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First steps in jshell

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Kay Kasemir edited this page Feb 27, 2021 · 4 revisions

The Java 'shell' allows testing simple one-line Java expressions without writing an actual program.

  • Open File Explorer and navigate to the folder C:\Users\Public\wpilib\2021\jdk\bin (in earlier years it was C:\Users\Public\frc2019\jdk\bin)
  • Start the jshell.exe

Things to try:

  • 2+3, followed by ENTER
  • Same, but with many spaces between 2, + and 3
  • 2*(3 + 4), followed by ENTER
  • 2*(3, ENTER, + 4), followed by ENTER
  • Math.sin(Math.PI/2), ENTER

==> The computer can compute, it understands how formulas "work", and spaces usually don't matter.

At the end of each line, remember to type ENTER:

  • 10/5
  • 10/3
  • 10%3
  • 10.0/3
  • "Hello !"
  • "Two and two make " + 2 + 2
  • "Two and two make " + (2 + 2)

==> There are different data types. Integer vs. floating point numbers, Strings.

  • int points=0
  • points
  • points = points + 10
  • points += 5
  • "I have " + points + " points"
  • String name = "Freddy"
  • "Hello, " + name + ", nice to meet you!"

==> Variables can store values.

When you define a variable, you need to specify its type: int for whole numbers, double for floating point numbers, String for text (more details will follow in https://github.com/team2393/FRC/blob/master/src/main/java/first/Types.java).

When you count people, you tend to count whole people. Do you count whole apples, or is there such a thing as half an apple? How many apples will there be per person?

  • int people = 2
  • double apples = 5
  • apples / people
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