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 document created by Marty Stepp, Stuart Reges and Whitaker Brand

Lab goals

Goals for this problem set:

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.

Type Description Example Result
int integers (up to 231 - 1) 3 + 4 * 5 23
double real numbers (up to 10308) 3.0 / 2.0 + 4.1 5.6
String text characters "hi" + (1 + 1) + "u" "hi2u"

Exercise : Expressions (2.1)

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

12 / 5 + 8 / 4
4
3 * 4 + 15 / 2
19
-(1 + 2 * 3 + (1 + 2) * 3)
-16
42 % 5 + 16 % 3
3
2.5 * 2 + 17 / 4
9.0
4.5 / 3 / 2 + 1
1.75

Exercise : More expressions (2.1)

Write the results of each of the following expressions.

2 + 6 + "cse 142"
"8cse 142"
"cse 142" + 2 + 6
"cse 14226"
1 + 9 / 2 * 2.0
9.0
46 / 3 / 2.0 / 3 * 4/5
2.0
50 / 9 / 2.0 + 200 / 10 / (5.0 / 2)
10.5

jGRASP Interactions Pane

continued on the next slide...

Exercise : Using jGRASP Interactions Pane

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

123 * 456 - 789
55299
3.14 + 1.59 * 2.65
7.3535
2 * 2 * 2 * 2 * 2 * 2 * 2 * 2 * 2 * 2
1024
2 + 2 + "xyz" + 3 + 3
"4xyz33"

(For the last expression, the Interactions Pane doesn't put "" quotes around Strings when displaying results, so you must add those yourself if needed. For example, if the Interactions Pane gives you a result of abc123, it should be written here as "abc123".)

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?

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?

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?

Exercise : a, b, and c practice-it

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

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

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 jGRASP 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