          y = x + 1;
          System.out.println("x and y are " + x + " and " + y);
      }
  }
  

Exercise : i, j, and k

What are the values of i, j, and k after the following statements?

int i = 2;
int j = 3;
int k = 4;
int x = i + j + k;

i = x - i - j;            // i? 4
j = x - j - k;            // j? 2
k = x - i - k;            // k? 1

Exercise : Equation

Suppose you have a real number variable x. Write a Java expression that computes a variable named y storing the following value:

y = 12.3x⁴ - 9.1x³ + 19.3x² - 4.6x + 34.2

(We haven't learned a way to do exponents yet, but you can simulate them using several multiplications.)

Use the example program on the next slide to test your code.

Exercise - Example code

Copy/paste this program into your editor to test your solution.

// expected output:
// y is 7043.7

public class EquationY {
    public static void main(String[] args) {
        double x = 5;

        double y = put your expression for y here ;

        System.out.println("y is " + y);
    }
}

(answer on next slide)

Exercise - answer

double y = 12.3*x*x*x*x - 9.1*x*x*x + 19.3*x*x - 4.6*x + 34.2;

If you want an added challenge, try to come up with a way to compute the above value while using the * operator no more than 4 times.

(click Next → for answer)

double y = (((12.3 * x - 9.1) * x + 19.3) * x - 4.6) * x + 34.2;

Exercise : Birthday variables

• Create a complete Java program in a class named Bday that declares four variables and assigns appropriate values to them.
  • your birthday month (1-12)
  • your birthday day (1-31)
  • the birthday month of another student sitting near you today (1-12)
  • the birthday day of that same student near you (1-31)
• Ask your neighbor for their name and for the proper numbers to store in the variables for his/her birthday. Then produce output in this format using your four variables:

  My birthday is 9/19, and Suzy's is 6/14.