Building Java Programs

Lab 3: Ch. 3: Parameters, Return

lab document created by Marty Stepp and Stuart Reges

Today's lab

Goals for today:

learn the basics of declaring and passing parameters to methods
use return values to send data between methods
Where you see this icon, you can click it to check the problem in Practice-It!

Parameters

A parameter allows you to pass in a value to a method as you call it.

public static void name(type name) {   // declare

methodName(expression);                // call

Example:

public static void squared(int num) {
    System.out.println(num + " times " + num + " is " + (num * num));
}
...
squared(3);          // 3 times 3 is 9
squared(8);          // 8 times 8 is 64

Exercise : Parameters output

What two values are printed by each call below? Write them in the boxes provided.

public class MysteryNums {
    public static void main(String[] args) {
        int x = 15;
        sentence(x, 42);      // 15 42

        int y = x - 5;
        sentence(y, x + y);   // 10 25
    }
    
    public static void sentence(int num1, int num2) {
        System.out.println(num1 + " " + num2);
    }
}

Solving "Parameter Mystery" problems

Note the values passed, the order in which they are passed, and the order the method prints them.

public class MysterySoda {
    public static void main(String[] args) {
        String soda = "coke";
        String pop = "pepsi";
        String pepsi = "soda";
        
        // #1 = "coke", #2 = "pepsi", #3 = "soda"
        carbonated(soda, pop, pepsi);
    }
    //                                    #1           #2           #3
    public static void carbonated(String coke, String soda, String pop) {
        //                           #2               #3             #1
        System.out.println("say " + soda + " not " + pop + " or " + coke);
    }
}

output: // say#2pepsi not#3soda or#1coke

Exercise : Parameter Mystery

Fill in the boxes with the output that each method call would produce:

public class MysterySoda {
    public static void main(String[] args) {
        String soda = "coke";
        String pop = "pepsi";
        String coke = "pop";
        String pepsi = "soda";
        String say = pop;

        carbonated(soda, pop, pepsi);        // say pepsi not soda or coke
        carbonated(coke, soda, pop);         // say coke not pepsi or pop
        carbonated(pop, pepsi, pepsi);       // say soda not soda or pepsi
        carbonated("pop", pop, "koolaid");   // say pepsi not koolaid or pop
        carbonated(say, "say", pop);         // say say not pepsi or pepsi
    }
    public static void carbonated(String coke, String soda, String pop) {
        System.out.println("say " + soda + " not " + pop + " or " + coke);
    }
}

Exercise : Syntax errors

The following Java program has 9 errors. Can you find all of them?

public class Parameters {
    public static void main() {
        double bubble = 867.5309;
        double x = 10.01;
        printer(double x, double y);
        printer(x);
        printer("barack", "obama");
        System.out.println("z = " + z);
    }

    public static void printer(x, y double) {
        int z = 5;
        System.out.println("x = " + double x + " and y = " + y);
        System.out.println("The value from main is: " + bubble);
    }
}

Copy and paste the code into jGrasp and see if you can fix the errors.

Exercise - answer

line 5: cannot use variable y without declaring and initializing it
line 5: cannot declare the type of y in the method call
line 6: cannot call printer without the correct number of parameters (2, in this case)
line 7: cannot call printer by passing the correct type of parameters (double, in this case)
line 8: cannot refer to the variable z: it is in scope inside printer, not main
line 11: must provide a type for x
line 11: must provide a variable name for the second parameter
line 12: must refer to the parameters using the exact same spelling
line 13: cannot refer to variables in main that were not passed into printer as a parameter

Exercise - Corrected version

Here is a corrected version of the program:

public class Parameters {
    public static void main() {
        double bubble = 867.5309;
        double x = 10.01;
        double y = 5.3;
        printer(double x, double y);
        printer(x, 0.0);
        printer("barack", "obama");
        int z = 5;
        System.out.println("z = " + z);
    }

    public static void printer(double x, double y) {
        System.out.println("x = " + x + " and y = " + y);
        System.out.println("The value from main is: " + bubble);
        int z = 5;
    }
}

Exercise : `printSquare`

Write a method printSquare that accepts min and max integers and prints a square of lines of increasing numbers. The first line should start with the minimum; each line that follows should start with the next-higher number. The numbers on a line wrap back to the minimum after it hits the maximum. For example, the call:
```
printSquare(3, 6);
```
should produce the following output:
```
3456
4563
5634
6345
```
If the maximum passed is less than the minimum, the method produces no output. Check your answer using Practice-It from the check-mark icon above.

Exercise : `printGrid`

Write a method named printGrid that accepts two integers representing a number of rows and columns and prints a grid of integers from 1 to (rows*columns) in column-major order.

For example, the call:

printGrid(4, 6);

should produce the following output:

1 5 9 13 17 21
2 6 10 14 18 22
3 7 11 15 19 23
4 8 12 16 20 24

Check your answer using Practice-It from the check-mark icon above.

