JShell is helpful for exploratory programming for calculations, error checking, building gui on the fly.
1. Start
PS F:\> jshell
| Welcome to JShell -- Version 11.0.14
| For an introduction type: /help intro
jshell> /help intro
2. variable assignment
jshell> int x = 8
x ==> 8
3. expression evaluation
jshell> x + x
$2 ==> 16
jshell> y = 2* x
| Error:
| cannot find symbol
| symbol: variable y
| y = 2* x
| ^
jshell> float y = 2 * x
y ==> 16.0
jshell> x = 2 * y
| Error:
| incompatible types: possible lossy conversion from float to int
| x = 2 * y
| ^---^
jshell> System.out.println("Hello");
Hello
4. Definitions
jshell> String twice(String s) {
...> return s + s;
...> }
| created method twice(String)
jshell> twice("Ocean")
$6 ==> "OceanOcean"
jshell> String twice(String s) {
...> return "Twice: " + s;
...> }
| modified method twice(String)
jshell> twice("Forest");
$8 ==> "Twice: Forest"
and a more involved example
jshell> double volume(double radius) {
...> return 4.0 / 3.0 * PI * cube(radius);
...> }
| created method volume(double), however, it cannot be invoked until variable PI, and method cube(double) are declared
jshell> double PI = 3.14145926535
PI ==> 3.14145926535
jshell> volume(2)
| attempted to call method volume(double) which cannot be invoked until method cube(double) is declared
jshell> double cube(double x) {return x * x * x; }
| created method cube(double)
jshell> volume(2)
$17 ==> 33.5088988304
jshell> BigDecimal PI = new BigDecimal("3.141592653589793238462643383")
PI ==> 3.141592653589793238462643383
jshell> volume(2)
| attempted to call method volume(double) which cannot be invoked until this error is corrected:
| bad operand types for binary operator '*'
| first type: double
| second type: java.math.BigDecimal
| return 4.0 / 3.0 * PI * cube(radius);
| ^------------^
5. Reassignment
jshell> String x
x ==> null
jshell> int x = 10
x ==> 10
jshell> String x
x ==> null
jshell> x
x ==> null
6. Error checking
Find out the exceptions specific to an operation to be handled.
jshell> int divide(int x, int y) {
...> return x / y;
...> }
| created method divide(int,int)
jshell> divide(5,0)
| Exception java.lang.ArithmeticException: / by zero
| at divide (#20:2)
| at (#21:1)
jshell> System.identityHashCode(PI)
$23 ==> 1987083830
7. Build GUI
Build a simple visible frame 300x200
jshell> JFrame frame = new JFrame("Demo")
frame ==> javax.swing.JFrame[frame0,0,0,0x0,invalid,hidden, ... tPaneCheckingEnabled=true]
jshell> frame.setSize(300, 200)
jshell> frame.setVisible(true)
jshell> Graphics graphic = frame.getGraphics()
graphic ==> sun.java2d.SunGraphics2D[font=java.awt.Font[famil ... java.awt.SystemColor[i=9]]
8. Program data
List all the variables
jshell> /vars
| int $2 = 16
| float y = 16.0
| String $6 = "OceanOcean"
| String $8 = "Twice: Forest"
| String x = null
| double $15 = 0.0
| double $17 = 33.5088988304
| BigDecimal PI = 3.141592653589793238462643383
| int $23 = 1987083830
| JFrame frame = javax.swing.JFrame[frame0,0,0,300x200,hidden,layout=java.awt.BorderLayout,title=Demo,resizable,normal,defaultCloseOperation=HIDE_ON_CLOSE,rootPane=javax.swing.JRootPane[,8,31,284x161,layout=javax.swing.JRootPane$RootLayout,alignmentX=0.0,alignmentY=0.0,border=,flags=16777673,maximumSize=,minimumSize=,preferredSize=],rootPaneCheckingEnabled=true]
| Graphics graphic = sun.java2d.SunGraphics2D[font=java.awt.Font[family=Dialog,name=Dialog,style=plain,size=12],color=java.awt.SystemColor[i=9]]
List all the methods in the current JShell session.
jshell> /methods
| String twice(String)
| double volume(double)
| which cannot be invoked until this error is corrected:
| bad operand types for binary operator '*'
| first type: double
| second type: java.math.BigDecimal
| return 4.0 / 3.0 * PI * cube(radius);
| ^------------^
| double cube(double)
| int divide(int,int)
9. Load libraries
Loading libraries with --class-path option.
PS F:\> jshell --class-path 'F:/Repo/sds/systemds/target'
| Welcome to JShell -- Version 11.0.14
| For an introduction type: /help intro
jshell> import org.apache.sysds.*;
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