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JavaSmall

simple java exercises using jshell

Lab Brief

Learn to use jshell to run Java code.

Steps

Startup jshell

  1. Open a terminal
  2. Type jshell
  3. Type System.out.println("Hello, World!");
  4. Type int x = 5;
  5. Type x
  6. Type x + 5
  7. Type x
  8. Type x = x + 10
  9. Type x
  10. Type void println(String s) { System.out.println(s); }
  11. Type println("your name here!");

Let's save this script to a file.

/save helloWorld.java

and then exit jshell. Use /exit or Ctrl-D.

Section 0

In the steps below, you will be using jshell to run Java code.

Start training your brain by TYPING the code into jshell. DO NOT COPY AND PASTE.

CopyPasta is the enemy of learning. It's cheating yourself. And at this point, it's cheating in this course.

Using jshell to run a simple Java program

Start jshell and type the following code:

Let's use a simple while loop to print out the numbers from 1 to 10.

int i = 1;
while (i <= 10) {
    System.out.println(i);
    i++;
}

Now TYPE it into jshell. Notice how when you type return after that } the loop will run.

Section 1

Using jshell to run a simple Java program with a method

A method in jshell is a little different than a method in a Java program. In jshell, you don't need a class to define a method.

A method is a block of code that performs a specific task. You use methods so you don't have to write the same code over and over again. (Programmers like to be lazy and efficient that way.)

Let's create a simple method to print out the numbers from 1 to 10.

void printNumbers() {
    int i = 1;
    while (i <= 10) {
        System.out.println(i);
        i++;
    }
}

Now TYPE it into jshell. and invoke the method by typing printNumbers(); and then return key.

Using jshell to run a simple Java program with a method that takes a parameter

Let's use a simple method to print out the numbers from 1 to n.

void printNumbers(int n) {
    int i = 1;
    while (i <= n) {
        System.out.println(i);
        i++;
    }
}

Now TYPE it into jshell. and invoke the method by typing printNumbers(5); and then return key. and then printNumbers(10); and then return key. and then printNumbers(20); and then return key.

Using jshell to run a simple Java program with a method that takes a parameter and returns a value

Let's use a simple method to return the sum of the numbers from 1 to n.

int sumNumbers(int n) {
    int i = 1;
    int sum = 0;
    while (i <= n) {
        sum = sum + i;
        i++;
    }
    return sum;
}

Now TYPE it into jshell. and invoke the method by typing sumNumbers(5); and then return key. and then sumNumbers(11); and then return key. and then sumNumbers(23); and then return key. and then sumNumbers(5683); and then return key. and then sumNumbers(106839); and then return key.

Using jshell to run a simple Java program with a method that takes a parameter and returns a value and uses a for loop

Let's use a simple method to return the sum of the numbers from 1 to n.

int sumNumbers(int n) {
    int sum = 0;
    for (int i = 1; i <= n; i++) {
        sum = sum + i;
    }
    return sum;
}

Now TYPE it into jshell. and invoke the method by typing sumNumbers(5); and then return key. and then sumNumbers(10); and then return key. and then sumNumbers(20); and then return key. and then sumNumbers(106839); and then return key.

Using jshell to run a simple Java program with a method that takes a parameter and returns a value and uses a for loop and a method that calls the first method

Let's use a simple method to return the sum of the numbers from 1 to n.

int sumNumbers(int n) {
    int sum = 0;
    for (int i = 1; i <= n; i++) {
        sum = sum + i;
    }
    return sum;
}

int sumNumbersTo10() {
    return sumNumbers(10);
}

Now TYPE it into jshell. and invoke the method by typing sumNumbersTo10(); and then return key.

Use jshel to save the code to a file

/save sumNumbers.java

Use jshell to load the code from a file

/open sumNumbers.java

Use jshell to edit the code

/edit sumNumbers.java

in the popup window, make a change, and then save the file. add a sumNumbersTo13 method that calls sumNumbers(13).

No, I'm not giving you that code, you have to write it yourself.

when done, click the exit button in the popup window.

now, run the sumNumbersTo13 method in jshell.

Use jshell to save the code to a file

/save sumNumbers.java

Write a few methods that takes a parameter and returns a value

Write a method that takes a parameter and returns a value.

int addTwoNumber(int x, int y) {
    return 0; // fix this
}

Now TYPE it into jshell. and invoke the method by typing addTwoNumber(5, 10); and then return key. Notice how the answer is wrong. Fix the method by using /edit and then run it again.

Now /edit and create 3 methods that takes two parameters and returns a math operation of the two numbers.

  1. divide divideTwoNumbers(int x, int y)
  2. multiply multiplyTwoNumbers(int x, int y)
  3. subtract subtractTwoNumbers(int x, int y)

Save the changes.

Section 2

Let's do some methods with if statements.

Write a method that takes a parameter and returns a value

Write a method that takes a parameter and returns a value.

int isEven(int x) {
    return 0; // fix this
}

Now TYPE it into jshell.

and invoke the method by typing isEven(5); and then return key.

Notice how the answer is wrong. Fix the method by using /edit and then run it again. (what might be wrong with the method?) maybe if the number is even, return 1, otherwise return 0.

But, what if we want to return a boolean instead of an int?

Write a method that takes a parameter and returns a boolean

Write a method that takes a parameter and returns a boolean.

boolean isEven(int x) {
    return false; // fix this
}

Now TYPE it into jshell.

and invoke the method by typing isEven(5); and then return key. So... what's wrong with the method? maybe if the number is even, return true, otherwise return false.

Write a method that takes a parameter and returns a boolean

Write a method that takes a parameter and returns a boolean.

boolean isOdd(int x) {
    return false; // fix this
}

Now TYPE it into jshell. and invoke the method by typing isOdd(5); and then return key.

Lets talk about odd and even numbers

How can we tell if a number is odd or even? The most common way is to use the modulo operator %.

See how it works:

var x = 5;
var y = 2;
x % y

What does it return? What does it mean?

now write a while loop that prints out the numbers from 1 to 10 and prints out the modulo of each number with 2.

int i = 1;

while (i <= 10) {
    System.out.println(i + " % 2 = " + i % 2);
    i++;
}

Now TYPE it into jshell.

Hmm. It appears that the % (mod or modulo) operator returns 0 if the number is even and 1 if the number is odd.

How can we use that in isEven and isOdd methods?

Write isOdd and isEven methods using the modulo operator

You'll need an if statement to check if the number is even or odd.

boolean isEven(int x) {
    return x % 2 == 0;
}

And now you write the isOdd method. Remember != means "not equal to".

You could also use == and make the 0 a 1, right?

Hey, why not do a /save?

/save sumNumbers.java

Write a method that prints "odd" or "even" for the numbers from 1 to 10

Write a method that prints "odd" or "even" for the numbers from 1 to 10.

void printOddEven() {
    int i = 1;
    while (i <= 10) {
        if (isEven(i)) {
            System.out.println(i + " is even");
        } else {
            System.out.println(i + " is odd");
        }
        i++;
    }
}

see the else part? what if you remove it? (puzzle over it in your mind. c'mon experiment!)

Now TYPE it into jshell.

Ok, enough here

You've done a lot of work. You've learned a lot about jshell and Java methods. Make sure you save it all to github.

git add .
git commit -m "done with jshell and JavaSmall"
git push

Now, switch to VARS.md and learn about variables in Java.