Exercise : Parameter Mystery

Fill in the boxes with the output that each method call would produce:

public class MysteryNumbers {
    public static void main(String[] args) {
        String one = "two";
        String two = "three";
        String three = "1";
        int number = 20;

        sentence(one, two, 3);                  // three times two = 6
        sentence(two, three, 14);               // 1 times three = 28
        sentence(three, three, number + 1);     // 1 times 1 = 42
        sentence(three, two, 1);                // three times 1 = 2
        sentence("eight", three, number / 2);   // 1 times eight = 20
    }
    public static void sentence(String three, String one, int number) {
        System.out.println(one + " times " + three + " = " + (number * 2));
    }
}

Exercise : Parameter Mystery

Fill in the boxes with the output that each method call would produce:

public class Mystery {
    public static void main(String[] args) {
        String hear = "bad";
        String song = "good";
        String good = "hear";
        String walk = "talk";
        String talk = "feel";
        String feel = "walk";
        claim(feel, song, feel);       // to walk the walk is good
        claim(good, hear, song);       // to hear the good is bad
        claim(talk, "song", feel);     // to feel the walk is song
        claim("claim", talk, walk);    // to claim the talk is feel
    }

    public static void claim(String hear, String good, String song) {
        System.out.println("to " + hear + " the " + song + " is " + good);
    }
}

Returning Values

A return value is when a method sends a value back to the code that called it.

public static type name(parameters) {      // declare
    ...
    return expression;
}

variableName = methodName(parameters);     // call

Example:

public static double fToC(double tempF) {
    return (tempF - 32) * 5.0 / 9.0;
}
...
double bodyTemp = fToC(98.6);          // bodyTemp stores 37.0
double freezing = fToC(32);            // freezing stores  0.0

Math expression syntax

Method	Description	Example
`Math.abs`	absolute value	`Math.abs(-308)` returns `308`
`Math.ceil`	ceiling (rounds upward)	`Math.ceil(2.13)` returns `3.0`
`Math.floor`	floor (rounds downward)	`Math.floor(2.93)` returns `2.0`
`Math.max`	max of two values	`Math.max(45, 207)` returns `207`
`Math.min`	min of two values	`Math.min(3.8, 2.75)` returns `2.75`
`Math.pow`	power	`Math.pow(3, 4)` returns `81.0`
`Math.round`	round to nearest integer	`Math.round(2.718)` returns `3`
`Math.sqrt`	square root	`Math.sqrt(81)` returns `9.0`

Exercise - Math expressions

Write the results of each expression. Use the proper type (such as .0 for a double). Note that a variable's value changes only if you re-assign it using the = operator.

double grade = 2.7;
Math.round(grade);                               // grade = 2.7
grade = Math.round(grade);                       // grade = 3.0

double min = Math.min(grade, Math.floor(2.9));   //   min = 2.0

double x = Math.pow(2, 4);                       //     x = 16.0
x = Math.sqrt(64);                               //     x = 8.0

int count = 25;
Math.sqrt(count);                                // count = 25
count = (int) Math.sqrt(count);                  // count = 5

int a = Math.abs(Math.min(-1, -3));              //     a = 3

Exercise : area

Consider the following method for converting milliseconds into days:

// converts milliseconds to days
public static double toDays(double millis) {
    return millis / 1000.0 / 60.0 / 60.0 / 24.0;
}

Write a similar method named area that takes as a parameter the radius of a circle and that returns the area of the circle. For example, the call area(2.0) should return 12.566370614359172. Recall that area can be computed as π times the radius squared and that Java has a constant called Math.PI.

(Try solving this problem in Practice-It by clicking the icon above.)

Exercise : pay

Write a method named pay that accepts two parameters: a real number for a TA's salary, and an integer for the number of hours the TA worked this week. The method should return how much money to pay the TA. For example, the call pay(5.50, 6) should return 33.0.

The TA should receive "overtime" pay of 1 ½ normal salary for any hours above 8. For example, the call pay(4.00, 11) should return (4.00 * 8) + (6.00 * 3) or 50.0.

(Try solving this problem in Practice-It by clicking the icon above.)

`String` methods

Method name	Description
`charAt(index)`	character at given index
`indexOf(str)`	index where the start of the given `String` appears in this string (-1 if not found)
`length()`	number of characters in this `String`
`replace(str1, str2)`	a new string with all occurrences of `str1` changed to `str2`
`substring(index1, index2)` or `substring(index1)`	the characters in this string from `index1` (inclusive) to `index2` (exclusive); if `index2` is omitted, grabs till end of string
`toLowerCase()`	a new string with all lowercase letters
`toUpperCase()`	a new string with all uppercase letters

Exercise : String expressions

Write the results of each expression with Strings in "quotes" and characters in single quotes ('a')

//       index 0123456789012345
String str1 = "Frodo Baggins";
String str2 = "Gandalf the GRAY";

`str1.length()`		13
`str1.charAt(7)`		'a'
`str2.charAt(0)`		'G'
`str1.indexOf("o")`		2
`str2.toUpperCase()`		"GANDALF THE GRAY"
`str1.toLowerCase().indexOf("B")`		-1
`str1.substring(4)`		"o Baggins"
`str2.substring(3, 14)`		"dalf the GR"
`str2.replace("a", "oo")`		"Goondoolf the GRAY"
`str2.replace("gray", "white")`		"Gandalf the GRAY"
`"str1".replace("r", "range")`		"strange1"

If you finish them all...

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Building Java Programs

Lab 3: Ch. 3: Parameters, Return

Building Java Programs

Lab 3: Ch. 3: Parameters, Return

Today's lab

Parameters

Exercise : Parameters output

Solving "Parameter Mystery" problems

Exercise : Parameter Mystery

Exercise : Syntax errors

Exercise - answer

Exercise - Corrected version

Exercise : printSquare

Exercise : printGrid

Exercise : Parameter Mystery

Exercise : Parameter Mystery

Returning Values

Math expression syntax

Exercise - Math expressions

Exercise : area

Exercise : pay

String methods

Exercise : String expressions

If you finish them all...

Exercise : `printSquare`

Exercise : `printGrid`

`String` methods